TOMOYO Linux Cross Reference
Linux/mm/vmscan.c

   /*
    *  linux/mm/vmscan.c
    *
    *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
    *
    *  Swap reorganised 29.12.95, Stephen Tweedie.
    *  kswapd added: 7.1.96  sct
    *  Removed kswapd_ctl limits, and swap out as many pages as needed
    *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
   *  Zone aware kswapd started 02/00, Kanoj Sarcar (kanoj@sgi.com).
   *  Multiqueue VM started 5.8.00, Rik van Riel.
   */
  
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/gfp.h>
  #include <linux/kernel_stat.h>
  #include <linux/swap.h>
  #include <linux/pagemap.h>
  #include <linux/init.h>
  #include <linux/highmem.h>
  #include <linux/vmpressure.h>
  #include <linux/vmstat.h>
  #include <linux/file.h>
  #include <linux/writeback.h>
  #include <linux/blkdev.h>
  #include <linux/buffer_head.h>  /* for try_to_release_page(),
                                          buffer_heads_over_limit */
  #include <linux/mm_inline.h>
  #include <linux/backing-dev.h>
  #include <linux/rmap.h>
  #include <linux/topology.h>
  #include <linux/cpu.h>
  #include <linux/cpuset.h>
  #include <linux/compaction.h>
  #include <linux/notifier.h>
  #include <linux/rwsem.h>
  #include <linux/delay.h>
  #include <linux/kthread.h>
  #include <linux/freezer.h>
  #include <linux/memcontrol.h>
  #include <linux/delayacct.h>
  #include <linux/sysctl.h>
  #include <linux/oom.h>
  #include <linux/prefetch.h>
  
  #include <asm/tlbflush.h>
  #include <asm/div64.h>
  
  #include <linux/swapops.h>
  #include <linux/balloon_compaction.h>
  
  #include "internal.h"
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/vmscan.h>
  
  struct scan_control {
          /* Incremented by the number of inactive pages that were scanned */
          unsigned long nr_scanned;
  
          /* Number of pages freed so far during a call to shrink_zones() */
          unsigned long nr_reclaimed;
  
          /* How many pages shrink_list() should reclaim */
          unsigned long nr_to_reclaim;
  
          unsigned long hibernation_mode;
  
          /* This context's GFP mask */
          gfp_t gfp_mask;
  
          int may_writepage;
  
          /* Can mapped pages be reclaimed? */
          int may_unmap;
  
          /* Can pages be swapped as part of reclaim? */
          int may_swap;
  
          int order;
  
          /* Scan (total_size >> priority) pages at once */
          int priority;
  
          /*
           * The memory cgroup that hit its limit and as a result is the
           * primary target of this reclaim invocation.
           */
          struct mem_cgroup *target_mem_cgroup;
  
          /*
           * Nodemask of nodes allowed by the caller. If NULL, all nodes
           * are scanned.
           */
          nodemask_t      *nodemask;
  };
  
  #define lru_to_page(_head) (list_entry((_head)->prev, struct page, lru))
 
 #ifdef ARCH_HAS_PREFETCH
 #define prefetch_prev_lru_page(_page, _base, _field)                    \
         do {                                                            \
                 if ((_page)->lru.prev != _base) {                       \
                         struct page *prev;                              \
                                                                         \
                         prev = lru_to_page(&(_page->lru));              \
                         prefetch(&prev->_field);                        \
                 }                                                       \
         } while (0)
 #else
 #define prefetch_prev_lru_page(_page, _base, _field) do { } while (0)
 #endif
 
 #ifdef ARCH_HAS_PREFETCHW
 #define prefetchw_prev_lru_page(_page, _base, _field)                   \
         do {                                                            \
                 if ((_page)->lru.prev != _base) {                       \
                         struct page *prev;                              \
                                                                         \
                         prev = lru_to_page(&(_page->lru));              \
                         prefetchw(&prev->_field);                       \
                 }                                                       \
         } while (0)
 #else
 #define prefetchw_prev_lru_page(_page, _base, _field) do { } while (0)
 #endif
 
 /*
  * From 0 .. 100.  Higher means more swappy.
  */
 int vm_swappiness = 60;
 unsigned long vm_total_pages;   /* The total number of pages which the VM controls */
 
 static LIST_HEAD(shrinker_list);
 static DECLARE_RWSEM(shrinker_rwsem);
 
 #ifdef CONFIG_MEMCG
 static bool global_reclaim(struct scan_control *sc)
 {
         return !sc->target_mem_cgroup;
 }
 #else
 static bool global_reclaim(struct scan_control *sc)
 {
         return true;
 }
 #endif
 
 static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
 {
         if (!mem_cgroup_disabled())
                 return mem_cgroup_get_lru_size(lruvec, lru);
 
         return zone_page_state(lruvec_zone(lruvec), NR_LRU_BASE + lru);
 }
 
 /*
  * Add a shrinker callback to be called from the vm
  */
 void register_shrinker(struct shrinker *shrinker)
 {
         atomic_long_set(&shrinker->nr_in_batch, 0);
         down_write(&shrinker_rwsem);
         list_add_tail(&shrinker->list, &shrinker_list);
         up_write(&shrinker_rwsem);
 }
 EXPORT_SYMBOL(register_shrinker);
 
 /*
  * Remove one
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
         down_write(&shrinker_rwsem);
         list_del(&shrinker->list);
         up_write(&shrinker_rwsem);
 }
 EXPORT_SYMBOL(unregister_shrinker);
 
 static inline int do_shrinker_shrink(struct shrinker *shrinker,
                                      struct shrink_control *sc,
                                      unsigned long nr_to_scan)
 {
         sc->nr_to_scan = nr_to_scan;
         return (*shrinker->shrink)(shrinker, sc);
 }
 
 #define SHRINK_BATCH 128
 /*
  * Call the shrink functions to age shrinkable caches
  *
  * Here we assume it costs one seek to replace a lru page and that it also
  * takes a seek to recreate a cache object.  With this in mind we age equal
  * percentages of the lru and ageable caches.  This should balance the seeks
  * generated by these structures.
  *
  * If the vm encountered mapped pages on the LRU it increase the pressure on
  * slab to avoid swapping.
  *
  * We do weird things to avoid (scanned*seeks*entries) overflowing 32 bits.
  *
  * `lru_pages' represents the number of on-LRU pages in all the zones which
  * are eligible for the caller's allocation attempt.  It is used for balancing
  * slab reclaim versus page reclaim.
  *
  * Returns the number of slab objects which we shrunk.
  */
 unsigned long shrink_slab(struct shrink_control *shrink,
                           unsigned long nr_pages_scanned,
                           unsigned long lru_pages)
 {
         struct shrinker *shrinker;
         unsigned long ret = 0;
 
         if (nr_pages_scanned == 0)
                 nr_pages_scanned = SWAP_CLUSTER_MAX;
 
         if (!down_read_trylock(&shrinker_rwsem)) {
                 /* Assume we'll be able to shrink next time */
                 ret = 1;
                 goto out;
         }
 
         list_for_each_entry(shrinker, &shrinker_list, list) {
                 unsigned long long delta;
                 long total_scan;
                 long max_pass;
                 int shrink_ret = 0;
                 long nr;
                 long new_nr;
                 long batch_size = shrinker->batch ? shrinker->batch
                                                   : SHRINK_BATCH;
 
                 max_pass = do_shrinker_shrink(shrinker, shrink, 0);
                 if (max_pass <= 0)
                         continue;
 
                 /*
                  * copy the current shrinker scan count into a local variable
                  * and zero it so that other concurrent shrinker invocations
                  * don't also do this scanning work.
                  */
                 nr = atomic_long_xchg(&shrinker->nr_in_batch, 0);
 
                 total_scan = nr;
                 delta = (4 * nr_pages_scanned) / shrinker->seeks;
                 delta *= max_pass;
                 do_div(delta, lru_pages + 1);
                 total_scan += delta;
                 if (total_scan < 0) {
                         printk(KERN_ERR "shrink_slab: %pF negative objects to "
                                "delete nr=%ld\n",
                                shrinker->shrink, total_scan);
                         total_scan = max_pass;
                 }
 
                 /*
                  * We need to avoid excessive windup on filesystem shrinkers
                  * due to large numbers of GFP_NOFS allocations causing the
                  * shrinkers to return -1 all the time. This results in a large
                  * nr being built up so when a shrink that can do some work
                  * comes along it empties the entire cache due to nr >>>
                  * max_pass.  This is bad for sustaining a working set in
                  * memory.
                  *
                  * Hence only allow the shrinker to scan the entire cache when
                  * a large delta change is calculated directly.
                  */
                 if (delta < max_pass / 4)
                         total_scan = min(total_scan, max_pass / 2);
 
                 /*
                  * Avoid risking looping forever due to too large nr value:
                  * never try to free more than twice the estimate number of
                  * freeable entries.
                  */
                 if (total_scan > max_pass * 2)
                         total_scan = max_pass * 2;
 
                 trace_mm_shrink_slab_start(shrinker, shrink, nr,
                                         nr_pages_scanned, lru_pages,
                                         max_pass, delta, total_scan);
 
                 while (total_scan >= batch_size) {
                         int nr_before;
 
                         nr_before = do_shrinker_shrink(shrinker, shrink, 0);
                         shrink_ret = do_shrinker_shrink(shrinker, shrink,
                                                         batch_size);
                         if (shrink_ret == -1)
                                 break;
                         if (shrink_ret < nr_before)
                                 ret += nr_before - shrink_ret;
                         count_vm_events(SLABS_SCANNED, batch_size);
                         total_scan -= batch_size;
 
                         cond_resched();
                 }
 
                 /*
                  * move the unused scan count back into the shrinker in a
                  * manner that handles concurrent updates. If we exhausted the
                  * scan, there is no need to do an update.
                  */
                 if (total_scan > 0)
                         new_nr = atomic_long_add_return(total_scan,
                                         &shrinker->nr_in_batch);
                 else
                         new_nr = atomic_long_read(&shrinker->nr_in_batch);
 
                 trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
         }
         up_read(&shrinker_rwsem);
 out:
         cond_resched();
         return ret;
 }
 
 static inline int is_page_cache_freeable(struct page *page)
 {
         /*
          * A freeable page cache page is referenced only by the caller
          * that isolated the page, the page cache radix tree and
          * optional buffer heads at page->private.
          */
         return page_count(page) - page_has_private(page) == 2;
 }
 
 static int may_write_to_queue(struct backing_dev_info *bdi,
                               struct scan_control *sc)
 {
         if (current->flags & PF_SWAPWRITE)
                 return 1;
         if (!bdi_write_congested(bdi))
                 return 1;
         if (bdi == current->backing_dev_info)
                 return 1;
         return 0;
 }
 
 /*
  * We detected a synchronous write error writing a page out.  Probably
  * -ENOSPC.  We need to propagate that into the address_space for a subsequent
  * fsync(), msync() or close().
  *
  * The tricky part is that after writepage we cannot touch the mapping: nothing
  * prevents it from being freed up.  But we have a ref on the page and once
  * that page is locked, the mapping is pinned.
  *
  * We're allowed to run sleeping lock_page() here because we know the caller has
  * __GFP_FS.
  */
 static void handle_write_error(struct address_space *mapping,
                                 struct page *page, int error)
 {
         lock_page(page);
         if (page_mapping(page) == mapping)
                 mapping_set_error(mapping, error);
         unlock_page(page);
 }
 
 /* possible outcome of pageout() */
 typedef enum {
         /* failed to write page out, page is locked */
         PAGE_KEEP,
         /* move page to the active list, page is locked */
         PAGE_ACTIVATE,
         /* page has been sent to the disk successfully, page is unlocked */
         PAGE_SUCCESS,
         /* page is clean and locked */
         PAGE_CLEAN,
 } pageout_t;
 
 /*
  * pageout is called by shrink_page_list() for each dirty page.
  * Calls ->writepage().
  */
 static pageout_t pageout(struct page *page, struct address_space *mapping,
                          struct scan_control *sc)
 {
         /*
          * If the page is dirty, only perform writeback if that write
          * will be non-blocking.  To prevent this allocation from being
          * stalled by pagecache activity.  But note that there may be
          * stalls if we need to run get_block().  We could test
          * PagePrivate for that.
          *
          * If this process is currently in __generic_file_aio_write() against
          * this page's queue, we can perform writeback even if that
          * will block.
          *
          * If the page is swapcache, write it back even if that would
          * block, for some throttling. This happens by accident, because
          * swap_backing_dev_info is bust: it doesn't reflect the
          * congestion state of the swapdevs.  Easy to fix, if needed.
          */
         if (!is_page_cache_freeable(page))
                 return PAGE_KEEP;
         if (!mapping) {
                 /*
                  * Some data journaling orphaned pages can have
                  * page->mapping == NULL while being dirty with clean buffers.
                  */
                 if (page_has_private(page)) {
                         if (try_to_free_buffers(page)) {
                                 ClearPageDirty(page);
                                 printk("%s: orphaned page\n", __func__);
                                 return PAGE_CLEAN;
                         }
                 }
                 return PAGE_KEEP;
         }
         if (mapping->a_ops->writepage == NULL)
                 return PAGE_ACTIVATE;
         if (!may_write_to_queue(mapping->backing_dev_info, sc))
                 return PAGE_KEEP;
 
         if (clear_page_dirty_for_io(page)) {
                 int res;
                 struct writeback_control wbc = {
                         .sync_mode = WB_SYNC_NONE,
                         .nr_to_write = SWAP_CLUSTER_MAX,
                         .range_start = 0,
                         .range_end = LLONG_MAX,
                         .for_reclaim = 1,
                 };
 
