TOMOYO Linux Cross Reference
Linux/mm/page-writeback.c

   /*
    * mm/page-writeback.c
    *
    * Copyright (C) 2002, Linus Torvalds.
    * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
    *
    * Contains functions related to writing back dirty pages at the
    * address_space level.
    *
   * 10Apr2002    Andrew Morton
   *              Initial version
   */
  
  #include <linux/kernel.h>
  #include <linux/export.h>
  #include <linux/spinlock.h>
  #include <linux/fs.h>
  #include <linux/mm.h>
  #include <linux/swap.h>
  #include <linux/slab.h>
  #include <linux/pagemap.h>
  #include <linux/writeback.h>
  #include <linux/init.h>
  #include <linux/backing-dev.h>
  #include <linux/task_io_accounting_ops.h>
  #include <linux/blkdev.h>
  #include <linux/mpage.h>
  #include <linux/rmap.h>
  #include <linux/percpu.h>
  #include <linux/notifier.h>
  #include <linux/smp.h>
  #include <linux/sysctl.h>
  #include <linux/cpu.h>
  #include <linux/syscalls.h>
  #include <linux/buffer_head.h> /* __set_page_dirty_buffers */
  #include <linux/pagevec.h>
  #include <linux/timer.h>
  #include <linux/sched/rt.h>
  #include <linux/mm_inline.h>
  #include <trace/events/writeback.h>
  
  #include "internal.h"
  
  /*
   * Sleep at most 200ms at a time in balance_dirty_pages().
   */
  #define MAX_PAUSE               max(HZ/5, 1)
  
  /*
   * Try to keep balance_dirty_pages() call intervals higher than this many pages
   * by raising pause time to max_pause when falls below it.
   */
  #define DIRTY_POLL_THRESH       (128 >> (PAGE_SHIFT - 10))
  
  /*
   * Estimate write bandwidth at 200ms intervals.
   */
  #define BANDWIDTH_INTERVAL      max(HZ/5, 1)
  
  #define RATELIMIT_CALC_SHIFT    10
  
  /*
   * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
   * will look to see if it needs to force writeback or throttling.
   */
  static long ratelimit_pages = 32;
  
  /* The following parameters are exported via /proc/sys/vm */
  
  /*
   * Start background writeback (via writeback threads) at this percentage
   */
  int dirty_background_ratio = 10;
  
  /*
   * dirty_background_bytes starts at 0 (disabled) so that it is a function of
   * dirty_background_ratio * the amount of dirtyable memory
   */
  unsigned long dirty_background_bytes;
  
  /*
   * free highmem will not be subtracted from the total free memory
   * for calculating free ratios if vm_highmem_is_dirtyable is true
   */
  int vm_highmem_is_dirtyable;
  
  /*
   * The generator of dirty data starts writeback at this percentage
   */
  int vm_dirty_ratio = 20;
  
  /*
   * vm_dirty_bytes starts at 0 (disabled) so that it is a function of
   * vm_dirty_ratio * the amount of dirtyable memory
   */
  unsigned long vm_dirty_bytes;
  
  /*
   * The interval between `kupdate'-style writebacks
  */
 unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
 
 EXPORT_SYMBOL_GPL(dirty_writeback_interval);
 
 /*
  * The longest time for which data is allowed to remain dirty
  */
 unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
 
 /*
  * Flag that makes the machine dump writes/reads and block dirtyings.
  */
 int block_dump;
 
 /*
  * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
  * a full sync is triggered after this time elapses without any disk activity.
  */
 int laptop_mode;
 
 EXPORT_SYMBOL(laptop_mode);
 
 /* End of sysctl-exported parameters */
 
 unsigned long global_dirty_limit;
 
 /*
  * Scale the writeback cache size proportional to the relative writeout speeds.
  *
  * We do this by keeping a floating proportion between BDIs, based on page
  * writeback completions [end_page_writeback()]. Those devices that write out
  * pages fastest will get the larger share, while the slower will get a smaller
  * share.
  *
  * We use page writeout completions because we are interested in getting rid of
  * dirty pages. Having them written out is the primary goal.
  *
  * We introduce a concept of time, a period over which we measure these events,
  * because demand can/will vary over time. The length of this period itself is
  * measured in page writeback completions.
  *
  */
 static struct fprop_global writeout_completions;
 
 static void writeout_period(unsigned long t);
 /* Timer for aging of writeout_completions */
 static struct timer_list writeout_period_timer =
                 TIMER_DEFERRED_INITIALIZER(writeout_period, 0, 0);
 static unsigned long writeout_period_time = 0;
 
 /*
  * Length of period for aging writeout fractions of bdis. This is an
  * arbitrarily chosen number. The longer the period, the slower fractions will
  * reflect changes in current writeout rate.
  */
 #define VM_COMPLETIONS_PERIOD_LEN (3*HZ)
 
 /*
  * In a memory zone, there is a certain amount of pages we consider
  * available for the page cache, which is essentially the number of
  * free and reclaimable pages, minus some zone reserves to protect
  * lowmem and the ability to uphold the zone's watermarks without
  * requiring writeback.
  *
  * This number of dirtyable pages is the base value of which the
  * user-configurable dirty ratio is the effictive number of pages that
  * are allowed to be actually dirtied.  Per individual zone, or
  * globally by using the sum of dirtyable pages over all zones.
  *
  * Because the user is allowed to specify the dirty limit globally as
  * absolute number of bytes, calculating the per-zone dirty limit can
  * require translating the configured limit into a percentage of
  * global dirtyable memory first.
  */
 
 /**
  * zone_dirtyable_memory - number of dirtyable pages in a zone
  * @zone: the zone
  *
  * Returns the zone's number of pages potentially available for dirty
  * page cache.  This is the base value for the per-zone dirty limits.
  */
 static unsigned long zone_dirtyable_memory(struct zone *zone)
 {
         unsigned long nr_pages;
 
         nr_pages = zone_page_state(zone, NR_FREE_PAGES);
         nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
 
         nr_pages += zone_page_state(zone, NR_INACTIVE_FILE);
         nr_pages += zone_page_state(zone, NR_ACTIVE_FILE);
 
         return nr_pages;
 }
 
 static unsigned long highmem_dirtyable_memory(unsigned long total)
 {
 #ifdef CONFIG_HIGHMEM
         int node;
         unsigned long x = 0;
 
         for_each_node_state(node, N_HIGH_MEMORY) {
                 struct zone *z = &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
 
                 x += zone_dirtyable_memory(z);
         }
         /*
          * Unreclaimable memory (kernel memory or anonymous memory
          * without swap) can bring down the dirtyable pages below
          * the zone's dirty balance reserve and the above calculation
          * will underflow.  However we still want to add in nodes
          * which are below threshold (negative values) to get a more
          * accurate calculation but make sure that the total never
          * underflows.
          */
         if ((long)x < 0)
                 x = 0;
 
         /*
          * Make sure that the number of highmem pages is never larger
          * than the number of the total dirtyable memory. This can only
          * occur in very strange VM situations but we want to make sure
          * that this does not occur.
          */
         return min(x, total);
 #else
         return 0;
 #endif
 }
 