                 SetPageReclaim(page);
                 res = mapping->a_ops->writepage(page, &wbc);
                 if (res < 0)
                         handle_write_error(mapping, page, res);
                 if (res == AOP_WRITEPAGE_ACTIVATE) {
                         ClearPageReclaim(page);
                         return PAGE_ACTIVATE;
                 }
 
                 if (!PageWriteback(page)) {
                         /* synchronous write or broken a_ops? */
                         ClearPageReclaim(page);
                 }
                 trace_mm_vmscan_writepage(page, trace_reclaim_flags(page));
                 inc_zone_page_state(page, NR_VMSCAN_WRITE);
                 return PAGE_SUCCESS;
         }
 
         return PAGE_CLEAN;
 }
 
 /*
  * Same as remove_mapping, but if the page is removed from the mapping, it
  * gets returned with a refcount of 0.
  */
 static int __remove_mapping(struct address_space *mapping, struct page *page)
 {
         BUG_ON(!PageLocked(page));
         BUG_ON(mapping != page_mapping(page));
 
         spin_lock_irq(&mapping->tree_lock);
         /*
          * The non racy check for a busy page.
          *
          * Must be careful with the order of the tests. When someone has
          * a ref to the page, it may be possible that they dirty it then
          * drop the reference. So if PageDirty is tested before page_count
          * here, then the following race may occur:
          *
          * get_user_pages(&page);
          * [user mapping goes away]
          * write_to(page);
          *                              !PageDirty(page)    [good]
          * SetPageDirty(page);
          * put_page(page);
          *                              !page_count(page)   [good, discard it]
          *
          * [oops, our write_to data is lost]
          *
          * Reversing the order of the tests ensures such a situation cannot
          * escape unnoticed. The smp_rmb is needed to ensure the page->flags
          * load is not satisfied before that of page->_count.
          *
          * Note that if SetPageDirty is always performed via set_page_dirty,
          * and thus under tree_lock, then this ordering is not required.
          */
         if (!page_freeze_refs(page, 2))
                 goto cannot_free;
         /* note: atomic_cmpxchg in page_freeze_refs provides the smp_rmb */
         if (unlikely(PageDirty(page))) {
                 page_unfreeze_refs(page, 2);
                 goto cannot_free;
         }
 
         if (PageSwapCache(page)) {
                 swp_entry_t swap = { .val = page_private(page) };
                 __delete_from_swap_cache(page);
                 spin_unlock_irq(&mapping->tree_lock);
                 swapcache_free(swap, page);
         } else {
                 void (*freepage)(struct page *);
 
                 freepage = mapping->a_ops->freepage;
 
                 __delete_from_page_cache(page);
                 spin_unlock_irq(&mapping->tree_lock);
                 mem_cgroup_uncharge_cache_page(page);
 
                 if (freepage != NULL)
                         freepage(page);
         }
 
         return 1;
 
 cannot_free:
         spin_unlock_irq(&mapping->tree_lock);
         return 0;
 }
 
 /*
  * Attempt to detach a locked page from its ->mapping.  If it is dirty or if
  * someone else has a ref on the page, abort and return 0.  If it was
  * successfully detached, return 1.  Assumes the caller has a single ref on
  * this page.
  */
 int remove_mapping(struct address_space *mapping, struct page *page)
 {
         if (__remove_mapping(mapping, page)) {
                 /*
                  * Unfreezing the refcount with 1 rather than 2 effectively
                  * drops the pagecache ref for us without requiring another
                  * atomic operation.
                  */
                 page_unfreeze_refs(page, 1);
                 return 1;
         }
         return 0;
 }
 
 /**
  * putback_lru_page - put previously isolated page onto appropriate LRU list
  * @page: page to be put back to appropriate lru list
  *
  * Add previously isolated @page to appropriate LRU list.
  * Page may still be unevictable for other reasons.
  *
  * lru_lock must not be held, interrupts must be enabled.
  */
 void putback_lru_page(struct page *page)
 {
         int lru;
         int active = !!TestClearPageActive(page);
         int was_unevictable = PageUnevictable(page);
 
         VM_BUG_ON(PageLRU(page));
 
 redo:
         ClearPageUnevictable(page);
 
         if (page_evictable(page)) {
                 /*
                  * For evictable pages, we can use the cache.
                  * In event of a race, worst case is we end up with an
                  * unevictable page on [in]active list.
                  * We know how to handle that.
                  */
                 lru = active + page_lru_base_type(page);
                 lru_cache_add_lru(page, lru);
         } else {
                 /*
                  * Put unevictable pages directly on zone's unevictable
                  * list.
                  */
                 lru = LRU_UNEVICTABLE;
                 add_page_to_unevictable_list(page);
                 /*
                  * When racing with an mlock or AS_UNEVICTABLE clearing
                  * (page is unlocked) make sure that if the other thread
                  * does not observe our setting of PG_lru and fails
                  * isolation/check_move_unevictable_pages,
                  * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
                  * the page back to the evictable list.
                  *
                  * The other side is TestClearPageMlocked() or shmem_lock().
                  */
                 smp_mb();
         }
 
         /*
          * page's status can change while we move it among lru. If an evictable
          * page is on unevictable list, it never be freed. To avoid that,
          * check after we added it to the list, again.
          */
         if (lru == LRU_UNEVICTABLE && page_evictable(page)) {
                 if (!isolate_lru_page(page)) {
                         put_page(page);
                         goto redo;
                 }
                 /* This means someone else dropped this page from LRU
                  * So, it will be freed or putback to LRU again. There is
                  * nothing to do here.
                  */
         }
 
         if (was_unevictable && lru != LRU_UNEVICTABLE)
                 count_vm_event(UNEVICTABLE_PGRESCUED);
         else if (!was_unevictable && lru == LRU_UNEVICTABLE)
                 count_vm_event(UNEVICTABLE_PGCULLED);
 
         put_page(page);         /* drop ref from isolate */
 }
 
 enum page_references {
         PAGEREF_RECLAIM,
         PAGEREF_RECLAIM_CLEAN,
         PAGEREF_KEEP,
         PAGEREF_ACTIVATE,
 };
 
 static enum page_references page_check_references(struct page *page,
                                                   struct scan_control *sc)
 {
         int referenced_ptes, referenced_page;
         unsigned long vm_flags;
 
         referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
                                           &vm_flags);
         referenced_page = TestClearPageReferenced(page);
 
         /*
          * Mlock lost the isolation race with us.  Let try_to_unmap()
          * move the page to the unevictable list.
          */
         if (vm_flags & VM_LOCKED)
                 return PAGEREF_RECLAIM;
 
         if (referenced_ptes) {
                 if (PageSwapBacked(page))
                         return PAGEREF_ACTIVATE;
                 /*
                  * All mapped pages start out with page table
                  * references from the instantiating fault, so we need
                  * to look twice if a mapped file page is used more
                  * than once.
                  *
                  * Mark it and spare it for another trip around the
                  * inactive list.  Another page table reference will
                  * lead to its activation.
                  *
                  * Note: the mark is set for activated pages as well
                  * so that recently deactivated but used pages are
                  * quickly recovered.
                  */
                 SetPageReferenced(page);
 
                 if (referenced_page || referenced_ptes > 1)
                         return PAGEREF_ACTIVATE;
 
                 /*
                  * Activate file-backed executable pages after first usage.
                  */
                 if (vm_flags & VM_EXEC)
                         return PAGEREF_ACTIVATE;
 
                 return PAGEREF_KEEP;
         }
 
         /* Reclaim if clean, defer dirty pages to writeback */
         if (referenced_page && !PageSwapBacked(page))
                 return PAGEREF_RECLAIM_CLEAN;
 
         return PAGEREF_RECLAIM;
 }
 
 /*
  * shrink_page_list() returns the number of reclaimed pages
  */
 static unsigned long shrink_page_list(struct list_head *page_list,
                                       struct zone *zone,
                                       struct scan_control *sc,
                                       enum ttu_flags ttu_flags,
                                       unsigned long *ret_nr_dirty,
                                       unsigned long *ret_nr_writeback,
                                       bool force_reclaim)
 {
         LIST_HEAD(ret_pages);
         LIST_HEAD(free_pages);
         int pgactivate = 0;
         unsigned long nr_dirty = 0;
         unsigned long nr_congested = 0;
         unsigned long nr_reclaimed = 0;
         unsigned long nr_writeback = 0;
 
         cond_resched();
 
         mem_cgroup_uncharge_start();
         while (!list_empty(page_list)) {
                 struct address_space *mapping;
                 struct page *page;
                 int may_enter_fs;
                 enum page_references references = PAGEREF_RECLAIM_CLEAN;
 
                 cond_resched();
 
                 page = lru_to_page(page_list);
                 list_del(&page->lru);
 
                 if (!trylock_page(page))
                         goto keep;
 
                 VM_BUG_ON(PageActive(page));
                 VM_BUG_ON(page_zone(page) != zone);
 
                 sc->nr_scanned++;
 
                 if (unlikely(!page_evictable(page)))
                         goto cull_mlocked;
 
                 if (!sc->may_unmap && page_mapped(page))
                         goto keep_locked;
 
                 /* Double the slab pressure for mapped and swapcache pages */
                 if (page_mapped(page) || PageSwapCache(page))
                         sc->nr_scanned++;
 
                 may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
                         (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
 
                 if (PageWriteback(page)) {
                         /*
                          * memcg doesn't have any dirty pages throttling so we
                          * could easily OOM just because too many pages are in
                          * writeback and there is nothing else to reclaim.
                          *
                          * Require may_enter_fs to wait on writeback, because
                          * fs may not have submitted IO yet. And a loop driver
                          * thread might enter reclaim, and deadlock if it waits
                          * on a page for which it is needed to do the write
                          * (loop masks off __GFP_IO|__GFP_FS for this reason);
                          * but more thought would probably show more reasons.
                          */
                         if (global_reclaim(sc) ||
                             !PageReclaim(page) || !may_enter_fs) {
                                 /*
                                  * This is slightly racy - end_page_writeback()
                                  * might have just cleared PageReclaim, then
                                  * setting PageReclaim here end up interpreted
                                  * as PageReadahead - but that does not matter
                                  * enough to care.  What we do want is for this
                                  * page to have PageReclaim set next time memcg
                                  * reclaim reaches the tests above, so it will
                                  * then wait_on_page_writeback() to avoid OOM;
                                  * and it's also appropriate in global reclaim.
                                  */
                                 SetPageReclaim(page);
                                 nr_writeback++;
                                 goto keep_locked;
                         }
                         wait_on_page_writeback(page);
                 }
 
                 if (!force_reclaim)
                         references = page_check_references(page, sc);
 
                 switch (references) {
                 case PAGEREF_ACTIVATE:
                         goto activate_locked;
                 case PAGEREF_KEEP:
                         goto keep_locked;
                 case PAGEREF_RECLAIM:
                 case PAGEREF_RECLAIM_CLEAN:
                         ; /* try to reclaim the page below */
                 }
 
                 /*
                  * Anonymous process memory has backing store?
                  * Try to allocate it some swap space here.
                  */
                 if (PageAnon(page) && !PageSwapCache(page)) {
                         if (!(sc->gfp_mask & __GFP_IO))
                                 goto keep_locked;
                         if (!add_to_swap(page, page_list))
                                 goto activate_locked;
                         may_enter_fs = 1;
                 }
 
                 mapping = page_mapping(page);
 
                 /*
                  * The page is mapped into the page tables of one or more
                  * processes. Try to unmap it here.
                  */
                 if (page_mapped(page) && mapping) {
                         switch (try_to_unmap(page, ttu_flags)) {
                         case SWAP_FAIL:
                                 goto activate_locked;
                         case SWAP_AGAIN:
                                 goto keep_locked;
                         case SWAP_MLOCK:
                                 goto cull_mlocked;
                         case SWAP_SUCCESS:
                                 ; /* try to free the page below */
                         }
                 }
 
                 if (PageDirty(page)) {
                         nr_dirty++;
 
                         /*
                          * Only kswapd can writeback filesystem pages to
                          * avoid risk of stack overflow but do not writeback
                          * unless under significant pressure.
                          */
                         if (page_is_file_cache(page) &&
                                         (!current_is_kswapd() ||
                                          sc->priority >= DEF_PRIORITY - 2)) {
                                 /*
                                  * Immediately reclaim when written back.
                                  * Similar in principal to deactivate_page()
                                  * except we already have the page isolated
                                  * and know it's dirty
                                  */
                                 inc_zone_page_state(page, NR_VMSCAN_IMMEDIATE);
                                 SetPageReclaim(page);
 
                                 goto keep_locked;
                         }
 
                         if (references == PAGEREF_RECLAIM_CLEAN)
                                 goto keep_locked;
                         if (!may_enter_fs)
                                 goto keep_locked;
                         if (!sc->may_writepage)
                                 goto keep_locked;
 
                         /* Page is dirty, try to write it out here */
                         switch (pageout(page, mapping, sc)) {
                         case PAGE_KEEP:
                                 nr_congested++;
                                 goto keep_locked;
                         case PAGE_ACTIVATE:
                                 goto activate_locked;
                         case PAGE_SUCCESS:
                                 if (PageWriteback(page))
                                         goto keep;
                                 if (PageDirty(page))
                                         goto keep;
 
                                 /*
                                  * A synchronous write - probably a ramdisk.  Go
                                  * ahead and try to reclaim the page.
                                  */
                                 if (!trylock_page(page))
                                         goto keep;
                                 if (PageDirty(page) || PageWriteback(page))
                                         goto keep_locked;
                                 mapping = page_mapping(page);
                         case PAGE_CLEAN:
                                 ; /* try to free the page below */
                         }
                 }
 
                 /*
                  * If the page has buffers, try to free the buffer mappings
                  * associated with this page. If we succeed we try to free
                  * the page as well.
                  *
                  * We do this even if the page is PageDirty().
                  * try_to_release_page() does not perform I/O, but it is
                  * possible for a page to have PageDirty set, but it is actually
                  * clean (all its buffers are clean).  This happens if the
                  * buffers were written out directly, with submit_bh(). ext3
                  * will do this, as well as the blockdev mapping.
                  * try_to_release_page() will discover that cleanness and will
                  * drop the buffers and mark the page clean - it can be freed.
                  *
                  * Rarely, pages can have buffers and no ->mapping.  These are
                  * the pages which were not successfully invalidated in
                  * truncate_complete_page().  We try to drop those buffers here
                  * and if that worked, and the page is no longer mapped into
                  * process address space (page_count == 1) it can be freed.
                  * Otherwise, leave the page on the LRU so it is swappable.
                  */
                 if (page_has_private(page)) {
                         if (!try_to_release_page(page, sc->gfp_mask))
                                 goto activate_locked;
                         if (!mapping && page_count(page) == 1) {
                                 unlock_page(page);
                                 if (put_page_testzero(page))
                                         goto free_it;
                                 else {
                                         /*
                                          * rare race with speculative reference.
                                          * the speculative reference will free
                                          * this page shortly, so we may
                                          * increment nr_reclaimed here (and
                                          * leave it off the LRU).
                                          */
                                         nr_reclaimed++;
                                         continue;
                                 }
                         }
                 }
 
                 if (!mapping || !__remove_mapping(mapping, page))
                         goto keep_locked;
 
                 /*
                  * At this point, we have no other references and there is
                  * no way to pick any more up (removed from LRU, removed
                  * from pagecache). Can use non-atomic bitops now (and
                  * we obviously don't have to worry about waking up a process
                  * waiting on the page lock, because there are no references.
                  */
                 __clear_page_locked(page);
 free_it:
                 nr_reclaimed++;
 
                 /*
                  * Is there need to periodically free_page_list? It would
                  * appear not as the counts should be low
                  */
                 list_add(&page->lru, &free_pages);
                 continue;
 
 cull_mlocked:
                 if (PageSwapCache(page))
                         try_to_free_swap(page);
                 unlock_page(page);
                 list_add(&page->lru, &ret_pages);
                 continue;
 
 activate_locked:
                 /* Not a candidate for swapping, so reclaim swap space. */
                 if (PageSwapCache(page) && vm_swap_full())
                         try_to_free_swap(page);
                 VM_BUG_ON(PageActive(page));
                 SetPageActive(page);
                 pgactivate++;
 keep_locked:
                 unlock_page(page);
 keep:
                 list_add(&page->lru, &ret_pages);
                 VM_BUG_ON(PageLRU(page) || PageUnevictable(page));
         }
 
         /*
          * Tag a zone as congested if all the dirty pages encountered were
          * backed by a congested BDI. In this case, reclaimers should just
          * back off and wait for congestion to clear because further reclaim
          * will encounter the same problem
          */
         if (nr_dirty && nr_dirty == nr_congested && global_reclaim(sc))
                 zone_set_flag(zone, ZONE_CONGESTED);
 
         free_hot_cold_page_list(&free_pages, 1);
 
         list_splice(&ret_pages, page_list);
         count_vm_events(PGACTIVATE, pgactivate);
         mem_cgroup_uncharge_end();
         *ret_nr_dirty += nr_dirty;
         *ret_nr_writeback += nr_writeback;
         return nr_reclaimed;
 }
 
 unsigned long reclaim_clean_pages_from_list(struct zone *zone,
                                             struct list_head *page_list)
 {
         struct scan_control sc = {
                 .gfp_mask = GFP_KERNEL,
                 .priority = DEF_PRIORITY,
                 .may_unmap = 1,
         };
         unsigned long ret, dummy1, dummy2;
         struct page *page, *next;
         LIST_HEAD(clean_pages);
 
         list_for_each_entry_safe(page, next, page_list, lru) {
                 if (page_is_file_cache(page) && !PageDirty(page) &&
                     !isolated_balloon_page(page)) {
                         ClearPageActive(page);
                         list_move(&page->lru, &clean_pages);
                 }
         }
 
         ret = shrink_page_list(&clean_pages, zone, &sc,
                                 TTU_UNMAP|TTU_IGNORE_ACCESS,
                                 &dummy1, &dummy2, true);
         list_splice(&clean_pages, page_list);
         __mod_zone_page_state(zone, NR_ISOLATED_FILE, -ret);
         return ret;
 }
 
 /*
  * Attempt to remove the specified page from its LRU.  Only take this page
  * if it is of the appropriate PageActive status.  Pages which are being
  * freed elsewhere are also ignored.
  *
  * page:        page to consider
  * mode:        one of the LRU isolation modes defined above
  *
  * returns 0 on success, -ve errno on failure.
  */
 int __isolate_lru_page(struct page *page, isolate_mode_t mode)
 {
         int ret = -EINVAL;
 
         /* Only take pages on the LRU. */
         if (!PageLRU(page))
                 return ret;
 
         /* Compaction should not handle unevictable pages but CMA can do so */
         if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
                 return ret;
 
         ret = -EBUSY;
 
         /*
          * To minimise LRU disruption, the caller can indicate that it only
          * wants to isolate pages it will be able to operate on without
          * blocking - clean pages for the most part.
          *
          * ISOLATE_CLEAN means that only clean pages should be isolated. This
          * is used by reclaim when it is cannot write to backing storage
          *
          * ISOLATE_ASYNC_MIGRATE is used to indicate that it only wants to pages
          * that it is possible to migrate without blocking
          */
         if (mode & (ISOLATE_CLEAN|ISOLATE_ASYNC_MIGRATE)) {
                 /* All the caller can do on PageWriteback is block */
                 if (PageWriteback(page))
                         return ret;
 
                 if (PageDirty(page)) {
                         struct address_space *mapping;
 
                         /* ISOLATE_CLEAN means only clean pages */
                         if (mode & ISOLATE_CLEAN)
                                 return ret;
 
                         /*
                          * Only pages without mappings or that have a
                          * ->migratepage callback are possible to migrate
                          * without blocking
                          */
                         mapping = page_mapping(page);
                         if (mapping && !mapping->a_ops->migratepage)
                                 return ret;
                 }
         }
 
         if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
                 return ret;
 
         if (likely(get_page_unless_zero(page))) {
                 /*
                  * Be careful not to clear PageLRU until after we're
                  * sure the page is not being freed elsewhere -- the
                  * page release code relies on it.
                  */
                 ClearPageLRU(page);
                 ret = 0;
         }
 
         return ret;
 }
 
 /*
  * zone->lru_lock is heavily contended.  Some of the functions that
  * shrink the lists perform better by taking out a batch of pages
  * and working on them outside the LRU lock.
  *
  * For pagecache intensive workloads, this function is the hottest
  * spot in the kernel (apart from copy_*_user functions).
  *
  * Appropriate locks must be held before calling this function.
  *
  * @nr_to_scan: The number of pages to look through on the list.
  * @lruvec:     The LRU vector to pull pages from.
  * @dst:        The temp list to put pages on to.
  * @nr_scanned: The number of pages that were scanned.
  * @sc:         The scan_control struct for this reclaim session
  * @mode:       One of the LRU isolation modes
  * @lru:        LRU list id for isolating
  *
  * returns how many pages were moved onto *@dst.
  */
 static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
                 struct lruvec *lruvec, struct list_head *dst,
                 unsigned long *nr_scanned, struct scan_control *sc,
                 isolate_mode_t mode, enum lru_list lru)
 {
         struct list_head *src = &lruvec->lists[lru];
         unsigned long nr_taken = 0;
         unsigned long scan;
 
         for (scan = 0; scan < nr_to_scan && !list_empty(src); scan++) {
                 struct page *page;
                 int nr_pages;
 
                 page = lru_to_page(src);
                 prefetchw_prev_lru_page(page, src, flags);
 
                 VM_BUG_ON(!PageLRU(page));
 
                 switch (__isolate_lru_page(page, mode)) {
                 case 0:
                         nr_pages = hpage_nr_pages(page);
                         mem_cgroup_update_lru_size(lruvec, lru, -nr_pages);
                         list_move(&page->lru, dst);
                         nr_taken += nr_pages;
                         break;
 
                 case -EBUSY:
                         /* else it is being freed elsewhere */
                         list_move(&page->lru, src);
                         continue;
 
                 default:
                         BUG();
                 }
         }
 
         *nr_scanned = scan;
         trace_mm_vmscan_lru_isolate(sc->order, nr_to_scan, scan,
                                     nr_taken, mode, is_file_lru(lru));
         return nr_taken;
 }
 
 /**
  * isolate_lru_page - tries to isolate a page from its LRU list
  * @page: page to isolate from its LRU list
  *
  * Isolates a @page from an LRU list, clears PageLRU and adjusts the
  * vmstat statistic corresponding to whatever LRU list the page was on.
  *
  * Returns 0 if the page was removed from an LRU list.
  * Returns -EBUSY if the page was not on an LRU list.
  *
  * The returned page will have PageLRU() cleared.  If it was found on
  * the active list, it will have PageActive set.  If it was found on
  * the unevictable list, it will have the PageUnevictable bit set. That flag
  * may need to be cleared by the caller before letting the page go.
  *
  * The vmstat statistic corresponding to the list on which the page was
  * found will be decremented.
  *
  * Restrictions:
  * (1) Must be called with an elevated refcount on the page. This is a
  *     fundamentnal difference from isolate_lru_pages (which is called
  *     without a stable reference).
  * (2) the lru_lock must not be held.
  * (3) interrupts must be enabled.
  */
 int isolate_lru_page(struct page *page)
 {
         int ret = -EBUSY;
 
         VM_BUG_ON(!page_count(page));
 
         if (PageLRU(page)) {
                 struct zone *zone = page_zone(page);
                 struct lruvec *lruvec;
 
                 spin_lock_irq(&zone->lru_lock);
                 lruvec = mem_cgroup_page_lruvec(page, zone);
                 if (PageLRU(page)) {
                         int lru = page_lru(page);
                         get_page(page);
                         ClearPageLRU(page);
                         del_page_from_lru_list(page, lruvec, lru);
                         ret = 0;
                 }
                 spin_unlock_irq(&zone->lru_lock);
         }
         return ret;
 }
 
 /*
  * A direct reclaimer may isolate SWAP_CLUSTER_MAX pages from the LRU list and
  * then get resheduled. When there are massive number of tasks doing page
  * allocation, such sleeping direct reclaimers may keep piling up on each CPU,
  * the LRU list will go small and be scanned faster than necessary, leading to
  * unnecessary swapping, thrashing and OOM.
  */
 static int too_many_isolated(struct zone *zone, int file,
                 struct scan_control *sc)
 {
         unsigned long inactive, isolated;
 
         if (current_is_kswapd())
                 return 0;
 
         if (!global_reclaim(sc))
                 return 0;
 
         if (file) {
                 inactive = zone_page_state(zone, NR_INACTIVE_FILE);
                 isolated = zone_page_state(zone, NR_ISOLATED_FILE);
         } else {
                 inactive = zone_page_state(zone, NR_INACTIVE_ANON);
                 isolated = zone_page_state(zone, NR_ISOLATED_ANON);
         }
 
         /*
          * GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so they
          * won't get blocked by normal direct-reclaimers, forming a circular
          * deadlock.
          */
         if ((sc->gfp_mask & GFP_IOFS) == GFP_IOFS)
                 inactive >>= 3;
 
         return isolated > inactive;
 }
 
 static noinline_for_stack void
 putback_inactive_pages(struct lruvec *lruvec, struct list_head *page_list)
 {
         struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
         struct zone *zone = lruvec_zone(lruvec);
         LIST_HEAD(pages_to_free);
 
         /*
          * Put back any unfreeable pages.
          */
         while (!list_empty(page_list)) {
                 struct page *page = lru_to_page(page_list);
                 int lru;
 
                 VM_BUG_ON(PageLRU(page));
                 list_del(&page->lru);
                 if (unlikely(!page_evictable(page))) {
                         spin_unlock_irq(&zone->lru_lock);
                         putback_lru_page(page);
                         spin_lock_irq(&zone->lru_lock);
                         continue;
                 }
 
                 lruvec = mem_cgroup_page_lruvec(page, zone);
 
                 SetPageLRU(page);
                 lru = page_lru(page);
                 add_page_to_lru_list(page, lruvec, lru);
 
                 if (is_active_lru(lru)) {
                         int file = is_file_lru(lru);
                         int numpages = hpage_nr_pages(page);
                         reclaim_stat->recent_rotated[file] += numpages;
                 }
                 if (put_page_testzero(page)) {
                         __ClearPageLRU(page);
                         __ClearPageActive(page);
                         del_page_from_lru_list(page, lruvec, lru);
 
                         if (unlikely(PageCompound(page))) {
                                 spin_unlock_irq(&zone->lru_lock);
                                 (*get_compound_page_dtor(page))(page);
                                 spin_lock_irq(&zone->lru_lock);
                         } else
                                 list_add(&page->lru, &pages_to_free);
                 }
         }
 
         /*
          * To save our caller's stack, now use input list for pages to free.
          */
         list_splice(&pages_to_free, page_list);
 }
 
 /*
  * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
  * of reclaimed pages
  */
 static noinline_for_stack unsigned long
 shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
                      struct scan_control *sc, enum lru_list lru)
 {
         LIST_HEAD(page_list);
         unsigned long nr_scanned;
         unsigned long nr_reclaimed = 0;
         unsigned long nr_taken;
         unsigned long nr_dirty = 0;
         unsigned long nr_writeback = 0;
         isolate_mode_t isolate_mode = 0;
         int file = is_file_lru(lru);
         struct zone *zone = lruvec_zone(lruvec);
         struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
 
         while (unlikely(too_many_isolated(zone, file, sc))) {
                 congestion_wait(BLK_RW_ASYNC, HZ/10);
 
                 /* We are about to die and free our memory. Return now. */
                 if (fatal_signal_pending(current))
                         return SWAP_CLUSTER_MAX;
         }
 
         lru_add_drain();
 
         if (!sc->may_unmap)
                 isolate_mode |= ISOLATE_UNMAPPED;
         if (!sc->may_writepage)
                 isolate_mode |= ISOLATE_CLEAN;
 
         spin_lock_irq(&zone->lru_lock);
 
         nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,
                                      &nr_scanned, sc, isolate_mode, lru);
 
         __mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
         __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
 
         if (global_reclaim(sc)) {
                 zone->pages_scanned += nr_scanned;
                 if (current_is_kswapd())
                         __count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scanned);
                 else
                         __count_zone_vm_events(PGSCAN_DIRECT, zone, nr_scanned);
         }
         spin_unlock_irq(&zone->lru_lock);
 
         if (nr_taken == 0)
                 return 0;
 
         nr_reclaimed = shrink_page_list(&page_list, zone, sc, TTU_UNMAP,
                                         &nr_dirty, &nr_writeback, false);
 
         spin_lock_irq(&zone->lru_lock);
 
         reclaim_stat->recent_scanned[file] += nr_taken;
 
         if (global_reclaim(sc)) {
                 if (current_is_kswapd())
                         __count_zone_vm_events(PGSTEAL_KSWAPD, zone,
                                                nr_reclaimed);
                 else
                         __count_zone_vm_events(PGSTEAL_DIRECT, zone,
                                                nr_reclaimed);
         }
 
         putback_inactive_pages(lruvec, &page_list);
 
         __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
 
         spin_unlock_irq(&zone->lru_lock);
 
         free_hot_cold_page_list(&page_list, 1);
 