 /**
  * global_dirtyable_memory - number of globally dirtyable pages
  *
  * Returns the global number of pages potentially available for dirty
  * page cache.  This is the base value for the global dirty limits.
  */
 static unsigned long global_dirtyable_memory(void)
 {
         unsigned long x;
 
         x = global_page_state(NR_FREE_PAGES);
         x -= min(x, dirty_balance_reserve);
 
         x += global_page_state(NR_INACTIVE_FILE);
         x += global_page_state(NR_ACTIVE_FILE);
 
         if (!vm_highmem_is_dirtyable)
                 x -= highmem_dirtyable_memory(x);
 
         return x + 1;   /* Ensure that we never return 0 */
 }
 
 /*
  * global_dirty_limits - background-writeback and dirty-throttling thresholds
  *
  * Calculate the dirty thresholds based on sysctl parameters
  * - vm.dirty_background_ratio  or  vm.dirty_background_bytes
  * - vm.dirty_ratio             or  vm.dirty_bytes
  * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
  * real-time tasks.
  */
 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
         const unsigned long available_memory = global_dirtyable_memory();
         unsigned long background;
         unsigned long dirty;
         struct task_struct *tsk;
 
         if (vm_dirty_bytes)
                 dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
         else
                 dirty = (vm_dirty_ratio * available_memory) / 100;
 
         if (dirty_background_bytes)
                 background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
         else
                 background = (dirty_background_ratio * available_memory) / 100;
 
         if (background >= dirty)
                 background = dirty / 2;
         tsk = current;
         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
                 background += background / 4;
                 dirty += dirty / 4;
         }
         *pbackground = background;
         *pdirty = dirty;
         trace_global_dirty_state(background, dirty);
 }
 
 /**
  * zone_dirty_limit - maximum number of dirty pages allowed in a zone
  * @zone: the zone
  *
  * Returns the maximum number of dirty pages allowed in a zone, based
  * on the zone's dirtyable memory.
  */
 static unsigned long zone_dirty_limit(struct zone *zone)
 {
         unsigned long zone_memory = zone_dirtyable_memory(zone);
         struct task_struct *tsk = current;
         unsigned long dirty;
 
         if (vm_dirty_bytes)
                 dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE) *
                         zone_memory / global_dirtyable_memory();
         else
                 dirty = vm_dirty_ratio * zone_memory / 100;
 
         if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
                 dirty += dirty / 4;
 
         return dirty;
 }
 
 /**
  * zone_dirty_ok - tells whether a zone is within its dirty limits
  * @zone: the zone to check
  *
  * Returns %true when the dirty pages in @zone are within the zone's
  * dirty limit, %false if the limit is exceeded.
  */
 bool zone_dirty_ok(struct zone *zone)
 {
         unsigned long limit = zone_dirty_limit(zone);
 
         return zone_page_state(zone, NR_FILE_DIRTY) +
                zone_page_state(zone, NR_UNSTABLE_NFS) +
                zone_page_state(zone, NR_WRITEBACK) <= limit;
 }
 
 int dirty_background_ratio_handler(struct ctl_table *table, int write,
                 void __user *buffer, size_t *lenp,
                 loff_t *ppos)
 {
         int ret;
 
         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
         if (ret == 0 && write)
                 dirty_background_bytes = 0;
         return ret;
 }
 
 int dirty_background_bytes_handler(struct ctl_table *table, int write,
                 void __user *buffer, size_t *lenp,
                 loff_t *ppos)
 {
         int ret;
 
         ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
         if (ret == 0 && write)
                 dirty_background_ratio = 0;
         return ret;
 }
 
 int dirty_ratio_handler(struct ctl_table *table, int write,
                 void __user *buffer, size_t *lenp,
                 loff_t *ppos)
 {
         int old_ratio = vm_dirty_ratio;
         int ret;
 
         ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
         if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
                 writeback_set_ratelimit();
                 vm_dirty_bytes = 0;
         }
         return ret;
 }
 
 int dirty_bytes_handler(struct ctl_table *table, int write,
                 void __user *buffer, size_t *lenp,
                 loff_t *ppos)
 {
         unsigned long old_bytes = vm_dirty_bytes;
         int ret;
 
         ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
         if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
                 writeback_set_ratelimit();
                 vm_dirty_ratio = 0;
         }
         return ret;
 }
 
 static unsigned long wp_next_time(unsigned long cur_time)
 {
         cur_time += VM_COMPLETIONS_PERIOD_LEN;
         /* 0 has a special meaning... */
         if (!cur_time)
                 return 1;
         return cur_time;
 }
 
 /*
  * Increment the BDI's writeout completion count and the global writeout
  * completion count. Called from test_clear_page_writeback().
  */
 static inline void __bdi_writeout_inc(struct backing_dev_info *bdi)
 {
         __inc_bdi_stat(bdi, BDI_WRITTEN);
         __fprop_inc_percpu_max(&writeout_completions, &bdi->completions,
                                bdi->max_prop_frac);
         /* First event after period switching was turned off? */
         if (!unlikely(writeout_period_time)) {
                 /*
                  * We can race with other __bdi_writeout_inc calls here but
                  * it does not cause any harm since the resulting time when
                  * timer will fire and what is in writeout_period_time will be
                  * roughly the same.
                  */
                 writeout_period_time = wp_next_time(jiffies);
                 mod_timer(&writeout_period_timer, writeout_period_time);
         }
 }
 
 void bdi_writeout_inc(struct backing_dev_info *bdi)
 {
         unsigned long flags;
 
         local_irq_save(flags);
         __bdi_writeout_inc(bdi);
         local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(bdi_writeout_inc);
 
 /*
  * Obtain an accurate fraction of the BDI's portion.
  */
 static void bdi_writeout_fraction(struct backing_dev_info *bdi,
                 long *numerator, long *denominator)
 {
         fprop_fraction_percpu(&writeout_completions, &bdi->completions,
                                 numerator, denominator);
 }
 
 /*
  * On idle system, we can be called long after we scheduled because we use
  * deferred timers so count with missed periods.
  */
 static void writeout_period(unsigned long t)
 {
         int miss_periods = (jiffies - writeout_period_time) /
                                                  VM_COMPLETIONS_PERIOD_LEN;
 
         if (fprop_new_period(&writeout_completions, miss_periods + 1)) {
                 writeout_period_time = wp_next_time(writeout_period_time +
                                 miss_periods * VM_COMPLETIONS_PERIOD_LEN);
                 mod_timer(&writeout_period_timer, writeout_period_time);
         } else {
                 /*
                  * Aging has zeroed all fractions. Stop wasting CPU on period
                  * updates.
                  */
                 writeout_period_time = 0;
         }
 }
 
 /*
  * bdi_min_ratio keeps the sum of the minimum dirty shares of all
  * registered backing devices, which, for obvious reasons, can not
  * exceed 100%.
  */
 static unsigned int bdi_min_ratio;
 
 int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 {
         int ret = 0;
 
         spin_lock_bh(&bdi_lock);
         if (min_ratio > bdi->max_ratio) {
                 ret = -EINVAL;
         } else {
                 min_ratio -= bdi->min_ratio;
                 if (bdi_min_ratio + min_ratio < 100) {
                         bdi_min_ratio += min_ratio;
                         bdi->min_ratio += min_ratio;
                 } else {
                         ret = -EINVAL;
                 }
         }
         spin_unlock_bh(&bdi_lock);
 
         return ret;
 }
 
 int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 {
         int ret = 0;
 
         if (max_ratio > 100)
                 return -EINVAL;
 
         spin_lock_bh(&bdi_lock);
         if (bdi->min_ratio > max_ratio) {
                 ret = -EINVAL;
         } else {
                 bdi->max_ratio = max_ratio;
                 bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
         }
         spin_unlock_bh(&bdi_lock);
 
         return ret;
 }
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
 static unsigned long dirty_freerun_ceiling(unsigned long thresh,
                                            unsigned long bg_thresh)
 {
         return (thresh + bg_thresh) / 2;
 }
 
 static unsigned long hard_dirty_limit(unsigned long thresh)
 {
         return max(thresh, global_dirty_limit);
 }
 