         /*
          * If reclaim is isolating dirty pages under writeback, it implies
          * that the long-lived page allocation rate is exceeding the page
          * laundering rate. Either the global limits are not being effective
          * at throttling processes due to the page distribution throughout
          * zones or there is heavy usage of a slow backing device. The
          * only option is to throttle from reclaim context which is not ideal
          * as there is no guarantee the dirtying process is throttled in the
          * same way balance_dirty_pages() manages.
          *
          * This scales the number of dirty pages that must be under writeback
          * before throttling depending on priority. It is a simple backoff
          * function that has the most effect in the range DEF_PRIORITY to
          * DEF_PRIORITY-2 which is the priority reclaim is considered to be
          * in trouble and reclaim is considered to be in trouble.
          *
          * DEF_PRIORITY   100% isolated pages must be PageWriteback to throttle
          * DEF_PRIORITY-1  50% must be PageWriteback
          * DEF_PRIORITY-2  25% must be PageWriteback, kswapd in trouble
          * ...
          * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any
          *                     isolated page is PageWriteback
          */
         if (nr_writeback && nr_writeback >=
                         (nr_taken >> (DEF_PRIORITY - sc->priority)))
                 wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
 
         trace_mm_vmscan_lru_shrink_inactive(zone->zone_pgdat->node_id,
                 zone_idx(zone),
                 nr_scanned, nr_reclaimed,
                 sc->priority,
                 trace_shrink_flags(file));
         return nr_reclaimed;
 }
 
 /*
  * This moves pages from the active list to the inactive list.
  *
  * We move them the other way if the page is referenced by one or more
  * processes, from rmap.
  *
  * If the pages are mostly unmapped, the processing is fast and it is
  * appropriate to hold zone->lru_lock across the whole operation.  But if
  * the pages are mapped, the processing is slow (page_referenced()) so we
  * should drop zone->lru_lock around each page.  It's impossible to balance
  * this, so instead we remove the pages from the LRU while processing them.
  * It is safe to rely on PG_active against the non-LRU pages in here because
  * nobody will play with that bit on a non-LRU page.
  *
  * The downside is that we have to touch page->_count against each page.
  * But we had to alter page->flags anyway.
  */
 
 static void move_active_pages_to_lru(struct lruvec *lruvec,
                                      struct list_head *list,
                                      struct list_head *pages_to_free,
                                      enum lru_list lru)
 {
         struct zone *zone = lruvec_zone(lruvec);
         unsigned long pgmoved = 0;
         struct page *page;
         int nr_pages;
 
         while (!list_empty(list)) {
                 page = lru_to_page(list);
                 lruvec = mem_cgroup_page_lruvec(page, zone);
 
                 VM_BUG_ON(PageLRU(page));
                 SetPageLRU(page);
 
                 nr_pages = hpage_nr_pages(page);
                 mem_cgroup_update_lru_size(lruvec, lru, nr_pages);
                 list_move(&page->lru, &lruvec->lists[lru]);
                 pgmoved += nr_pages;
 
                 if (put_page_testzero(page)) {
                         __ClearPageLRU(page);
                         __ClearPageActive(page);
                         del_page_from_lru_list(page, lruvec, lru);
 
                         if (unlikely(PageCompound(page))) {
                                 spin_unlock_irq(&zone->lru_lock);
                                 (*get_compound_page_dtor(page))(page);
                                 spin_lock_irq(&zone->lru_lock);
                         } else
                                 list_add(&page->lru, pages_to_free);
                 }
         }
         __mod_zone_page_state(zone, NR_LRU_BASE + lru, pgmoved);
         if (!is_active_lru(lru))
                 __count_vm_events(PGDEACTIVATE, pgmoved);
 }
 
 static void shrink_active_list(unsigned long nr_to_scan,
                                struct lruvec *lruvec,
                                struct scan_control *sc,
                                enum lru_list lru)
 {
         unsigned long nr_taken;
         unsigned long nr_scanned;
         unsigned long vm_flags;
         LIST_HEAD(l_hold);      /* The pages which were snipped off */
         LIST_HEAD(l_active);
         LIST_HEAD(l_inactive);
         struct page *page;
         struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
         unsigned long nr_rotated = 0;
         isolate_mode_t isolate_mode = 0;
         int file = is_file_lru(lru);
         struct zone *zone = lruvec_zone(lruvec);
 
         lru_add_drain();
 
         if (!sc->may_unmap)
                 isolate_mode |= ISOLATE_UNMAPPED;
         if (!sc->may_writepage)
                 isolate_mode |= ISOLATE_CLEAN;
 
         spin_lock_irq(&zone->lru_lock);
 
         nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
                                      &nr_scanned, sc, isolate_mode, lru);
         if (global_reclaim(sc))
                 zone->pages_scanned += nr_scanned;
 
         reclaim_stat->recent_scanned[file] += nr_taken;
 
         __count_zone_vm_events(PGREFILL, zone, nr_scanned);
         __mod_zone_page_state(zone, NR_LRU_BASE + lru, -nr_taken);
         __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
         spin_unlock_irq(&zone->lru_lock);
 
         while (!list_empty(&l_hold)) {
                 cond_resched();
                 page = lru_to_page(&l_hold);
                 list_del(&page->lru);
 
                 if (unlikely(!page_evictable(page))) {
                         putback_lru_page(page);
                         continue;
                 }
 
                 if (unlikely(buffer_heads_over_limit)) {
                         if (page_has_private(page) && trylock_page(page)) {
                                 if (page_has_private(page))
                                         try_to_release_page(page, 0);
                                 unlock_page(page);
                         }
                 }
 
                 if (page_referenced(page, 0, sc->target_mem_cgroup,
                                     &vm_flags)) {
                         nr_rotated += hpage_nr_pages(page);
                         /*
                          * Identify referenced, file-backed active pages and
                          * give them one more trip around the active list. So
                          * that executable code get better chances to stay in
                          * memory under moderate memory pressure.  Anon pages
                          * are not likely to be evicted by use-once streaming
                          * IO, plus JVM can create lots of anon VM_EXEC pages,
                          * so we ignore them here.
                          */
                         if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
                                 list_add(&page->lru, &l_active);
                                 continue;
                         }
                 }
 
                 ClearPageActive(page);  /* we are de-activating */
                 list_add(&page->lru, &l_inactive);
         }
 
         /*
          * Move pages back to the lru list.
          */
         spin_lock_irq(&zone->lru_lock);
         /*
          * Count referenced pages from currently used mappings as rotated,
          * even though only some of them are actually re-activated.  This
          * helps balance scan pressure between file and anonymous pages in
          * get_scan_ratio.
          */
         reclaim_stat->recent_rotated[file] += nr_rotated;
 
         move_active_pages_to_lru(lruvec, &l_active, &l_hold, lru);
         move_active_pages_to_lru(lruvec, &l_inactive, &l_hold, lru - LRU_ACTIVE);
         __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, -nr_taken);
         spin_unlock_irq(&zone->lru_lock);
 
         free_hot_cold_page_list(&l_hold, 1);
 }
 
 #ifdef CONFIG_SWAP
 static int inactive_anon_is_low_global(struct zone *zone)
 {
         unsigned long active, inactive;
 
         active = zone_page_state(zone, NR_ACTIVE_ANON);
         inactive = zone_page_state(zone, NR_INACTIVE_ANON);
 
         if (inactive * zone->inactive_ratio < active)
                 return 1;
 
         return 0;
 }
 
 /**
  * inactive_anon_is_low - check if anonymous pages need to be deactivated
  * @lruvec: LRU vector to check
  *
  * Returns true if the zone does not have enough inactive anon pages,
  * meaning some active anon pages need to be deactivated.
  */
 static int inactive_anon_is_low(struct lruvec *lruvec)
 {
         /*
          * If we don't have swap space, anonymous page deactivation
          * is pointless.
          */
         if (!total_swap_pages)
                 return 0;
 
         if (!mem_cgroup_disabled())
                 return mem_cgroup_inactive_anon_is_low(lruvec);
 
         return inactive_anon_is_low_global(lruvec_zone(lruvec));
 }
 #else
 static inline int inactive_anon_is_low(struct lruvec *lruvec)
 {
         return 0;
 }
 #endif
 
 /**
  * inactive_file_is_low - check if file pages need to be deactivated
  * @lruvec: LRU vector to check
  *
  * When the system is doing streaming IO, memory pressure here
  * ensures that active file pages get deactivated, until more
  * than half of the file pages are on the inactive list.
  *
  * Once we get to that situation, protect the system's working
  * set from being evicted by disabling active file page aging.
  *
  * This uses a different ratio than the anonymous pages, because
  * the page cache uses a use-once replacement algorithm.
  */
 static int inactive_file_is_low(struct lruvec *lruvec)
 {
         unsigned long inactive;
         unsigned long active;
 
         inactive = get_lru_size(lruvec, LRU_INACTIVE_FILE);
         active = get_lru_size(lruvec, LRU_ACTIVE_FILE);
 
         return active > inactive;
 }
 
 static int inactive_list_is_low(struct lruvec *lruvec, enum lru_list lru)
 {
         if (is_file_lru(lru))
                 return inactive_file_is_low(lruvec);
         else
                 return inactive_anon_is_low(lruvec);
 }
 
 static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
                                  struct lruvec *lruvec, struct scan_control *sc)
 {
         if (is_active_lru(lru)) {
                 if (inactive_list_is_low(lruvec, lru))
                         shrink_active_list(nr_to_scan, lruvec, sc, lru);
                 return 0;
         }
 
         return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);
 }
 
 static int vmscan_swappiness(struct scan_control *sc)
 {
         if (global_reclaim(sc))
                 return vm_swappiness;
         return mem_cgroup_swappiness(sc->target_mem_cgroup);
 }
 
 enum scan_balance {
         SCAN_EQUAL,
         SCAN_FRACT,
         SCAN_ANON,
         SCAN_FILE,
 };
 
 /*
  * Determine how aggressively the anon and file LRU lists should be
  * scanned.  The relative value of each set of LRU lists is determined
  * by looking at the fraction of the pages scanned we did rotate back
  * onto the active list instead of evict.
  *
  * nr[0] = anon inactive pages to scan; nr[1] = anon active pages to scan
  * nr[2] = file inactive pages to scan; nr[3] = file active pages to scan
  */
 static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
                            unsigned long *nr)
 {
         struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
         u64 fraction[2];
         u64 denominator = 0;    /* gcc */
         struct zone *zone = lruvec_zone(lruvec);
         unsigned long anon_prio, file_prio;
         enum scan_balance scan_balance;
         unsigned long anon, file, free;
         bool force_scan = false;
         unsigned long ap, fp;
         enum lru_list lru;
 
         /*
          * If the zone or memcg is small, nr[l] can be 0.  This
          * results in no scanning on this priority and a potential
          * priority drop.  Global direct reclaim can go to the next
          * zone and tends to have no problems. Global kswapd is for
          * zone balancing and it needs to scan a minimum amount. When
          * reclaiming for a memcg, a priority drop can cause high
          * latencies, so it's better to scan a minimum amount there as
          * well.
          */
         if (current_is_kswapd() && zone->all_unreclaimable)
                 force_scan = true;
         if (!global_reclaim(sc))
                 force_scan = true;
 
         /* If we have no swap space, do not bother scanning anon pages. */
         if (!sc->may_swap || (get_nr_swap_pages() <= 0)) {
                 scan_balance = SCAN_FILE;
                 goto out;
         }
 
         /*
          * Global reclaim will swap to prevent OOM even with no
          * swappiness, but memcg users want to use this knob to
          * disable swapping for individual groups completely when
          * using the memory controller's swap limit feature would be
          * too expensive.
          */
         if (!global_reclaim(sc) && !vmscan_swappiness(sc)) {
                 scan_balance = SCAN_FILE;
                 goto out;
         }
 
         /*
          * Do not apply any pressure balancing cleverness when the
          * system is close to OOM, scan both anon and file equally
          * (unless the swappiness setting disagrees with swapping).
          */
         if (!sc->priority && vmscan_swappiness(sc)) {
                 scan_balance = SCAN_EQUAL;
                 goto out;
         }
 
         anon  = get_lru_size(lruvec, LRU_ACTIVE_ANON) +
                 get_lru_size(lruvec, LRU_INACTIVE_ANON);
         file  = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
                 get_lru_size(lruvec, LRU_INACTIVE_FILE);
 
         /*
          * If it's foreseeable that reclaiming the file cache won't be
          * enough to get the zone back into a desirable shape, we have
          * to swap.  Better start now and leave the - probably heavily
          * thrashing - remaining file pages alone.
          */
         if (global_reclaim(sc)) {
                 free = zone_page_state(zone, NR_FREE_PAGES);
                 if (unlikely(file + free <= high_wmark_pages(zone))) {
                         scan_balance = SCAN_ANON;
                         goto out;
                 }
         }
 
         /*
          * There is enough inactive page cache, do not reclaim
          * anything from the anonymous working set right now.
          */
         if (!inactive_file_is_low(lruvec)) {
                 scan_balance = SCAN_FILE;
                 goto out;
         }
 
         scan_balance = SCAN_FRACT;
 
         /*
          * With swappiness at 100, anonymous and file have the same priority.
          * This scanning priority is essentially the inverse of IO cost.
          */
         anon_prio = vmscan_swappiness(sc);
         file_prio = 200 - anon_prio;
 
         /*
          * OK, so we have swap space and a fair amount of page cache
          * pages.  We use the recently rotated / recently scanned
          * ratios to determine how valuable each cache is.
          *
          * Because workloads change over time (and to avoid overflow)
          * we keep these statistics as a floating average, which ends
          * up weighing recent references more than old ones.
          *
          * anon in [0], file in [1]
          */
         spin_lock_irq(&zone->lru_lock);
         if (unlikely(reclaim_stat->recent_scanned[0] > anon / 4)) {
                 reclaim_stat->recent_scanned[0] /= 2;
                 reclaim_stat->recent_rotated[0] /= 2;
         }
 
         if (unlikely(reclaim_stat->recent_scanned[1] > file / 4)) {
                 reclaim_stat->recent_scanned[1] /= 2;
                 reclaim_stat->recent_rotated[1] /= 2;
         }
 
         /*
          * The amount of pressure on anon vs file pages is inversely
          * proportional to the fraction of recently scanned pages on
          * each list that were recently referenced and in active use.
          */
         ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
         ap /= reclaim_stat->recent_rotated[0] + 1;
 
         fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
         fp /= reclaim_stat->recent_rotated[1] + 1;
         spin_unlock_irq(&zone->lru_lock);
 
         fraction[0] = ap;
         fraction[1] = fp;
         denominator = ap + fp + 1;
 out:
         for_each_evictable_lru(lru) {
                 int file = is_file_lru(lru);
                 unsigned long size;
                 unsigned long scan;
 
                 size = get_lru_size(lruvec, lru);
                 scan = size >> sc->priority;
 
                 if (!scan && force_scan)
                         scan = min(size, SWAP_CLUSTER_MAX);
 
                 switch (scan_balance) {
                 case SCAN_EQUAL:
                         /* Scan lists relative to size */
                         break;
                 case SCAN_FRACT:
                         /*
                          * Scan types proportional to swappiness and
                          * their relative recent reclaim efficiency.
                          */
                         scan = div64_u64(scan * fraction[file], denominator);
                         break;
                 case SCAN_FILE:
                 case SCAN_ANON:
                         /* Scan one type exclusively */
                         if ((scan_balance == SCAN_FILE) != file)
                                 scan = 0;
                         break;
                 default:
                         /* Look ma, no brain */
                         BUG();
                 }
                 nr[lru] = scan;
         }
 }
 
 /*
  * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
  */
 static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 {
         unsigned long nr[NR_LRU_LISTS];
         unsigned long nr_to_scan;
         enum lru_list lru;
         unsigned long nr_reclaimed = 0;
         unsigned long nr_to_reclaim = sc->nr_to_reclaim;
         struct blk_plug plug;
 
         get_scan_count(lruvec, sc, nr);
 
         blk_start_plug(&plug);
         while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
                                         nr[LRU_INACTIVE_FILE]) {
                 for_each_evictable_lru(lru) {
                         if (nr[lru]) {
                                 nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
                                 nr[lru] -= nr_to_scan;
 
                                 nr_reclaimed += shrink_list(lru, nr_to_scan,
                                                             lruvec, sc);
                         }
                 }
                 /*
                  * On large memory systems, scan >> priority can become
                  * really large. This is fine for the starting priority;
                  * we want to put equal scanning pressure on each zone.
                  * However, if the VM has a harder time of freeing pages,
                  * with multiple processes reclaiming pages, the total
                  * freeing target can get unreasonably large.
                  */
                 if (nr_reclaimed >= nr_to_reclaim &&
                     sc->priority < DEF_PRIORITY)
                         break;
         }
         blk_finish_plug(&plug);
         sc->nr_reclaimed += nr_reclaimed;
 
         /*
          * Even if we did not try to evict anon pages at all, we want to
          * rebalance the anon lru active/inactive ratio.
          */
         if (inactive_anon_is_low(lruvec))
                 shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                                    sc, LRU_ACTIVE_ANON);
 
         throttle_vm_writeout(sc->gfp_mask);
 }
 
 /* Use reclaim/compaction for costly allocs or under memory pressure */
 static bool in_reclaim_compaction(struct scan_control *sc)
 {
         if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
                         (sc->order > PAGE_ALLOC_COSTLY_ORDER ||
                          sc->priority < DEF_PRIORITY - 2))
                 return true;
 
         return false;
 }
 
 /*
  * Reclaim/compaction is used for high-order allocation requests. It reclaims
  * order-0 pages before compacting the zone. should_continue_reclaim() returns
  * true if more pages should be reclaimed such that when the page allocator
  * calls try_to_compact_zone() that it will have enough free pages to succeed.
  * It will give up earlier than that if there is difficulty reclaiming pages.
  */
 static inline bool should_continue_reclaim(struct zone *zone,
                                         unsigned long nr_reclaimed,
                                         unsigned long nr_scanned,
                                         struct scan_control *sc)
 {
         unsigned long pages_for_compaction;
         unsigned long inactive_lru_pages;
 
         /* If not in reclaim/compaction mode, stop */
         if (!in_reclaim_compaction(sc))
                 return false;
 
         /* Consider stopping depending on scan and reclaim activity */
         if (sc->gfp_mask & __GFP_REPEAT) {
                 /*
                  * For __GFP_REPEAT allocations, stop reclaiming if the
                  * full LRU list has been scanned and we are still failing
                  * to reclaim pages. This full LRU scan is potentially
                  * expensive but a __GFP_REPEAT caller really wants to succeed
                  */
                 if (!nr_reclaimed && !nr_scanned)
                         return false;
         } else {
                 /*
                  * For non-__GFP_REPEAT allocations which can presumably
                  * fail without consequence, stop if we failed to reclaim
                  * any pages from the last SWAP_CLUSTER_MAX number of
                  * pages that were scanned. This will return to the
                  * caller faster at the risk reclaim/compaction and
                  * the resulting allocation attempt fails
                  */
                 if (!nr_reclaimed)
                         return false;
         }
 
         /*
          * If we have not reclaimed enough pages for compaction and the
          * inactive lists are large enough, continue reclaiming
          */
         pages_for_compaction = (2UL << sc->order);
         inactive_lru_pages = zone_page_state(zone, NR_INACTIVE_FILE);
         if (get_nr_swap_pages() > 0)
                 inactive_lru_pages += zone_page_state(zone, NR_INACTIVE_ANON);
         if (sc->nr_reclaimed < pages_for_compaction &&
                         inactive_lru_pages > pages_for_compaction)
                 return true;
 
         /* If compaction would go ahead or the allocation would succeed, stop */
         switch (compaction_suitable(zone, sc->order)) {
         case COMPACT_PARTIAL:
         case COMPACT_CONTINUE:
                 return false;
         default:
                 return true;
         }
 }
 
 static void shrink_zone(struct zone *zone, struct scan_control *sc)
 {
         unsigned long nr_reclaimed, nr_scanned;
 
         do {
                 struct mem_cgroup *root = sc->target_mem_cgroup;
                 struct mem_cgroup_reclaim_cookie reclaim = {
                         .zone = zone,
                         .priority = sc->priority,
                 };
                 struct mem_cgroup *memcg;
 
                 nr_reclaimed = sc->nr_reclaimed;
                 nr_scanned = sc->nr_scanned;
 
                 memcg = mem_cgroup_iter(root, NULL, &reclaim);
                 do {
                         struct lruvec *lruvec;
 
                         lruvec = mem_cgroup_zone_lruvec(zone, memcg);
 
                         shrink_lruvec(lruvec, sc);
 
                         /*
                          * Direct reclaim and kswapd have to scan all memory
                          * cgroups to fulfill the overall scan target for the
                          * zone.
                          *
                          * Limit reclaim, on the other hand, only cares about
                          * nr_to_reclaim pages to be reclaimed and it will
                          * retry with decreasing priority if one round over the
                          * whole hierarchy is not sufficient.
                          */
                         if (!global_reclaim(sc) &&
                                         sc->nr_reclaimed >= sc->nr_to_reclaim) {
                                 mem_cgroup_iter_break(root, memcg);
                                 break;
                         }
                         memcg = mem_cgroup_iter(root, memcg, &reclaim);
                 } while (memcg);
 
                 vmpressure(sc->gfp_mask, sc->target_mem_cgroup,
                            sc->nr_scanned - nr_scanned,
                            sc->nr_reclaimed - nr_reclaimed);
 
         } while (should_continue_reclaim(zone, sc->nr_reclaimed - nr_reclaimed,
                                          sc->nr_scanned - nr_scanned, sc));
 }
 
 /* Returns true if compaction should go ahead for a high-order request */
 static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
 {
         unsigned long balance_gap, watermark;
         bool watermark_ok;
 
         /* Do not consider compaction for orders reclaim is meant to satisfy */
         if (sc->order <= PAGE_ALLOC_COSTLY_ORDER)
                 return false;
 
         /*
          * Compaction takes time to run and there are potentially other
          * callers using the pages just freed. Continue reclaiming until
          * there is a buffer of free pages available to give compaction
          * a reasonable chance of completing and allocating the page
          */
         balance_gap = min(low_wmark_pages(zone),
                 (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
                         KSWAPD_ZONE_BALANCE_GAP_RATIO);
         watermark = high_wmark_pages(zone) + balance_gap + (2UL << sc->order);
         watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, 0, 0);
 
         /*
          * If compaction is deferred, reclaim up to a point where
          * compaction will have a chance of success when re-enabled
          */
         if (compaction_deferred(zone, sc->order))
                 return watermark_ok;
 
         /* If compaction is not ready to start, keep reclaiming */
         if (!compaction_suitable(zone, sc->order))
                 return false;
 
         return watermark_ok;
 }
 
 /*
  * This is the direct reclaim path, for page-allocating processes.  We only
  * try to reclaim pages from zones which will satisfy the caller's allocation
  * request.
  *
  * We reclaim from a zone even if that zone is over high_wmark_pages(zone).
  * Because:
  * a) The caller may be trying to free *extra* pages to satisfy a higher-order
  *    allocation or
  * b) The target zone may be at high_wmark_pages(zone) but the lower zones
  *    must go *over* high_wmark_pages(zone) to satisfy the `incremental min'
  *    zone defense algorithm.
  *
  * If a zone is deemed to be full of pinned pages then just give it a light
  * scan then give up on it.
  *
  * This function returns true if a zone is being reclaimed for a costly
  * high-order allocation and compaction is ready to begin. This indicates to
  * the caller that it should consider retrying the allocation instead of
  * further reclaim.
  */
 static bool shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
 {
         struct zoneref *z;
         struct zone *zone;
         unsigned long nr_soft_reclaimed;
         unsigned long nr_soft_scanned;
         bool aborted_reclaim = false;
 
         /*
          * If the number of buffer_heads in the machine exceeds the maximum
          * allowed level, force direct reclaim to scan the highmem zone as
          * highmem pages could be pinning lowmem pages storing buffer_heads
          */
         if (buffer_heads_over_limit)
                 sc->gfp_mask |= __GFP_HIGHMEM;
 
         for_each_zone_zonelist_nodemask(zone, z, zonelist,
                                         gfp_zone(sc->gfp_mask), sc->nodemask) {
                 if (!populated_zone(zone))
                         continue;
                 /*
                  * Take care memory controller reclaiming has small influence
                  * to global LRU.
                  */
                 if (global_reclaim(sc)) {
                         if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                                 continue;
                         if (zone->all_unreclaimable &&
                                         sc->priority != DEF_PRIORITY)
                                 continue;       /* Let kswapd poll it */
                         if (IS_ENABLED(CONFIG_COMPACTION)) {
                                 /*
                                  * If we already have plenty of memory free for
                                  * compaction in this zone, don't free any more.
                                  * Even though compaction is invoked for any
                                  * non-zero order, only frequent costly order
                                  * reclamation is disruptive enough to become a
                                  * noticeable problem, like transparent huge
                                  * page allocations.
                                  */
                                 if (compaction_ready(zone, sc)) {
                                         aborted_reclaim = true;
                                         continue;
                                 }
                         }
                         /*
                          * This steals pages from memory cgroups over softlimit
                          * and returns the number of reclaimed pages and
                          * scanned pages. This works for global memory pressure
                          * and balancing, not for a memcg's limit.
                          */
                         nr_soft_scanned = 0;
                         nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
                                                 sc->order, sc->gfp_mask,
                                                 &nr_soft_scanned);
                         sc->nr_reclaimed += nr_soft_reclaimed;
                         sc->nr_scanned += nr_soft_scanned;
                         /* need some check for avoid more shrink_zone() */
                 }
 
                 shrink_zone(zone, sc);
         }
 
         return aborted_reclaim;
 }
 
 static unsigned long zone_reclaimable_pages(struct zone *zone)
 {
         int nr;
 
         nr = zone_page_state(zone, NR_ACTIVE_FILE) +
              zone_page_state(zone, NR_INACTIVE_FILE);
 
         if (get_nr_swap_pages() > 0)
                 nr += zone_page_state(zone, NR_ACTIVE_ANON) +
                       zone_page_state(zone, NR_INACTIVE_ANON);
 
         return nr;
 }
 
 static bool zone_reclaimable(struct zone *zone)
 {
         return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
 }
 
 /* All zones in zonelist are unreclaimable? */
 static bool all_unreclaimable(struct zonelist *zonelist,
                 struct scan_control *sc)
 {
         struct zoneref *z;
         struct zone *zone;
 
         for_each_zone_zonelist_nodemask(zone, z, zonelist,
                         gfp_zone(sc->gfp_mask), sc->nodemask) {
                 if (!populated_zone(zone))
                         continue;
                 if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                         continue;
                 if (!zone->all_unreclaimable)
                         return false;
         }
 
         return true;
 }
 
 /*
  * This is the main entry point to direct page reclaim.
  *
  * If a full scan of the inactive list fails to free enough memory then we
  * are "out of memory" and something needs to be killed.
  *
  * If the caller is !__GFP_FS then the probability of a failure is reasonably
  * high - the zone may be full of dirty or under-writeback pages, which this
  * caller can't do much about.  We kick the writeback threads and take explicit
  * naps in the hope that some of these pages can be written.  But if the
  * allocating task holds filesystem locks which prevent writeout this might not
  * work, and the allocation attempt will fail.
  *
  * returns:     0, if no pages reclaimed
  *              else, the number of pages reclaimed
  */
 static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
                                         struct scan_control *sc,
                                         struct shrink_control *shrink)
 {
         unsigned long total_scanned = 0;
         struct reclaim_state *reclaim_state = current->reclaim_state;
         struct zoneref *z;
         struct zone *zone;
         unsigned long writeback_threshold;
         bool aborted_reclaim;
 
         delayacct_freepages_start();
 
         if (global_reclaim(sc))
                 count_vm_event(ALLOCSTALL);
 
         do {
                 vmpressure_prio(sc->gfp_mask, sc->target_mem_cgroup,
                                 sc->priority);
                 sc->nr_scanned = 0;
                 aborted_reclaim = shrink_zones(zonelist, sc);
 
                 /*
                  * Don't shrink slabs when reclaiming memory from
                  * over limit cgroups
                  */
                 if (global_reclaim(sc)) {
                         unsigned long lru_pages = 0;
                         for_each_zone_zonelist(zone, z, zonelist,
                                         gfp_zone(sc->gfp_mask)) {
                                 if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                                         continue;
 
                                 lru_pages += zone_reclaimable_pages(zone);
                         }
 
                         shrink_slab(shrink, sc->nr_scanned, lru_pages);
                         if (reclaim_state) {
                                 sc->nr_reclaimed += reclaim_state->reclaimed_slab;
                                 reclaim_state->reclaimed_slab = 0;
                         }
                 }
                 total_scanned += sc->nr_scanned;
                 if (sc->nr_reclaimed >= sc->nr_to_reclaim)
                         goto out;
 
                 /*
                  * If we're getting trouble reclaiming, start doing
                  * writepage even in laptop mode.
                  */
                 if (sc->priority < DEF_PRIORITY - 2)
                         sc->may_writepage = 1;
 