 /**
  * bdi_dirty_limit - @bdi's share of dirty throttling threshold
  * @bdi: the backing_dev_info to query
  * @dirty: global dirty limit in pages
  *
  * Returns @bdi's dirty limit in pages. The term "dirty" in the context of
  * dirty balancing includes all PG_dirty, PG_writeback and NFS unstable pages.
  *
  * Note that balance_dirty_pages() will only seriously take it as a hard limit
  * when sleeping max_pause per page is not enough to keep the dirty pages under
  * control. For example, when the device is completely stalled due to some error
  * conditions, or when there are 1000 dd tasks writing to a slow 10MB/s USB key.
  * In the other normal situations, it acts more gently by throttling the tasks
  * more (rather than completely block them) when the bdi dirty pages go high.
  *
  * It allocates high/low dirty limits to fast/slow devices, in order to prevent
  * - starving fast devices
  * - piling up dirty pages (that will take long time to sync) on slow devices
  *
  * The bdi's share of dirty limit will be adapting to its throughput and
  * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
  */
 unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
 {
         u64 bdi_dirty;
         long numerator, denominator;
 
         /*
          * Calculate this BDI's share of the dirty ratio.
          */
         bdi_writeout_fraction(bdi, &numerator, &denominator);
 
         bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
         bdi_dirty *= numerator;
         do_div(bdi_dirty, denominator);
 
         bdi_dirty += (dirty * bdi->min_ratio) / 100;
         if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
                 bdi_dirty = dirty * bdi->max_ratio / 100;
 
         return bdi_dirty;
 }
 
 /*
  *                           setpoint - dirty 3
  *        f(dirty) := 1.0 + (----------------)
  *                           limit - setpoint
  *
  * it's a 3rd order polynomial that subjects to
  *
  * (1) f(freerun)  = 2.0 => rampup dirty_ratelimit reasonably fast
  * (2) f(setpoint) = 1.0 => the balance point
  * (3) f(limit)    = 0   => the hard limit
  * (4) df/dx      <= 0   => negative feedback control
  * (5) the closer to setpoint, the smaller |df/dx| (and the reverse)
  *     => fast response on large errors; small oscillation near setpoint
  */
 static long long pos_ratio_polynom(unsigned long setpoint,
                                           unsigned long dirty,
                                           unsigned long limit)
 {
         long long pos_ratio;
         long x;
 
         x = div64_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
                       (limit - setpoint) | 1);
         pos_ratio = x;
         pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
         pos_ratio = pos_ratio * x >> RATELIMIT_CALC_SHIFT;
         pos_ratio += 1 << RATELIMIT_CALC_SHIFT;
 
         return clamp(pos_ratio, 0LL, 2LL << RATELIMIT_CALC_SHIFT);
 }
 
 /*
  * Dirty position control.
  *
  * (o) global/bdi setpoints
  *
  * We want the dirty pages be balanced around the global/bdi setpoints.
  * When the number of dirty pages is higher/lower than the setpoint, the
  * dirty position control ratio (and hence task dirty ratelimit) will be
  * decreased/increased to bring the dirty pages back to the setpoint.
  *
  *     pos_ratio = 1 << RATELIMIT_CALC_SHIFT
  *
  *     if (dirty < setpoint) scale up   pos_ratio
  *     if (dirty > setpoint) scale down pos_ratio
  *
  *     if (bdi_dirty < bdi_setpoint) scale up   pos_ratio
  *     if (bdi_dirty > bdi_setpoint) scale down pos_ratio
  *
  *     task_ratelimit = dirty_ratelimit * pos_ratio >> RATELIMIT_CALC_SHIFT
  *
  * (o) global control line
  *
  *     ^ pos_ratio
  *     |
  *     |            |<===== global dirty control scope ======>|
  * 2.0 .............*
  *     |            .*
  *     |            . *
  *     |            .   *
  *     |            .     *
  *     |            .        *
  *     |            .            *
  * 1.0 ................................*
  *     |            .                  .     *
  *     |            .                  .          *
  *     |            .                  .              *
  *     |            .                  .                 *
  *     |            .                  .                    *
  *   0 +------------.------------------.----------------------*------------->
  *           freerun^          setpoint^                 limit^   dirty pages
  *
  * (o) bdi control line
  *
  *     ^ pos_ratio
  *     |
  *     |            *
  *     |              *
  *     |                *
  *     |                  *
  *     |                    * |<=========== span ============>|
  * 1.0 .......................*
  *     |                      . *
  *     |                      .   *
  *     |                      .     *
  *     |                      .       *
  *     |                      .         *
  *     |                      .           *
  *     |                      .             *
  *     |                      .               *
  *     |                      .                 *
  *     |                      .                   *
  *     |                      .                     *
  * 1/4 ...............................................* * * * * * * * * * * *
  *     |                      .                         .
  *     |                      .                           .
  *     |                      .                             .
  *   0 +----------------------.-------------------------------.------------->
  *                bdi_setpoint^                    x_intercept^
  *
  * The bdi control line won't drop below pos_ratio=1/4, so that bdi_dirty can
  * be smoothly throttled down to normal if it starts high in situations like
  * - start writing to a slow SD card and a fast disk at the same time. The SD
  *   card's bdi_dirty may rush to many times higher than bdi_setpoint.
  * - the bdi dirty thresh drops quickly due to change of JBOD workload
  */
 static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
                                         unsigned long thresh,
                                         unsigned long bg_thresh,
                                         unsigned long dirty,
                                         unsigned long bdi_thresh,
                                         unsigned long bdi_dirty)
 {
         unsigned long write_bw = bdi->avg_write_bandwidth;
         unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
         unsigned long limit = hard_dirty_limit(thresh);
         unsigned long x_intercept;
         unsigned long setpoint;         /* dirty pages' target balance point */
         unsigned long bdi_setpoint;
         unsigned long span;
         long long pos_ratio;            /* for scaling up/down the rate limit */
         long x;
 
         if (unlikely(dirty >= limit))
                 return 0;
 
         /*
          * global setpoint
          *
          * See comment for pos_ratio_polynom().
          */
         setpoint = (freerun + limit) / 2;
         pos_ratio = pos_ratio_polynom(setpoint, dirty, limit);
 
         /*
          * The strictlimit feature is a tool preventing mistrusted filesystems
          * from growing a large number of dirty pages before throttling. For
          * such filesystems balance_dirty_pages always checks bdi counters
          * against bdi limits. Even if global "nr_dirty" is under "freerun".
          * This is especially important for fuse which sets bdi->max_ratio to
          * 1% by default. Without strictlimit feature, fuse writeback may
          * consume arbitrary amount of RAM because it is accounted in
          * NR_WRITEBACK_TEMP which is not involved in calculating "nr_dirty".
          *
          * Here, in bdi_position_ratio(), we calculate pos_ratio based on
          * two values: bdi_dirty and bdi_thresh. Let's consider an example:
          * total amount of RAM is 16GB, bdi->max_ratio is equal to 1%, global
          * limits are set by default to 10% and 20% (background and throttle).
          * Then bdi_thresh is 1% of 20% of 16GB. This amounts to ~8K pages.
          * bdi_dirty_limit(bdi, bg_thresh) is about ~4K pages. bdi_setpoint is
          * about ~6K pages (as the average of background and throttle bdi
          * limits). The 3rd order polynomial will provide positive feedback if
          * bdi_dirty is under bdi_setpoint and vice versa.
          *
          * Note, that we cannot use global counters in these calculations
          * because we want to throttle process writing to a strictlimit BDI
          * much earlier than global "freerun" is reached (~23MB vs. ~2.3GB
          * in the example above).
          */
         if (unlikely(bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
                 long long bdi_pos_ratio;
                 unsigned long bdi_bg_thresh;
 
                 if (bdi_dirty < 8)
                         return min_t(long long, pos_ratio * 2,
                                      2 << RATELIMIT_CALC_SHIFT);
 
                 if (bdi_dirty >= bdi_thresh)
                         return 0;
 
                 bdi_bg_thresh = div_u64((u64)bdi_thresh * bg_thresh, thresh);
                 bdi_setpoint = dirty_freerun_ceiling(bdi_thresh,
                                                      bdi_bg_thresh);
 
                 if (bdi_setpoint == 0 || bdi_setpoint == bdi_thresh)
                         return 0;
 
                 bdi_pos_ratio = pos_ratio_polynom(bdi_setpoint, bdi_dirty,
                                                   bdi_thresh);
 