                 /*
                  * Try to write back as many pages as we just scanned.  This
                  * tends to cause slow streaming writers to write data to the
                  * disk smoothly, at the dirtying rate, which is nice.   But
                  * that's undesirable in laptop mode, where we *want* lumpy
                  * writeout.  So in laptop mode, write out the whole world.
                  */
                 writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
                 if (total_scanned > writeback_threshold) {
                         wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
                                                 WB_REASON_TRY_TO_FREE_PAGES);
                         sc->may_writepage = 1;
                 }
 
                 /* Take a nap, wait for some writeback to complete */
                 if (!sc->hibernation_mode && sc->nr_scanned &&
                     sc->priority < DEF_PRIORITY - 2) {
                         struct zone *preferred_zone;
 
                         first_zones_zonelist(zonelist, gfp_zone(sc->gfp_mask),
                                                 &cpuset_current_mems_allowed,
                                                 &preferred_zone);
                         wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/10);
                 }
         } while (--sc->priority >= 0);
 
 out:
         delayacct_freepages_end();
 
         if (sc->nr_reclaimed)
                 return sc->nr_reclaimed;
 
         /*
          * As hibernation is going on, kswapd is freezed so that it can't mark
          * the zone into all_unreclaimable. Thus bypassing all_unreclaimable
          * check.
          */
         if (oom_killer_disabled)
                 return 0;
 
         /* Aborted reclaim to try compaction? don't OOM, then */
         if (aborted_reclaim)
                 return 1;
 
         /* top priority shrink_zones still had more to do? don't OOM, then */
         if (global_reclaim(sc) && !all_unreclaimable(zonelist, sc))
                 return 1;
 
         return 0;
 }
 
 static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
 {
         struct zone *zone;
         unsigned long pfmemalloc_reserve = 0;
         unsigned long free_pages = 0;
         int i;
         bool wmark_ok;
 
         for (i = 0; i <= ZONE_NORMAL; i++) {
                 zone = &pgdat->node_zones[i];
                 if (!populated_zone(zone))
                         continue;
 
                 pfmemalloc_reserve += min_wmark_pages(zone);
                 free_pages += zone_page_state(zone, NR_FREE_PAGES);
         }
 
         /* If there are no reserves (unexpected config) then do not throttle */
         if (!pfmemalloc_reserve)
                 return true;
 
         wmark_ok = free_pages > pfmemalloc_reserve / 2;
 
         /* kswapd must be awake if processes are being throttled */
         if (!wmark_ok && waitqueue_active(&pgdat->kswapd_wait)) {
                 pgdat->classzone_idx = min(pgdat->classzone_idx,
                                                 (enum zone_type)ZONE_NORMAL);
                 wake_up_interruptible(&pgdat->kswapd_wait);
         }
 
         return wmark_ok;
 }
 
 /*
  * Throttle direct reclaimers if backing storage is backed by the network
  * and the PFMEMALLOC reserve for the preferred node is getting dangerously
  * depleted. kswapd will continue to make progress and wake the processes
  * when the low watermark is reached.
  *
  * Returns true if a fatal signal was delivered during throttling. If this
  * happens, the page allocator should not consider triggering the OOM killer.
  */
 static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
                                         nodemask_t *nodemask)
 {
         struct zoneref *z;
         struct zone *zone;
         pg_data_t *pgdat = NULL;
 
         /*
          * Kernel threads should not be throttled as they may be indirectly
          * responsible for cleaning pages necessary for reclaim to make forward
          * progress. kjournald for example may enter direct reclaim while
          * committing a transaction where throttling it could forcing other
          * processes to block on log_wait_commit().
          */
         if (current->flags & PF_KTHREAD)
                 goto out;
 
         /*
          * If a fatal signal is pending, this process should not throttle.
          * It should return quickly so it can exit and free its memory
          */
         if (fatal_signal_pending(current))
                 goto out;
 
         /*
          * Check if the pfmemalloc reserves are ok by finding the first node
          * with a usable ZONE_NORMAL or lower zone. The expectation is that
          * GFP_KERNEL will be required for allocating network buffers when
          * swapping over the network so ZONE_HIGHMEM is unusable.
          *
          * Throttling is based on the first usable node and throttled processes
          * wait on a queue until kswapd makes progress and wakes them. There
          * is an affinity then between processes waking up and where reclaim
          * progress has been made assuming the process wakes on the same node.
          * More importantly, processes running on remote nodes will not compete
          * for remote pfmemalloc reserves and processes on different nodes
          * should make reasonable progress.
          */
         for_each_zone_zonelist_nodemask(zone, z, zonelist,
                                         gfp_mask, nodemask) {
                 if (zone_idx(zone) > ZONE_NORMAL)
                         continue;
 
                 /* Throttle based on the first usable node */
                 pgdat = zone->zone_pgdat;
                 if (pfmemalloc_watermark_ok(pgdat))
                         goto out;
                 break;
         }
 
         /* If no zone was usable by the allocation flags then do not throttle */
         if (!pgdat)
                 goto out;
 
         /* Account for the throttling */
         count_vm_event(PGSCAN_DIRECT_THROTTLE);
 
         /*
          * If the caller cannot enter the filesystem, it's possible that it
          * is due to the caller holding an FS lock or performing a journal
          * transaction in the case of a filesystem like ext[3|4]. In this case,
          * it is not safe to block on pfmemalloc_wait as kswapd could be
          * blocked waiting on the same lock. Instead, throttle for up to a
          * second before continuing.
          */
         if (!(gfp_mask & __GFP_FS)) {
                 wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
                         pfmemalloc_watermark_ok(pgdat), HZ);
 
                 goto check_pending;
         }
 
         /* Throttle until kswapd wakes the process */
         wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
                 pfmemalloc_watermark_ok(pgdat));
 
 check_pending:
         if (fatal_signal_pending(current))
                 return true;
 
 out:
         return false;
 }
 
 unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
                                 gfp_t gfp_mask, nodemask_t *nodemask)
 {
         unsigned long nr_reclaimed;
         struct scan_control sc = {
                 .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
                 .may_writepage = !laptop_mode,
                 .nr_to_reclaim = SWAP_CLUSTER_MAX,
                 .may_unmap = 1,
                 .may_swap = 1,
                 .order = order,
                 .priority = DEF_PRIORITY,
                 .target_mem_cgroup = NULL,
                 .nodemask = nodemask,
         };
         struct shrink_control shrink = {
                 .gfp_mask = sc.gfp_mask,
         };
 
         /*
          * Do not enter reclaim if fatal signal was delivered while throttled.
          * 1 is returned so that the page allocator does not OOM kill at this
          * point.
          */
         if (throttle_direct_reclaim(gfp_mask, zonelist, nodemask))
                 return 1;
 
         trace_mm_vmscan_direct_reclaim_begin(order,
                                 sc.may_writepage,
                                 gfp_mask);
 
         nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);
 
         trace_mm_vmscan_direct_reclaim_end(nr_reclaimed);
 
         return nr_reclaimed;
 }
 
 #ifdef CONFIG_MEMCG
 
 unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *memcg,
                                                 gfp_t gfp_mask, bool noswap,
                                                 struct zone *zone,
                                                 unsigned long *nr_scanned)
 {
         struct scan_control sc = {
                 .nr_scanned = 0,
                 .nr_to_reclaim = SWAP_CLUSTER_MAX,
                 .may_writepage = !laptop_mode,
                 .may_unmap = 1,
                 .may_swap = !noswap,
                 .order = 0,
                 .priority = 0,
                 .target_mem_cgroup = memcg,
         };
         struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);
 
         sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                         (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
 
         trace_mm_vmscan_memcg_softlimit_reclaim_begin(sc.order,
                                                       sc.may_writepage,
                                                       sc.gfp_mask);
 
         /*
          * NOTE: Although we can get the priority field, using it
          * here is not a good idea, since it limits the pages we can scan.
          * if we don't reclaim here, the shrink_zone from balance_pgdat
          * will pick up pages from other mem cgroup's as well. We hack
          * the priority and make it zero.
          */
         shrink_lruvec(lruvec, &sc);
 
         trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
 
         *nr_scanned = sc.nr_scanned;
         return sc.nr_reclaimed;
 }
 
 unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
                                            gfp_t gfp_mask,
                                            bool noswap)
 {
         struct zonelist *zonelist;
         unsigned long nr_reclaimed;
         int nid;
         struct scan_control sc = {
                 .may_writepage = !laptop_mode,
                 .may_unmap = 1,
                 .may_swap = !noswap,
                 .nr_to_reclaim = SWAP_CLUSTER_MAX,
                 .order = 0,
                 .priority = DEF_PRIORITY,
                 .target_mem_cgroup = memcg,
                 .nodemask = NULL, /* we don't care the placement */
                 .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
                                 (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
         };
         struct shrink_control shrink = {
                 .gfp_mask = sc.gfp_mask,
         };
 
         /*
          * Unlike direct reclaim via alloc_pages(), memcg's reclaim doesn't
          * take care of from where we get pages. So the node where we start the
          * scan does not need to be the current node.
          */
         nid = mem_cgroup_select_victim_node(memcg);
 
         zonelist = NODE_DATA(nid)->node_zonelists;
 
         trace_mm_vmscan_memcg_reclaim_begin(0,
                                             sc.may_writepage,
                                             sc.gfp_mask);
 
         nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);
 
         trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
 
         return nr_reclaimed;
 }
 #endif
 
 static void age_active_anon(struct zone *zone, struct scan_control *sc)
 {
         struct mem_cgroup *memcg;
 
         if (!total_swap_pages)
                 return;
 
         memcg = mem_cgroup_iter(NULL, NULL, NULL);
         do {
                 struct lruvec *lruvec = mem_cgroup_zone_lruvec(zone, memcg);
 
                 if (inactive_anon_is_low(lruvec))
                         shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
                                            sc, LRU_ACTIVE_ANON);
 
                 memcg = mem_cgroup_iter(NULL, memcg, NULL);
         } while (memcg);
 }
 
 static bool zone_balanced(struct zone *zone, int order,
                           unsigned long balance_gap, int classzone_idx)
 {
         if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone) +
                                     balance_gap, classzone_idx, 0))
                 return false;
 
         if (IS_ENABLED(CONFIG_COMPACTION) && order &&
             !compaction_suitable(zone, order))
                 return false;
 
         return true;
 }
 
 /*
  * pgdat_balanced() is used when checking if a node is balanced.
  *
  * For order-0, all zones must be balanced!
  *
  * For high-order allocations only zones that meet watermarks and are in a
  * zone allowed by the callers classzone_idx are added to balanced_pages. The
  * total of balanced pages must be at least 25% of the zones allowed by
  * classzone_idx for the node to be considered balanced. Forcing all zones to
  * be balanced for high orders can cause excessive reclaim when there are
  * imbalanced zones.
  * The choice of 25% is due to
  *   o a 16M DMA zone that is balanced will not balance a zone on any
  *     reasonable sized machine
  *   o On all other machines, the top zone must be at least a reasonable
  *     percentage of the middle zones. For example, on 32-bit x86, highmem
  *     would need to be at least 256M for it to be balance a whole node.
  *     Similarly, on x86-64 the Normal zone would need to be at least 1G
  *     to balance a node on its own. These seemed like reasonable ratios.
  */
 static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
 {
         unsigned long managed_pages = 0;
         unsigned long balanced_pages = 0;
         int i;
 
         /* Check the watermark levels */
         for (i = 0; i <= classzone_idx; i++) {
                 struct zone *zone = pgdat->node_zones + i;
 
                 if (!populated_zone(zone))
                         continue;
 
                 managed_pages += zone->managed_pages;
 
                 /*
                  * A special case here:
                  *
                  * balance_pgdat() skips over all_unreclaimable after
                  * DEF_PRIORITY. Effectively, it considers them balanced so
                  * they must be considered balanced here as well!
                  */
                 if (zone->all_unreclaimable) {
                         balanced_pages += zone->managed_pages;
                         continue;
                 }
 
                 if (zone_balanced(zone, order, 0, i))
                         balanced_pages += zone->managed_pages;
                 else if (!order)
                         return false;
         }
 
         if (order)
                 return balanced_pages >= (managed_pages >> 2);
         else
                 return true;
 }
 
 /*
  * Prepare kswapd for sleeping. This verifies that there are no processes
  * waiting in throttle_direct_reclaim() and that watermarks have been met.
  *
  * Returns true if kswapd is ready to sleep
  */
 static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
                                         int classzone_idx)
 {
         /* If a direct reclaimer woke kswapd within HZ/10, it's premature */
         if (remaining)
                 return false;
 
         /*
          * The throttled processes are normally woken up in balance_pgdat() as
          * soon as pfmemalloc_watermark_ok() is true. But there is a potential
          * race between when kswapd checks the watermarks and a process gets
          * throttled. There is also a potential race if processes get
          * throttled, kswapd wakes, a large process exits thereby balancing the
          * zones, which causes kswapd to exit balance_pgdat() before reaching
          * the wake up checks. If kswapd is going to sleep, no process should
          * be sleeping on pfmemalloc_wait, so wake them now if necessary. If
          * the wake up is premature, processes will wake kswapd and get
          * throttled again. The difference from wake ups in balance_pgdat() is
          * that here we are under prepare_to_wait().
          */
         if (waitqueue_active(&pgdat->pfmemalloc_wait))
                 wake_up_all(&pgdat->pfmemalloc_wait);
 
         return pgdat_balanced(pgdat, order, classzone_idx);
 }
 
 /*
  * For kswapd, balance_pgdat() will work across all this node's zones until
  * they are all at high_wmark_pages(zone).
  *
  * Returns the final order kswapd was reclaiming at
  *
  * There is special handling here for zones which are full of pinned pages.
  * This can happen if the pages are all mlocked, or if they are all used by
  * device drivers (say, ZONE_DMA).  Or if they are all in use by hugetlb.
  * What we do is to detect the case where all pages in the zone have been
  * scanned twice and there has been zero successful reclaim.  Mark the zone as
  * dead and from now on, only perform a short scan.  Basically we're polling
  * the zone for when the problem goes away.
  *
  * kswapd scans the zones in the highmem->normal->dma direction.  It skips
  * zones which have free_pages > high_wmark_pages(zone), but once a zone is
  * found to have free_pages <= high_wmark_pages(zone), we scan that zone and the
  * lower zones regardless of the number of free pages in the lower zones. This
  * interoperates with the page allocator fallback scheme to ensure that aging
  * of pages is balanced across the zones.
  */
 static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                                                         int *classzone_idx)
 {
         bool pgdat_is_balanced = false;
         int i;
         int end_zone = 0;       /* Inclusive.  0 = ZONE_DMA */
         struct reclaim_state *reclaim_state = current->reclaim_state;
         unsigned long nr_soft_reclaimed;
         unsigned long nr_soft_scanned;
         struct scan_control sc = {
                 .gfp_mask = GFP_KERNEL,
                 .may_unmap = 1,
                 .may_swap = 1,
                 /*
                  * kswapd doesn't want to be bailed out while reclaim. because
                  * we want to put equal scanning pressure on each zone.
                  */
                 .nr_to_reclaim = ULONG_MAX,
                 .order = order,
                 .target_mem_cgroup = NULL,
         };
         struct shrink_control shrink = {
                 .gfp_mask = sc.gfp_mask,
         };
 loop_again:
         sc.priority = DEF_PRIORITY;
         sc.nr_reclaimed = 0;
         sc.may_writepage = !laptop_mode;
         count_vm_event(PAGEOUTRUN);
 
         do {
                 unsigned long lru_pages = 0;
 