                 /*
                  * Typically, for strictlimit case, bdi_setpoint << setpoint
                  * and pos_ratio >> bdi_pos_ratio. In the other words global
                  * state ("dirty") is not limiting factor and we have to
                  * make decision based on bdi counters. But there is an
                  * important case when global pos_ratio should get precedence:
                  * global limits are exceeded (e.g. due to activities on other
                  * BDIs) while given strictlimit BDI is below limit.
                  *
                  * "pos_ratio * bdi_pos_ratio" would work for the case above,
                  * but it would look too non-natural for the case of all
                  * activity in the system coming from a single strictlimit BDI
                  * with bdi->max_ratio == 100%.
                  *
                  * Note that min() below somewhat changes the dynamics of the
                  * control system. Normally, pos_ratio value can be well over 3
                  * (when globally we are at freerun and bdi is well below bdi
                  * setpoint). Now the maximum pos_ratio in the same situation
                  * is 2. We might want to tweak this if we observe the control
                  * system is too slow to adapt.
                  */
                 return min(pos_ratio, bdi_pos_ratio);
         }
 
         /*
          * We have computed basic pos_ratio above based on global situation. If
          * the bdi is over/under its share of dirty pages, we want to scale
          * pos_ratio further down/up. That is done by the following mechanism.
          */
 
         /*
          * bdi setpoint
          *
          *        f(bdi_dirty) := 1.0 + k * (bdi_dirty - bdi_setpoint)
          *
          *                        x_intercept - bdi_dirty
          *                     := --------------------------
          *                        x_intercept - bdi_setpoint
          *
          * The main bdi control line is a linear function that subjects to
          *
          * (1) f(bdi_setpoint) = 1.0
          * (2) k = - 1 / (8 * write_bw)  (in single bdi case)
          *     or equally: x_intercept = bdi_setpoint + 8 * write_bw
          *
          * For single bdi case, the dirty pages are observed to fluctuate
          * regularly within range
          *        [bdi_setpoint - write_bw/2, bdi_setpoint + write_bw/2]
          * for various filesystems, where (2) can yield in a reasonable 12.5%
          * fluctuation range for pos_ratio.
          *
          * For JBOD case, bdi_thresh (not bdi_dirty!) could fluctuate up to its
          * own size, so move the slope over accordingly and choose a slope that
          * yields 100% pos_ratio fluctuation on suddenly doubled bdi_thresh.
          */
         if (unlikely(bdi_thresh > thresh))
                 bdi_thresh = thresh;
         /*
          * It's very possible that bdi_thresh is close to 0 not because the
          * device is slow, but that it has remained inactive for long time.
          * Honour such devices a reasonable good (hopefully IO efficient)
          * threshold, so that the occasional writes won't be blocked and active
          * writes can rampup the threshold quickly.
          */
         bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
         /*
          * scale global setpoint to bdi's:
          *      bdi_setpoint = setpoint * bdi_thresh / thresh
          */
         x = div_u64((u64)bdi_thresh << 16, thresh | 1);
         bdi_setpoint = setpoint * (u64)x >> 16;
         /*
          * Use span=(8*write_bw) in single bdi case as indicated by
          * (thresh - bdi_thresh ~= 0) and transit to bdi_thresh in JBOD case.
          *
          *        bdi_thresh                    thresh - bdi_thresh
          * span = ---------- * (8 * write_bw) + ------------------- * bdi_thresh
          *          thresh                            thresh
          */
         span = (thresh - bdi_thresh + 8 * write_bw) * (u64)x >> 16;
         x_intercept = bdi_setpoint + span;
 
         if (bdi_dirty < x_intercept - span / 4) {
                 pos_ratio = div64_u64(pos_ratio * (x_intercept - bdi_dirty),
                                       (x_intercept - bdi_setpoint) | 1);
         } else
                 pos_ratio /= 4;
 
         /*
          * bdi reserve area, safeguard against dirty pool underrun and disk idle
          * It may push the desired control point of global dirty pages higher
          * than setpoint.
          */
         x_intercept = bdi_thresh / 2;
         if (bdi_dirty < x_intercept) {
                 if (bdi_dirty > x_intercept / 8)
                         pos_ratio = div_u64(pos_ratio * x_intercept, bdi_dirty);
                 else
                         pos_ratio *= 8;
         }
 
         return pos_ratio;
 }
 
 static void bdi_update_write_bandwidth(struct backing_dev_info *bdi,
                                        unsigned long elapsed,
                                        unsigned long written)
 {
         const unsigned long period = roundup_pow_of_two(3 * HZ);
         unsigned long avg = bdi->avg_write_bandwidth;
         unsigned long old = bdi->write_bandwidth;
         u64 bw;
 
         /*
          * bw = written * HZ / elapsed
          *
          *                   bw * elapsed + write_bandwidth * (period - elapsed)
          * write_bandwidth = ---------------------------------------------------
          *                                          period
          *
          * @written may have decreased due to account_page_redirty().
          * Avoid underflowing @bw calculation.
          */
         bw = written - min(written, bdi->written_stamp);
         bw *= HZ;
         if (unlikely(elapsed > period)) {
                 do_div(bw, elapsed);
                 avg = bw;
                 goto out;
         }
         bw += (u64)bdi->write_bandwidth * (period - elapsed);
         bw >>= ilog2(period);
 
         /*
          * one more level of smoothing, for filtering out sudden spikes
          */
         if (avg > old && old >= (unsigned long)bw)
                 avg -= (avg - old) >> 3;
 
         if (avg < old && old <= (unsigned long)bw)
                 avg += (old - avg) >> 3;
 
 out:
         bdi->write_bandwidth = bw;
         bdi->avg_write_bandwidth = avg;
 }
 
 /*
  * The global dirtyable memory and dirty threshold could be suddenly knocked
  * down by a large amount (eg. on the startup of KVM in a swapless system).
  * This may throw the system into deep dirty exceeded state and throttle
  * heavy/light dirtiers alike. To retain good responsiveness, maintain
  * global_dirty_limit for tracking slowly down to the knocked down dirty
  * threshold.
  */
 static void update_dirty_limit(unsigned long thresh, unsigned long dirty)
 {
         unsigned long limit = global_dirty_limit;
 
         /*
          * Follow up in one step.
          */
         if (limit < thresh) {
                 limit = thresh;
                 goto update;
         }
 
         /*
          * Follow down slowly. Use the higher one as the target, because thresh
          * may drop below dirty. This is exactly the reason to introduce
          * global_dirty_limit which is guaranteed to lie above the dirty pages.
          */
         thresh = max(thresh, dirty);
         if (limit > thresh) {
                 limit -= (limit - thresh) >> 5;
                 goto update;
         }
         return;
 update:
         global_dirty_limit = limit;
 }
 
 static void global_update_bandwidth(unsigned long thresh,
                                     unsigned long dirty,
                                     unsigned long now)
 {
         static DEFINE_SPINLOCK(dirty_lock);
         static unsigned long update_time = INITIAL_JIFFIES;
 
         /*
          * check locklessly first to optimize away locking for the most time
          */
         if (time_before(now, update_time + BANDWIDTH_INTERVAL))
                 return;
 
         spin_lock(&dirty_lock);
         if (time_after_eq(now, update_time + BANDWIDTH_INTERVAL)) {
                 update_dirty_limit(thresh, dirty);
                 update_time = now;
         }
         spin_unlock(&dirty_lock);
 }
 