                 /*
                  * Scan in the highmem->dma direction for the highest
                  * zone which needs scanning
                  */
                 for (i = pgdat->nr_zones - 1; i >= 0; i--) {
                         struct zone *zone = pgdat->node_zones + i;
 
                         if (!populated_zone(zone))
                                 continue;
 
                         if (zone->all_unreclaimable &&
                             sc.priority != DEF_PRIORITY)
                                 continue;
 
                         /*
                          * Do some background aging of the anon list, to give
                          * pages a chance to be referenced before reclaiming.
                          */
                         age_active_anon(zone, &sc);
 
                         /*
                          * If the number of buffer_heads in the machine
                          * exceeds the maximum allowed level and this node
                          * has a highmem zone, force kswapd to reclaim from
                          * it to relieve lowmem pressure.
                          */
                         if (buffer_heads_over_limit && is_highmem_idx(i)) {
                                 end_zone = i;
                                 break;
                         }
 
                         if (!zone_balanced(zone, order, 0, 0)) {
                                 end_zone = i;
                                 break;
                         } else {
                                 /* If balanced, clear the congested flag */
                                 zone_clear_flag(zone, ZONE_CONGESTED);
                         }
                 }
 
                 if (i < 0) {
                         pgdat_is_balanced = true;
                         goto out;
                 }
 
                 for (i = 0; i <= end_zone; i++) {
                         struct zone *zone = pgdat->node_zones + i;
 
                         lru_pages += zone_reclaimable_pages(zone);
                 }
 
                 /*
                  * Now scan the zone in the dma->highmem direction, stopping
                  * at the last zone which needs scanning.
                  *
                  * We do this because the page allocator works in the opposite
                  * direction.  This prevents the page allocator from allocating
                  * pages behind kswapd's direction of progress, which would
                  * cause too much scanning of the lower zones.
                  */
                 for (i = 0; i <= end_zone; i++) {
                         struct zone *zone = pgdat->node_zones + i;
                         int nr_slab, testorder;
                         unsigned long balance_gap;
 
                         if (!populated_zone(zone))
                                 continue;
 
                         if (zone->all_unreclaimable &&
                             sc.priority != DEF_PRIORITY)
                                 continue;
 
                         sc.nr_scanned = 0;
 
                         nr_soft_scanned = 0;
                         /*
                          * Call soft limit reclaim before calling shrink_zone.
                          */
                         nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
                                                         order, sc.gfp_mask,
                                                         &nr_soft_scanned);
                         sc.nr_reclaimed += nr_soft_reclaimed;
 
                         /*
                          * We put equal pressure on every zone, unless
                          * one zone has way too many pages free
                          * already. The "too many pages" is defined
                          * as the high wmark plus a "gap" where the
                          * gap is either the low watermark or 1%
                          * of the zone, whichever is smaller.
                          */
                         balance_gap = min(low_wmark_pages(zone),
                                 (zone->managed_pages +
                                         KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
                                 KSWAPD_ZONE_BALANCE_GAP_RATIO);
                         /*
                          * Kswapd reclaims only single pages with compaction
                          * enabled. Trying too hard to reclaim until contiguous
                          * free pages have become available can hurt performance
                          * by evicting too much useful data from memory.
                          * Do not reclaim more than needed for compaction.
                          */
                         testorder = order;
                         if (IS_ENABLED(CONFIG_COMPACTION) && order &&
                                         compaction_suitable(zone, order) !=
                                                 COMPACT_SKIPPED)
                                 testorder = 0;
 
                         if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
                             !zone_balanced(zone, testorder,
                                            balance_gap, end_zone)) {
                                 shrink_zone(zone, &sc);
 
                                 reclaim_state->reclaimed_slab = 0;
                                 nr_slab = shrink_slab(&shrink, sc.nr_scanned, lru_pages);
                                 sc.nr_reclaimed += reclaim_state->reclaimed_slab;
 
                                 if (nr_slab == 0 && !zone_reclaimable(zone))
                                         zone->all_unreclaimable = 1;
                         }
 
                         /*
                          * If we're getting trouble reclaiming, start doing
                          * writepage even in laptop mode.
                          */
                         if (sc.priority < DEF_PRIORITY - 2)
                                 sc.may_writepage = 1;
 
                         if (zone->all_unreclaimable) {
                                 if (end_zone && end_zone == i)
                                         end_zone--;
                                 continue;
                         }
 
                         if (zone_balanced(zone, testorder, 0, end_zone))
                                 /*
                                  * If a zone reaches its high watermark,
                                  * consider it to be no longer congested. It's
                                  * possible there are dirty pages backed by
                                  * congested BDIs but as pressure is relieved,
                                  * speculatively avoid congestion waits
                                  */
                                 zone_clear_flag(zone, ZONE_CONGESTED);
                 }
 
                 /*
                  * If the low watermark is met there is no need for processes
                  * to be throttled on pfmemalloc_wait as they should not be
                  * able to safely make forward progress. Wake them
                  */
                 if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
                                 pfmemalloc_watermark_ok(pgdat))
                         wake_up(&pgdat->pfmemalloc_wait);
 
                 if (pgdat_balanced(pgdat, order, *classzone_idx)) {
                         pgdat_is_balanced = true;
                         break;          /* kswapd: all done */
                 }
 
                 /*
                  * We do this so kswapd doesn't build up large priorities for
                  * example when it is freeing in parallel with allocators. It
                  * matches the direct reclaim path behaviour in terms of impact
                  * on zone->*_priority.
                  */
                 if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
                         break;
         } while (--sc.priority >= 0);
 
 out:
         if (!pgdat_is_balanced) {
                 cond_resched();
 
                 try_to_freeze();
 
                 /*
                  * Fragmentation may mean that the system cannot be
                  * rebalanced for high-order allocations in all zones.
                  * At this point, if nr_reclaimed < SWAP_CLUSTER_MAX,
                  * it means the zones have been fully scanned and are still
                  * not balanced. For high-order allocations, there is
                  * little point trying all over again as kswapd may
                  * infinite loop.
                  *
                  * Instead, recheck all watermarks at order-0 as they
                  * are the most important. If watermarks are ok, kswapd will go
                  * back to sleep. High-order users can still perform direct
                  * reclaim if they wish.
                  */
                 if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
                         order = sc.order = 0;
 
                 goto loop_again;
         }
 
         /*
          * If kswapd was reclaiming at a higher order, it has the option of
          * sleeping without all zones being balanced. Before it does, it must
          * ensure that the watermarks for order-0 on *all* zones are met and
          * that the congestion flags are cleared. The congestion flag must
          * be cleared as kswapd is the only mechanism that clears the flag
          * and it is potentially going to sleep here.
          */
         if (order) {
                 int zones_need_compaction = 1;
 
                 for (i = 0; i <= end_zone; i++) {
                         struct zone *zone = pgdat->node_zones + i;
 
                         if (!populated_zone(zone))
                                 continue;
 
                         /* Check if the memory needs to be defragmented. */
                         if (zone_watermark_ok(zone, order,
                                     low_wmark_pages(zone), *classzone_idx, 0))
                                 zones_need_compaction = 0;
                 }
 
                 if (zones_need_compaction)
                         compact_pgdat(pgdat, order);
         }
 
         /*
          * Return the order we were reclaiming at so prepare_kswapd_sleep()
          * makes a decision on the order we were last reclaiming at. However,
          * if another caller entered the allocator slow path while kswapd
          * was awake, order will remain at the higher level
          */
         *classzone_idx = end_zone;
         return order;
 }
 
 static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, int classzone_idx)
 {
         long remaining = 0;
         DEFINE_WAIT(wait);
 
         if (freezing(current) || kthread_should_stop())
                 return;
 
         prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
 
         /* Try to sleep for a short interval */
         if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
                 remaining = schedule_timeout(HZ/10);
                 finish_wait(&pgdat->kswapd_wait, &wait);
                 prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
         }
 
         /*
          * After a short sleep, check if it was a premature sleep. If not, then
          * go fully to sleep until explicitly woken up.
          */
         if (prepare_kswapd_sleep(pgdat, order, remaining, classzone_idx)) {
                 trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
 
                 /*
                  * vmstat counters are not perfectly accurate and the estimated
                  * value for counters such as NR_FREE_PAGES can deviate from the
                  * true value by nr_online_cpus * threshold. To avoid the zone
                  * watermarks being breached while under pressure, we reduce the
                  * per-cpu vmstat threshold while kswapd is awake and restore
                  * them before going back to sleep.
                  */
                 set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold);
 
                 /*
                  * Compaction records what page blocks it recently failed to
                  * isolate pages from and skips them in the future scanning.
                  * When kswapd is going to sleep, it is reasonable to assume
                  * that pages and compaction may succeed so reset the cache.
                  */
                 reset_isolation_suitable(pgdat);
 
                 if (!kthread_should_stop())
                         schedule();
 
                 set_pgdat_percpu_threshold(pgdat, calculate_pressure_threshold);
         } else {
                 if (remaining)
                         count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
                 else
                         count_vm_event(KSWAPD_HIGH_WMARK_HIT_QUICKLY);
         }
         finish_wait(&pgdat->kswapd_wait, &wait);
 }
 
 /*
  * The background pageout daemon, started as a kernel thread
  * from the init process.
  *
  * This basically trickles out pages so that we have _some_
  * free memory available even if there is no other activity
  * that frees anything up. This is needed for things like routing
  * etc, where we otherwise might have all activity going on in
  * asynchronous contexts that cannot page things out.
  *
  * If there are applications that are active memory-allocators
  * (most normal use), this basically shouldn't matter.
  */
 static int kswapd(void *p)
 {
         unsigned long order, new_order;
         unsigned balanced_order;
         int classzone_idx, new_classzone_idx;
         int balanced_classzone_idx;
         pg_data_t *pgdat = (pg_data_t*)p;
         struct task_struct *tsk = current;
 
         struct reclaim_state reclaim_state = {
                 .reclaimed_slab = 0,
         };
         const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
 
         lockdep_set_current_reclaim_state(GFP_KERNEL);
 
         if (!cpumask_empty(cpumask))
                 set_cpus_allowed_ptr(tsk, cpumask);
         current->reclaim_state = &reclaim_state;
 
         /*
          * Tell the memory management that we're a "memory allocator",
          * and that if we need more memory we should get access to it
          * regardless (see "__alloc_pages()"). "kswapd" should
          * never get caught in the normal page freeing logic.
          *
          * (Kswapd normally doesn't need memory anyway, but sometimes
          * you need a small amount of memory in order to be able to
          * page out something else, and this flag essentially protects
          * us from recursively trying to free more memory as we're
          * trying to free the first piece of memory in the first place).
          */
         tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
         set_freezable();
 
         order = new_order = 0;
         balanced_order = 0;
         classzone_idx = new_classzone_idx = pgdat->nr_zones - 1;
         balanced_classzone_idx = classzone_idx;
         for ( ; ; ) {
                 bool ret;
 
                 /*
                  * If the last balance_pgdat was unsuccessful it's unlikely a
                  * new request of a similar or harder type will succeed soon
                  * so consider going to sleep on the basis we reclaimed at
                  */
                 if (balanced_classzone_idx >= new_classzone_idx &&
                                         balanced_order == new_order) {
                         new_order = pgdat->kswapd_max_order;
                         new_classzone_idx = pgdat->classzone_idx;
                         pgdat->kswapd_max_order =  0;
                         pgdat->classzone_idx = pgdat->nr_zones - 1;
                 }
 
                 if (order < new_order || classzone_idx > new_classzone_idx) {
                         /*
                          * Don't sleep if someone wants a larger 'order'
                          * allocation or has tigher zone constraints
                          */
                         order = new_order;
                         classzone_idx = new_classzone_idx;
                 } else {
                         kswapd_try_to_sleep(pgdat, balanced_order,
                                                 balanced_classzone_idx);
                         order = pgdat->kswapd_max_order;
                         classzone_idx = pgdat->classzone_idx;
                         new_order = order;
                         new_classzone_idx = classzone_idx;
                         pgdat->kswapd_max_order = 0;
                         pgdat->classzone_idx = pgdat->nr_zones - 1;
                 }
 
                 ret = try_to_freeze();
                 if (kthread_should_stop())
                         break;
 
                 /*
                  * We can speed up thawing tasks if we don't call balance_pgdat
                  * after returning from the refrigerator
                  */
                 if (!ret) {
                         trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
                         balanced_classzone_idx = classzone_idx;
                         balanced_order = balance_pgdat(pgdat, order,
                                                 &balanced_classzone_idx);
                 }
         }
 
         tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
         current->reclaim_state = NULL;
         lockdep_clear_current_reclaim_state();
 
         return 0;
 }
 
 /*
  * A zone is low on free memory, so wake its kswapd task to service it.
  */
 void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
 {
         pg_data_t *pgdat;
 
         if (!populated_zone(zone))
                 return;
 
         if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
                 return;
         pgdat = zone->zone_pgdat;
         if (pgdat->kswapd_max_order < order) {
                 pgdat->kswapd_max_order = order;
                 pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
         }
         if (!waitqueue_active(&pgdat->kswapd_wait))
                 return;
         if (zone_watermark_ok_safe(zone, order, low_wmark_pages(zone), 0, 0))
                 return;
 
         trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
         wake_up_interruptible(&pgdat->kswapd_wait);
 }
 