 /*
  * Maintain bdi->dirty_ratelimit, the base dirty throttle rate.
  *
  * Normal bdi tasks will be curbed at or below it in long term.
  * Obviously it should be around (write_bw / N) when there are N dd tasks.
  */
 static void bdi_update_dirty_ratelimit(struct backing_dev_info *bdi,
                                        unsigned long thresh,
                                        unsigned long bg_thresh,
                                        unsigned long dirty,
                                        unsigned long bdi_thresh,
                                        unsigned long bdi_dirty,
                                        unsigned long dirtied,
                                        unsigned long elapsed)
 {
         unsigned long freerun = dirty_freerun_ceiling(thresh, bg_thresh);
         unsigned long limit = hard_dirty_limit(thresh);
         unsigned long setpoint = (freerun + limit) / 2;
         unsigned long write_bw = bdi->avg_write_bandwidth;
         unsigned long dirty_ratelimit = bdi->dirty_ratelimit;
         unsigned long dirty_rate;
         unsigned long task_ratelimit;
         unsigned long balanced_dirty_ratelimit;
         unsigned long pos_ratio;
         unsigned long step;
         unsigned long x;
         unsigned long shift;
 
         /*
          * The dirty rate will match the writeout rate in long term, except
          * when dirty pages are truncated by userspace or re-dirtied by FS.
          */
         dirty_rate = (dirtied - bdi->dirtied_stamp) * HZ / elapsed;
 
         pos_ratio = bdi_position_ratio(bdi, thresh, bg_thresh, dirty,
                                        bdi_thresh, bdi_dirty);
         /*
          * task_ratelimit reflects each dd's dirty rate for the past 200ms.
          */
         task_ratelimit = (u64)dirty_ratelimit *
                                         pos_ratio >> RATELIMIT_CALC_SHIFT;
         task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
 
         /*
          * A linear estimation of the "balanced" throttle rate. The theory is,
          * if there are N dd tasks, each throttled at task_ratelimit, the bdi's
          * dirty_rate will be measured to be (N * task_ratelimit). So the below
          * formula will yield the balanced rate limit (write_bw / N).
          *
          * Note that the expanded form is not a pure rate feedback:
          *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate)              (1)
          * but also takes pos_ratio into account:
          *      rate_(i+1) = rate_(i) * (write_bw / dirty_rate) * pos_ratio  (2)
          *
          * (1) is not realistic because pos_ratio also takes part in balancing
          * the dirty rate.  Consider the state
          *      pos_ratio = 0.5                                              (3)
          *      rate = 2 * (write_bw / N)                                    (4)
          * If (1) is used, it will stuck in that state! Because each dd will
          * be throttled at
          *      task_ratelimit = pos_ratio * rate = (write_bw / N)           (5)
          * yielding
          *      dirty_rate = N * task_ratelimit = write_bw                   (6)
          * put (6) into (1) we get
          *      rate_(i+1) = rate_(i)                                        (7)
          *
          * So we end up using (2) to always keep
          *      rate_(i+1) ~= (write_bw / N)                                 (8)
          * regardless of the value of pos_ratio. As long as (8) is satisfied,
          * pos_ratio is able to drive itself to 1.0, which is not only where
          * the dirty count meet the setpoint, but also where the slope of
          * pos_ratio is most flat and hence task_ratelimit is least fluctuated.
          */
         balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
                                            dirty_rate | 1);
         /*
          * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
          */
         if (unlikely(balanced_dirty_ratelimit > write_bw))
                 balanced_dirty_ratelimit = write_bw;
 
         /*
          * We could safely do this and return immediately:
          *
          *      bdi->dirty_ratelimit = balanced_dirty_ratelimit;
          *
          * However to get a more stable dirty_ratelimit, the below elaborated
          * code makes use of task_ratelimit to filter out singular points and
          * limit the step size.
          *
          * The below code essentially only uses the relative value of
          *
          *      task_ratelimit - dirty_ratelimit
          *      = (pos_ratio - 1) * dirty_ratelimit
          *
          * which reflects the direction and size of dirty position error.
          */
 
         /*
          * dirty_ratelimit will follow balanced_dirty_ratelimit iff
          * task_ratelimit is on the same side of dirty_ratelimit, too.
          * For example, when
          * - dirty_ratelimit > balanced_dirty_ratelimit
          * - dirty_ratelimit > task_ratelimit (dirty pages are above setpoint)
          * lowering dirty_ratelimit will help meet both the position and rate
          * control targets. Otherwise, don't update dirty_ratelimit if it will
          * only help meet the rate target. After all, what the users ultimately
          * feel and care are stable dirty rate and small position error.
          *
          * |task_ratelimit - dirty_ratelimit| is used to limit the step size
          * and filter out the singular points of balanced_dirty_ratelimit. Which
          * keeps jumping around randomly and can even leap far away at times
          * due to the small 200ms estimation period of dirty_rate (we want to
          * keep that period small to reduce time lags).
          */
         step = 0;
 
         /*
          * For strictlimit case, calculations above were based on bdi counters
          * and limits (starting from pos_ratio = bdi_position_ratio() and up to
          * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate).
          * Hence, to calculate "step" properly, we have to use bdi_dirty as
          * "dirty" and bdi_setpoint as "setpoint".
          *
          * We rampup dirty_ratelimit forcibly if bdi_dirty is low because
          * it's possible that bdi_thresh is close to zero due to inactivity
          * of backing device (see the implementation of bdi_dirty_limit()).
          */
         if (unlikely(bdi->capabilities & BDI_CAP_STRICTLIMIT)) {
                 dirty = bdi_dirty;
                 if (bdi_dirty < 8)
                         setpoint = bdi_dirty + 1;
                 else
                         setpoint = (bdi_thresh +
                                     bdi_dirty_limit(bdi, bg_thresh)) / 2;
         }
 
         if (dirty < setpoint) {
                 x = min3(bdi->balanced_dirty_ratelimit,
                          balanced_dirty_ratelimit, task_ratelimit);
                 if (dirty_ratelimit < x)
                         step = x - dirty_ratelimit;
         } else {
                 x = max3(bdi->balanced_dirty_ratelimit,
                          balanced_dirty_ratelimit, task_ratelimit);
                 if (dirty_ratelimit > x)
                         step = dirty_ratelimit - x;
         }
 
         /*
          * Don't pursue 100% rate matching. It's impossible since the balanced
          * rate itself is constantly fluctuating. So decrease the track speed
          * when it gets close to the target. Helps eliminate pointless tremors.
          */
         shift = dirty_ratelimit / (2 * step + 1);
         if (shift < BITS_PER_LONG)
                 step = DIV_ROUND_UP(step >> shift, 8);
         else
                 step = 0;
 
         if (dirty_ratelimit < balanced_dirty_ratelimit)
                 dirty_ratelimit += step;
         else
                 dirty_ratelimit -= step;
 
         bdi->dirty_ratelimit = max(dirty_ratelimit, 1UL);
         bdi->balanced_dirty_ratelimit = balanced_dirty_ratelimit;
 
         trace_bdi_dirty_ratelimit(bdi, dirty_rate, task_ratelimit);
 }
 
 void __bdi_update_bandwidth(struct backing_dev_info *bdi,
                             unsigned long thresh,
                             unsigned long bg_thresh,
                             unsigned long dirty,
                             unsigned long bdi_thresh,
                             unsigned long bdi_dirty,
                             unsigned long start_time)
 {
         unsigned long now = jiffies;
         unsigned long elapsed = now - bdi->bw_time_stamp;
         unsigned long dirtied;
         unsigned long written;
 
         /*
          * rate-limit, only update once every 200ms.
          */
         if (elapsed < BANDWIDTH_INTERVAL)
                 return;
 
         dirtied = percpu_counter_read(&bdi->bdi_stat[BDI_DIRTIED]);
         written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]);
 