 #ifdef CONFIG_HIBERNATION
 /*
  * Try to free `nr_to_reclaim' of memory, system-wide, and return the number of
  * freed pages.
  *
  * Rather than trying to age LRUs the aim is to preserve the overall
  * LRU order by reclaiming preferentially
  * inactive > active > active referenced > active mapped
  */
 unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
 {
         struct reclaim_state reclaim_state;
         struct scan_control sc = {
                 .gfp_mask = GFP_HIGHUSER_MOVABLE,
                 .may_swap = 1,
                 .may_unmap = 1,
                 .may_writepage = 1,
                 .nr_to_reclaim = nr_to_reclaim,
                 .hibernation_mode = 1,
                 .order = 0,
                 .priority = DEF_PRIORITY,
         };
         struct shrink_control shrink = {
                 .gfp_mask = sc.gfp_mask,
         };
         struct zonelist *zonelist = node_zonelist(numa_node_id(), sc.gfp_mask);
         struct task_struct *p = current;
         unsigned long nr_reclaimed;
 
         p->flags |= PF_MEMALLOC;
         lockdep_set_current_reclaim_state(sc.gfp_mask);
         reclaim_state.reclaimed_slab = 0;
         p->reclaim_state = &reclaim_state;
 
         nr_reclaimed = do_try_to_free_pages(zonelist, &sc, &shrink);
 
         p->reclaim_state = NULL;
         lockdep_clear_current_reclaim_state();
         p->flags &= ~PF_MEMALLOC;
 
         return nr_reclaimed;
 }
 #endif /* CONFIG_HIBERNATION */
 
 /* It's optimal to keep kswapds on the same CPUs as their memory, but
    not required for correctness.  So if the last cpu in a node goes
    away, we get changed to run anywhere: as the first one comes back,
    restore their cpu bindings. */
 static int cpu_callback(struct notifier_block *nfb, unsigned long action,
                         void *hcpu)
 {
         int nid;
 
         if (action == CPU_ONLINE || action == CPU_ONLINE_FROZEN) {
                 for_each_node_state(nid, N_MEMORY) {
                         pg_data_t *pgdat = NODE_DATA(nid);
                         const struct cpumask *mask;
 
                         mask = cpumask_of_node(pgdat->node_id);
 
                         if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
                                 /* One of our CPUs online: restore mask */
                                 set_cpus_allowed_ptr(pgdat->kswapd, mask);
                 }
         }
         return NOTIFY_OK;
 }
 
 /*
  * This kswapd start function will be called by init and node-hot-add.
  * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
  */
 int kswapd_run(int nid)
 {
         pg_data_t *pgdat = NODE_DATA(nid);
         int ret = 0;
 
         if (pgdat->kswapd)
                 return 0;
 
         pgdat->kswapd = kthread_run(kswapd, pgdat, "kswapd%d", nid);
         if (IS_ERR(pgdat->kswapd)) {
                 /* failure at boot is fatal */
                 BUG_ON(system_state == SYSTEM_BOOTING);
                 pr_err("Failed to start kswapd on node %d\n", nid);
                 ret = PTR_ERR(pgdat->kswapd);
                 pgdat->kswapd = NULL;
         }
         return ret;
 }
 
 /*
  * Called by memory hotplug when all memory in a node is offlined.  Caller must
  * hold lock_memory_hotplug().
  */
 void kswapd_stop(int nid)
 {
         struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
 
         if (kswapd) {
                 kthread_stop(kswapd);
                 NODE_DATA(nid)->kswapd = NULL;
         }
 }
 
 static int __init kswapd_init(void)
 {
         int nid;
 
         swap_setup();
         for_each_node_state(nid, N_MEMORY)
                 kswapd_run(nid);
         hotcpu_notifier(cpu_callback, 0);
         return 0;
 }
 
 module_init(kswapd_init)
 
 #ifdef CONFIG_NUMA
 /*
  * Zone reclaim mode
  *
  * If non-zero call zone_reclaim when the number of free pages falls below
  * the watermarks.
  */
 int zone_reclaim_mode __read_mostly;
 
 #define RECLAIM_OFF 0
 #define RECLAIM_ZONE (1<<0)     /* Run shrink_inactive_list on the zone */
 #define RECLAIM_WRITE (1<<1)    /* Writeout pages during reclaim */
 #define RECLAIM_SWAP (1<<2)     /* Swap pages out during reclaim */
 
 /*
  * Priority for ZONE_RECLAIM. This determines the fraction of pages
  * of a node considered for each zone_reclaim. 4 scans 1/16th of
  * a zone.
  */
 #define ZONE_RECLAIM_PRIORITY 4
 
 /*
  * Percentage of pages in a zone that must be unmapped for zone_reclaim to
  * occur.
  */
 int sysctl_min_unmapped_ratio = 1;
 
 /*
  * If the number of slab pages in a zone grows beyond this percentage then
  * slab reclaim needs to occur.
  */
 int sysctl_min_slab_ratio = 5;
 
 static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
 {
         unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
         unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) +
                 zone_page_state(zone, NR_ACTIVE_FILE);
 
         /*
          * It's possible for there to be more file mapped pages than
          * accounted for by the pages on the file LRU lists because
          * tmpfs pages accounted for as ANON can also be FILE_MAPPED
          */
         return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
 }
 
 /* Work out how many page cache pages we can reclaim in this reclaim_mode */
 static long zone_pagecache_reclaimable(struct zone *zone)
 {
         long nr_pagecache_reclaimable;
         long delta = 0;
 
         /*
          * If RECLAIM_SWAP is set, then all file pages are considered
          * potentially reclaimable. Otherwise, we have to worry about
          * pages like swapcache and zone_unmapped_file_pages() provides
          * a better estimate
          */
         if (zone_reclaim_mode & RECLAIM_SWAP)
                 nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
         else
                 nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
 
         /* If we can't clean pages, remove dirty pages from consideration */
         if (!(zone_reclaim_mode & RECLAIM_WRITE))
                 delta += zone_page_state(zone, NR_FILE_DIRTY);
 
         /* Watch for any possible underflows due to delta */
         if (unlikely(delta > nr_pagecache_reclaimable))
                 delta = nr_pagecache_reclaimable;
 
         return nr_pagecache_reclaimable - delta;
 }
 
 /*
  * Try to free up some pages from this zone through reclaim.
  */
 static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 {
         /* Minimum pages needed in order to stay on node */
         const unsigned long nr_pages = 1 << order;
         struct task_struct *p = current;
         struct reclaim_state reclaim_state;
         struct scan_control sc = {
                 .may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
                 .may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
                 .may_swap = 1,
                 .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
                 .gfp_mask = (gfp_mask = memalloc_noio_flags(gfp_mask)),
                 .order = order,
                 .priority = ZONE_RECLAIM_PRIORITY,
         };
         struct shrink_control shrink = {
                 .gfp_mask = sc.gfp_mask,
         };
         unsigned long nr_slab_pages0, nr_slab_pages1;
 
         cond_resched();
         /*
          * We need to be able to allocate from the reserves for RECLAIM_SWAP
          * and we also need to be able to write out pages for RECLAIM_WRITE
          * and RECLAIM_SWAP.
          */
         p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
         lockdep_set_current_reclaim_state(gfp_mask);
         reclaim_state.reclaimed_slab = 0;
         p->reclaim_state = &reclaim_state;
 
         if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
                 /*
                  * Free memory by calling shrink zone with increasing
                  * priorities until we have enough memory freed.
                  */
                 do {
                         shrink_zone(zone, &sc);
                 } while (sc.nr_reclaimed < nr_pages && --sc.priority >= 0);
         }
 
         nr_slab_pages0 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
         if (nr_slab_pages0 > zone->min_slab_pages) {
                 /*
                  * shrink_slab() does not currently allow us to determine how
                  * many pages were freed in this zone. So we take the current
                  * number of slab pages and shake the slab until it is reduced
                  * by the same nr_pages that we used for reclaiming unmapped
                  * pages.
                  *
                  * Note that shrink_slab will free memory on all zones and may
                  * take a long time.
                  */
                 for (;;) {
                         unsigned long lru_pages = zone_reclaimable_pages(zone);
 
                         /* No reclaimable slab or very low memory pressure */
                         if (!shrink_slab(&shrink, sc.nr_scanned, lru_pages))
                                 break;
 
                         /* Freed enough memory */
                         nr_slab_pages1 = zone_page_state(zone,
                                                         NR_SLAB_RECLAIMABLE);
                         if (nr_slab_pages1 + nr_pages <= nr_slab_pages0)
                                 break;
                 }
 
                 /*
                  * Update nr_reclaimed by the number of slab pages we
                  * reclaimed from this zone.
                  */
                 nr_slab_pages1 = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
                 if (nr_slab_pages1 < nr_slab_pages0)
                         sc.nr_reclaimed += nr_slab_pages0 - nr_slab_pages1;
         }
 
         p->reclaim_state = NULL;
         current->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE);
         lockdep_clear_current_reclaim_state();
         return sc.nr_reclaimed >= nr_pages;
 }
 
 int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 {
         int node_id;
         int ret;
 
         /*
          * Zone reclaim reclaims unmapped file backed pages and
          * slab pages if we are over the defined limits.
          *
          * A small portion of unmapped file backed pages is needed for
          * file I/O otherwise pages read by file I/O will be immediately
          * thrown out if the zone is overallocated. So we do not reclaim
          * if less than a specified percentage of the zone is used by
          * unmapped file backed pages.
          */
         if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
             zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
                 return ZONE_RECLAIM_FULL;
 
         if (zone->all_unreclaimable)
                 return ZONE_RECLAIM_FULL;
 
         /*
          * Do not scan if the allocation should not be delayed.
          */
         if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
                 return ZONE_RECLAIM_NOSCAN;
 
         /*
          * Only run zone reclaim on the local zone or on zones that do not
          * have associated processors. This will favor the local processor
          * over remote processors and spread off node memory allocations
          * as wide as possible.
          */
         node_id = zone_to_nid(zone);
         if (node_state(node_id, N_CPU) && node_id != numa_node_id())
                 return ZONE_RECLAIM_NOSCAN;
 
         if (zone_test_and_set_flag(zone, ZONE_RECLAIM_LOCKED))
                 return ZONE_RECLAIM_NOSCAN;
 
         ret = __zone_reclaim(zone, gfp_mask, order);
         zone_clear_flag(zone, ZONE_RECLAIM_LOCKED);
 
         if (!ret)
                 count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED);
 
         return ret;
 }
 #endif
 
 /*
  * page_evictable - test whether a page is evictable
  * @page: the page to test
  *
  * Test whether page is evictable--i.e., should be placed on active/inactive
  * lists vs unevictable list.
  *
  * Reasons page might not be evictable:
  * (1) page's mapping marked unevictable
  * (2) page is part of an mlocked VMA
  *
  */
 int page_evictable(struct page *page)
 {
         return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
 }
 
 #ifdef CONFIG_SHMEM
 /**
  * check_move_unevictable_pages - check pages for evictability and move to appropriate zone lru list
  * @pages:      array of pages to check
  * @nr_pages:   number of pages to check
  *
  * Checks pages for evictability and moves them to the appropriate lru list.
  *
  * This function is only used for SysV IPC SHM_UNLOCK.
  */
 void check_move_unevictable_pages(struct page **pages, int nr_pages)
 {
         struct lruvec *lruvec;
         struct zone *zone = NULL;
         int pgscanned = 0;
         int pgrescued = 0;
         int i;
 
         for (i = 0; i < nr_pages; i++) {
                 struct page *page = pages[i];
                 struct zone *pagezone;
 
                 pgscanned++;
                 pagezone = page_zone(page);
                 if (pagezone != zone) {
                         if (zone)
                                 spin_unlock_irq(&zone->lru_lock);
                         zone = pagezone;
                         spin_lock_irq(&zone->lru_lock);
                 }
                 lruvec = mem_cgroup_page_lruvec(page, zone);
 
                 if (!PageLRU(page) || !PageUnevictable(page))
                         continue;
 
                 if (page_evictable(page)) {
                         enum lru_list lru = page_lru_base_type(page);
 
                         VM_BUG_ON(PageActive(page));
                         ClearPageUnevictable(page);
                         del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
                         add_page_to_lru_list(page, lruvec, lru);
                         pgrescued++;
                 }
         }
 
         if (zone) {
                 __count_vm_events(UNEVICTABLE_PGRESCUED, pgrescued);
                 __count_vm_events(UNEVICTABLE_PGSCANNED, pgscanned);
                 spin_unlock_irq(&zone->lru_lock);
         }
 }
 #endif /* CONFIG_SHMEM */
 
 static void warn_scan_unevictable_pages(void)
 {
         printk_once(KERN_WARNING
                     "%s: The scan_unevictable_pages sysctl/node-interface has been "
                     "disabled for lack of a legitimate use case.  If you have "
                     "one, please send an email to linux-mm@kvack.org.\n",
                     current->comm);
 }
 
 /*
  * scan_unevictable_pages [vm] sysctl handler.  On demand re-scan of
  * all nodes' unevictable lists for evictable pages
  */
 unsigned long scan_unevictable_pages;
 
 int scan_unevictable_handler(struct ctl_table *table, int write,
                            void __user *buffer,
                            size_t *length, loff_t *ppos)
 {
         warn_scan_unevictable_pages();
         proc_doulongvec_minmax(table, write, buffer, length, ppos);
         scan_unevictable_pages = 0;
         return 0;
 }
 
 #ifdef CONFIG_NUMA
 /*
  * per node 'scan_unevictable_pages' attribute.  On demand re-scan of
  * a specified node's per zone unevictable lists for evictable pages.
  */
 
 static ssize_t read_scan_unevictable_node(struct device *dev,
                                           struct device_attribute *attr,
                                           char *buf)
 {
         warn_scan_unevictable_pages();
         return sprintf(buf, "\n");     /* always zero; should fit... */
 }
 
 static ssize_t write_scan_unevictable_node(struct device *dev,
                                            struct device_attribute *attr,
                                         const char *buf, size_t count)
 {
         warn_scan_unevictable_pages();
         return 1;
 }
 
 
 static DEVICE_ATTR(scan_unevictable_pages, S_IRUGO | S_IWUSR,
                         read_scan_unevictable_node,
                         write_scan_unevictable_node);
 
 int scan_unevictable_register_node(struct node *node)
 {
         return device_create_file(&node->dev, &dev_attr_scan_unevictable_pages);
 }
 
 void scan_unevictable_unregister_node(struct node *node)
 {
         device_remove_file(&node->dev, &dev_attr_scan_unevictable_pages);
 }
 #endif