         /*
          * Skip quiet periods when disk bandwidth is under-utilized.
          * (at least 1s idle time between two flusher runs)
          */
         if (elapsed > HZ && time_before(bdi->bw_time_stamp, start_time))
                 goto snapshot;
 
         if (thresh) {
                 global_update_bandwidth(thresh, dirty, now);
                 bdi_update_dirty_ratelimit(bdi, thresh, bg_thresh, dirty,
                                            bdi_thresh, bdi_dirty,
                                            dirtied, elapsed);
         }
         bdi_update_write_bandwidth(bdi, elapsed, written);
 
 snapshot:
         bdi->dirtied_stamp = dirtied;
         bdi->written_stamp = written;
         bdi->bw_time_stamp = now;
 }
 
 static void bdi_update_bandwidth(struct backing_dev_info *bdi,
                                  unsigned long thresh,
                                  unsigned long bg_thresh,
                                  unsigned long dirty,
                                  unsigned long bdi_thresh,
                                  unsigned long bdi_dirty,
                                  unsigned long start_time)
 {
         if (time_is_after_eq_jiffies(bdi->bw_time_stamp + BANDWIDTH_INTERVAL))
                 return;
         spin_lock(&bdi->wb.list_lock);
         __bdi_update_bandwidth(bdi, thresh, bg_thresh, dirty,
                                bdi_thresh, bdi_dirty, start_time);
         spin_unlock(&bdi->wb.list_lock);
 }
 
 /*
  * After a task dirtied this many pages, balance_dirty_pages_ratelimited()
  * will look to see if it needs to start dirty throttling.
  *
  * If dirty_poll_interval is too low, big NUMA machines will call the expensive
  * global_page_state() too often. So scale it near-sqrt to the safety margin
  * (the number of pages we may dirty without exceeding the dirty limits).
  */
 static unsigned long dirty_poll_interval(unsigned long dirty,
                                          unsigned long thresh)
 {
         if (thresh > dirty)
                 return 1UL << (ilog2(thresh - dirty) >> 1);
 
         return 1;
 }
 
 static unsigned long bdi_max_pause(struct backing_dev_info *bdi,
                                    unsigned long bdi_dirty)
 {
         unsigned long bw = bdi->avg_write_bandwidth;
         unsigned long t;
 
         /*
          * Limit pause time for small memory systems. If sleeping for too long
          * time, a small pool of dirty/writeback pages may go empty and disk go
          * idle.
          *
          * 8 serves as the safety ratio.
          */
         t = bdi_dirty / (1 + bw / roundup_pow_of_two(1 + HZ / 8));
         t++;
 
         return min_t(unsigned long, t, MAX_PAUSE);
 }
 
 static long bdi_min_pause(struct backing_dev_info *bdi,
                           long max_pause,
                           unsigned long task_ratelimit,
                           unsigned long dirty_ratelimit,
                           int *nr_dirtied_pause)
 {
         long hi = ilog2(bdi->avg_write_bandwidth);
         long lo = ilog2(bdi->dirty_ratelimit);
         long t;         /* target pause */
         long pause;     /* estimated next pause */
         int pages;      /* target nr_dirtied_pause */
 
         /* target for 10ms pause on 1-dd case */
         t = max(1, HZ / 100);
 
         /*
          * Scale up pause time for concurrent dirtiers in order to reduce CPU
          * overheads.
          *
          * (N * 10ms) on 2^N concurrent tasks.
          */
         if (hi > lo)
                 t += (hi - lo) * (10 * HZ) / 1024;
 
         /*
          * This is a bit convoluted. We try to base the next nr_dirtied_pause
          * on the much more stable dirty_ratelimit. However the next pause time
          * will be computed based on task_ratelimit and the two rate limits may
          * depart considerably at some time. Especially if task_ratelimit goes
          * below dirty_ratelimit/2 and the target pause is max_pause, the next
          * pause time will be max_pause*2 _trimmed down_ to max_pause.  As a
          * result task_ratelimit won't be executed faithfully, which could
          * eventually bring down dirty_ratelimit.
          *
          * We apply two rules to fix it up:
          * 1) try to estimate the next pause time and if necessary, use a lower
          *    nr_dirtied_pause so as not to exceed max_pause. When this happens,
          *    nr_dirtied_pause will be "dancing" with task_ratelimit.
          * 2) limit the target pause time to max_pause/2, so that the normal
          *    small fluctuations of task_ratelimit won't trigger rule (1) and
          *    nr_dirtied_pause will remain as stable as dirty_ratelimit.
          */
         t = min(t, 1 + max_pause / 2);
         pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
 
         /*
          * Tiny nr_dirtied_pause is found to hurt I/O performance in the test
          * case fio-mmap-randwrite-64k, which does 16*{sync read, async write}.
          * When the 16 consecutive reads are often interrupted by some dirty
          * throttling pause during the async writes, cfq will go into idles
          * (deadline is fine). So push nr_dirtied_pause as high as possible
          * until reaches DIRTY_POLL_THRESH=32 pages.
          */
         if (pages < DIRTY_POLL_THRESH) {
                 t = max_pause;
                 pages = dirty_ratelimit * t / roundup_pow_of_two(HZ);
                 if (pages > DIRTY_POLL_THRESH) {
                         pages = DIRTY_POLL_THRESH;
                         t = HZ * DIRTY_POLL_THRESH / dirty_ratelimit;
                 }
         }
 
         pause = HZ * pages / (task_ratelimit + 1);
         if (pause > max_pause) {
                 t = max_pause;
                 pages = task_ratelimit * t / roundup_pow_of_two(HZ);
         }
 
         *nr_dirtied_pause = pages;
         /*
          * The minimal pause time will normally be half the target pause time.
          */
         return pages >= DIRTY_POLL_THRESH ? 1 + t / 2 : t;
 }
 
 static inline void bdi_dirty_limits(struct backing_dev_info *bdi,
                                     unsigned long dirty_thresh,
                                     unsigned long background_thresh,
                                     unsigned long *bdi_dirty,
                                     unsigned long *bdi_thresh,
                                     unsigned long *bdi_bg_thresh)
 {
         unsigned long bdi_reclaimable;
 
         /*
          * bdi_thresh is not treated as some limiting factor as
          * dirty_thresh, due to reasons
          * - in JBOD setup, bdi_thresh can fluctuate a lot
          * - in a system with HDD and USB key, the USB key may somehow
          *   go into state (bdi_dirty >> bdi_thresh) either because
          *   bdi_dirty starts high, or because bdi_thresh drops low.
          *   In this case we don't want to hard throttle the USB key
          *   dirtiers for 100 seconds until bdi_dirty drops under
          *   bdi_thresh. Instead the auxiliary bdi control line in
          *   bdi_position_ratio() will let the dirtier task progress
          *   at some rate <= (write_bw / 2) for bringing down bdi_dirty.
          */
         *bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
 
         if (bdi_bg_thresh)
                 *bdi_bg_thresh = dirty_thresh ? div_u64((u64)*bdi_thresh *
                                                         background_thresh,
                                                         dirty_thresh) : 0;
 
         /*
          * In order to avoid the stacked BDI deadlock we need
          * to ensure we accurately count the 'dirty' pages when
          * the threshold is low.
          *
          * Otherwise it would be possible to get thresh+n pages
          * reported dirty, even though there are thresh-m pages
          * actually dirty; with m+n sitting in the percpu
          * deltas.
          */
         if (*bdi_thresh < 2 * bdi_stat_error(bdi)) {
                 bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
                 *bdi_dirty = bdi_reclaimable +
                         bdi_stat_sum(bdi, BDI_WRITEBACK);
         } else {
                 bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
                 *bdi_dirty = bdi_reclaimable +
                         bdi_stat(bdi, BDI_WRITEBACK);
         }
 }
 
 /*
  * balance_dirty_pages() must be called by processes which are generating dirty
  * data.  It looks at the number of dirty pages in the machine and will force
  * the caller to wait once crossing the (background_thresh + dirty_thresh) / 2.
  * If we're over `background_thresh' then the writeback threads are woken to
  * perform some writeout.
  */
 static void balance_dirty_pages(struct address_space *mapping,
                                 unsigned long pages_dirtied)
 {
         unsigned long nr_reclaimable;   /* = file_dirty + unstable_nfs */
         unsigned long nr_dirty;  /* = file_dirty + writeback + unstable_nfs */
         unsigned long background_thresh;
         unsigned long dirty_thresh;
         long period;
         long pause;
         long max_pause;
         long min_pause;
         int nr_dirtied_pause;
         bool dirty_exceeded = false;
         unsigned long task_ratelimit;
         unsigned long dirty_ratelimit;
         unsigned long pos_ratio;
         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
         bool strictlimit = bdi->capabilities & BDI_CAP_STRICTLIMIT;
         unsigned long start_time = jiffies;
 
         for (;;) {
                 unsigned long now = jiffies;
                 unsigned long uninitialized_var(bdi_thresh);
                 unsigned long thresh;
                 unsigned long uninitialized_var(bdi_dirty);
                 unsigned long dirty;
                 unsigned long bg_thresh;
 
                 /*
                  * Unstable writes are a feature of certain networked
                  * filesystems (i.e. NFS) in which data may have been
                  * written to the server's write cache, but has not yet
                  * been flushed to permanent storage.
                  */
                 nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
                                         global_page_state(NR_UNSTABLE_NFS);
                 nr_dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);
 
                 global_dirty_limits(&background_thresh, &dirty_thresh);
 
                 if (unlikely(strictlimit)) {
                         bdi_dirty_limits(bdi, dirty_thresh, background_thresh,
                                          &bdi_dirty, &bdi_thresh, &bg_thresh);
 
                         dirty = bdi_dirty;
                         thresh = bdi_thresh;
                 } else {
                         dirty = nr_dirty;
                         thresh = dirty_thresh;
                         bg_thresh = background_thresh;
                 }
 
                 /*
                  * Throttle it only when the background writeback cannot
                  * catch-up. This avoids (excessively) small writeouts
                  * when the bdi limits are ramping up in case of !strictlimit.
                  *
                  * In strictlimit case make decision based on the bdi counters
                  * and limits. Small writeouts when the bdi limits are ramping
                  * up are the price we consciously pay for strictlimit-ing.
                  */
                 if (dirty <= dirty_freerun_ceiling(thresh, bg_thresh)) {
                         current->dirty_paused_when = now;
                         current->nr_dirtied = 0;
                         current->nr_dirtied_pause =
                                 dirty_poll_interval(dirty, thresh);
                         break;
                 }
 
                 if (unlikely(!writeback_in_progress(bdi)))
                         bdi_start_background_writeback(bdi);
 
                 if (!strictlimit)
                         bdi_dirty_limits(bdi, dirty_thresh, background_thresh,
                                          &bdi_dirty, &bdi_thresh, NULL);
 
                 dirty_exceeded = (bdi_dirty > bdi_thresh) &&
                                  ((nr_dirty > dirty_thresh) || strictlimit);
                 if (dirty_exceeded && !bdi->dirty_exceeded)
                         bdi->dirty_exceeded = 1;
 
                 bdi_update_bandwidth(bdi, dirty_thresh, background_thresh,
                                      nr_dirty, bdi_thresh, bdi_dirty,
                                      start_time);
 
                 dirty_ratelimit = bdi->dirty_ratelimit;
                 pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
                                                background_thresh, nr_dirty,
                                                bdi_thresh, bdi_dirty);
                 task_ratelimit = ((u64)dirty_ratelimit * pos_ratio) >>
                                                         RATELIMIT_CALC_SHIFT;
                 max_pause = bdi_max_pause(bdi, bdi_dirty);
                 min_pause = bdi_min_pause(bdi, max_pause,
                                           task_ratelimit, dirty_ratelimit,
                                           &nr_dirtied_pause);
 
                 if (unlikely(task_ratelimit == 0)) {
                         period = max_pause;
                         pause = max_pause;
                         goto pause;
                 }
                 period = HZ * pages_dirtied / task_ratelimit;
                 pause = period;
                 if (current->dirty_paused_when)
                         pause -= now - current->dirty_paused_when;
                 /*
                  * For less than 1s think time (ext3/4 may block the dirtier
                  * for up to 800ms from time to time on 1-HDD; so does xfs,
                  * however at much less frequency), try to compensate it in
                  * future periods by updating the virtual time; otherwise just
                  * do a reset, as it may be a light dirtier.
                  */
                 if (pause < min_pause) {
                         trace_balance_dirty_pages(bdi,
                                                   dirty_thresh,
                                                   background_thresh,
                                                   nr_dirty,
                                                   bdi_thresh,
                                                   bdi_dirty,
                                                   dirty_ratelimit,
                                                   task_ratelimit,
                                                   pages_dirtied,
                                                   period,
                                                   min(pause, 0L),
                                                   start_time);
                         if (pause < -HZ) {
                                 current->dirty_paused_when = now;
                                 current->nr_dirtied = 0;
                         } else if (period) {
                                 current->dirty_paused_when += period;
                                 current->nr_dirtied = 0;
                         } else if (current->nr_dirtied_pause <= pages_dirtied)
                                 current->nr_dirtied_pause += pages_dirtied;
                         break;
                 }
                 if (unlikely(pause > max_pause)) {
                         /* for occasional dropped task_ratelimit */
                         now += min(pause - max_pause, max_pause);
                         pause = max_pause;
                 }
 
 pause:
                 trace_balance_dirty_pages(bdi,
                                           dirty_thresh,
                                           background_thresh,
                                           nr_dirty,
                                           bdi_thresh,
                                           bdi_dirty,
                                           dirty_ratelimit,
                                           task_ratelimit,
                                           pages_dirtied,
                                           period,
                                           pause,
                                           start_time);
                 __set_current_state(TASK_KILLABLE);
                 io_schedule_timeout(pause);
 
                 current->dirty_paused_when = now + pause;
                 current->nr_dirtied = 0;
                 current->nr_dirtied_pause = nr_dirtied_pause;
 
                 /*
                  * This is typically equal to (nr_dirty < dirty_thresh) and can
                  * also keep "1000+ dd on a slow USB stick" under control.
                  */
                 if (task_ratelimit)
                         break;
 
                 /*
                  * In the case of an unresponding NFS server and the NFS dirty
                  * pages exceeds dirty_thresh, give the other good bdi's a pipe
                  * to go through, so that tasks on them still remain responsive.
                  *
                  * In theory 1 page is enough to keep the comsumer-producer
                  * pipe going: the flusher cleans 1 page => the task dirties 1
                  * more page. However bdi_dirty has accounting errors.  So use
                  * the larger and more IO friendly bdi_stat_error.
                  */
                 if (bdi_dirty <= bdi_stat_error(bdi))
                         break;
 
                 if (fatal_signal_pending(current))
                         break;
         }
 
         if (!dirty_exceeded && bdi->dirty_exceeded)
                 bdi->dirty_exceeded = 0;
 
         if (writeback_in_progress(bdi))
                 return;
 
         /*
          * In laptop mode, we wait until hitting the higher threshold before
          * starting background writeout, and then write out all the way down
          * to the lower threshold.  So slow writers cause minimal disk activity.
          *
          * In normal mode, we start background writeout at the lower
          * background_thresh, to keep the amount of dirty memory low.
          */
         if (laptop_mode)
                 return;
 
         if (nr_reclaimable > background_thresh)
                 bdi_start_background_writeback(bdi);
 }
 
 static DEFINE_PER_CPU(int, bdp_ratelimits);
 
 /*
  * Normal tasks are throttled by
  *      loop {
  *              dirty tsk->nr_dirtied_pause pages;
  *              take a snap in balance_dirty_pages();
  *      }
  * However there is a worst case. If every task exit immediately when dirtied
  * (tsk->nr_dirtied_pause - 1) pages, balance_dirty_pages() will never be
  * called to throttle the page dirties. The solution is to save the not yet
  * throttled page dirties in dirty_throttle_leaks on task exit and charge them
  * randomly into the running tasks. This works well for the above worst case,
  * as the new task will pick up and accumulate the old task's leaked dirty
  * count and eventually get throttled.
  */
 DEFINE_PER_CPU(int, dirty_throttle_leaks) = 0;
 
 /**
  * balance_dirty_pages_ratelimited - balance dirty memory state
  * @mapping: address_space which was dirtied
  *
  * Processes which are dirtying memory should call in here once for each page
  * which was newly dirtied.  The function will periodically check the system's
  * dirty state and will initiate writeback if needed.
  *
  * On really big machines, get_writeback_state is expensive, so try to avoid
  * calling it too often (ratelimiting).  But once we're over the dirty memory
  * limit we decrease the ratelimiting by a lot, to prevent individual processes
  * from overshooting the limit by (ratelimit_pages) each.
  */
 void balance_dirty_pages_ratelimited(struct address_space *mapping)
 {
         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
         int ratelimit;
         int *p;
 
         if (!bdi_cap_account_dirty(bdi))
                 return;
 
         ratelimit = current->nr_dirtied_pause;
         if (bdi->dirty_exceeded)
                 ratelimit = min(ratelimit, 32 >> (PAGE_SHIFT - 10));
 
         preempt_disable();
         /*
          * This prevents one CPU to accumulate too many dirtied pages without
          * calling into balance_dirty_pages(), which can happen when there are
          * 1000+ tasks, all of them start dirtying pages at exactly the same
          * time, hence all honoured too large initial task->nr_dirtied_pause.
          */
         p =  this_cpu_ptr(&bdp_ratelimits);
         if (unlikely(current->nr_dirtied >= ratelimit))
                 *p = 0;
         else if (unlikely(*p >= ratelimit_pages)) {
                 *p = 0;
                 ratelimit = 0;
         }
         /*
          * Pick up the dirtied pages by the exited tasks. This avoids lots of
          * short-lived tasks (eg. gcc invocations in a kernel build) escaping
          * the dirty throttling and livelock other long-run dirtiers.
          */
         p = this_cpu_ptr(&dirty_throttle_leaks);
         if (*p > 0 && current->nr_dirtied < ratelimit) {
                 unsigned long nr_pages_dirtied;
                 nr_pages_dirtied = min(*p, ratelimit - current->nr_dirtied);
                 *p -= nr_pages_dirtied;
                 current->nr_dirtied += nr_pages_dirtied;
         }
         preempt_enable();
 
         if (unlikely(current->nr_dirtied >= ratelimit))
                 balance_dirty_pages(mapping, current->nr_dirtied);
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited);
 
 void throttle_vm_writeout(gfp_t gfp_mask)
 {
         unsigned long background_thresh;
         unsigned long dirty_thresh;
 
         for ( ; ; ) {
                 global_dirty_limits(&background_thresh, &dirty_thresh);
                 dirty_thresh = hard_dirty_limit(dirty_thresh);
 
                 /*
                  * Boost the allowable dirty threshold a bit for page
                  * allocators so they don't get DoS'ed by heavy writers
                  */
                 dirty_thresh += dirty_thresh / 10;      /* wheeee... */
 
                 if (global_page_state(NR_UNSTABLE_NFS) +
                         global_page_state(NR_WRITEBACK) <= dirty_thresh)
                                 break;
                 congestion_wait(BLK_RW_ASYNC, HZ/10);
 
                 /*
                  * The caller might hold locks which can prevent IO completion
                  * or progress in the filesystem.  So we cannot just sit here
                  * waiting for IO to complete.
                  */
                 if ((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO))
                         break;
         }
 }
 
 /*
  * sysctl handler for /proc/sys/vm/dirty_writeback_centisecs
  */
 int dirty_writeback_centisecs_handler(struct ctl_table *table, int write,
         void __user *buffer, size_t *length, loff_t *ppos)
 {
         proc_dointvec(table, write, buffer, length, ppos);
         return 0;
 }
 
 #ifdef CONFIG_BLOCK
 void laptop_mode_timer_fn(unsigned long data)
 {
         struct request_queue *q = (struct request_queue *)data;
         int nr_pages = global_page_state(NR_FILE_DIRTY) +
                 global_page_state(NR_UNSTABLE_NFS);
 
         /*
          * We want to write everything out, not just down to the dirty
          * threshold
          */
         if (bdi_has_dirty_io(&q->backing_dev_info))
                 bdi_start_writeback(&q->backing_dev_info, nr_pages,
                                         WB_REASON_LAPTOP_TIMER);
 }
 
 /*
  * We've spun up the disk and we're in laptop mode: schedule writeback
  * of all dirty data a few seconds from now.  If the flush is already scheduled
  * then push it back - the user is still using the disk.
  */
 void laptop_io_completion(struct backing_dev_info *info)
 {
         mod_timer(&info->laptop_mode_wb_timer, jiffies + laptop_mode);
 }
 
 /*
  * We're in laptop mode and we've just synced. The sync's writes will have
  * caused another writeback to be scheduled by laptop_io_completion.
  * Nothing needs to be written back anymore, so we unschedule the writeback.
  */
 void laptop_sync_completion(void)
 {
         struct backing_dev_info *bdi;
 
         rcu_read_lock();
 
         list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
                 del_timer(&bdi->laptop_mode_wb_timer);
 
         rcu_read_unlock();
 }
 #endif
 
 /*
  * If ratelimit_pages is too high then we can get into dirty-data overload
  * if a large number of processes all perform writes at the same time.
  * If it is too low then SMP machines will call the (expensive)
  * get_writeback_state too often.
  *
  * Here we set ratelimit_pages to a level which ensures that when all CPUs are
  * dirtying in parallel, we cannot go more than 3% (1/32) over the dirty memory
  * thresholds.
  */
 
 void writeback_set_ratelimit(void)
 {
         unsigned long background_thresh;
         unsigned long dirty_thresh;
         global_dirty_limits(&background_thresh, &dirty_thresh);
         global_dirty_limit = dirty_thresh;
         ratelimit_pages = dirty_thresh / (num_online_cpus() * 32);
         if (ratelimit_pages < 16)
                 ratelimit_pages = 16;
 }
 
 static int
 ratelimit_handler(struct notifier_block *self, unsigned long action,
                   void *hcpu)
 {
 
         switch (action & ~CPU_TASKS_FROZEN) {
         case CPU_ONLINE:
         case CPU_DEAD:
                 writeback_set_ratelimit();
                 return NOTIFY_OK;
         default:
                 return NOTIFY_DONE;
         }
 }
 
 static struct notifier_block ratelimit_nb = {
         .notifier_call  = ratelimit_handler,
         .next           = NULL,
 };
 
 /*
  * Called early on to tune the page writeback dirty limits.
  *
  * We used to scale dirty pages according to how total memory
  * related to pages that could be allocated for buffers (by
  * comparing nr_free_buffer_pages() to vm_total_pages.
  *
  * However, that was when we used "dirty_ratio" to scale with
  * all memory, and we don't do that any more. "dirty_ratio"
  * is now applied to total non-HIGHPAGE memory (by subtracting
  * totalhigh_pages from vm_total_pages), and as such we can't
  * get into the old insane situation any more where we had
  * large amounts of dirty pages compared to a small amount of
  * non-HIGHMEM memory.
  *
  * But we might still want to scale the dirty_ratio by how
  * much memory the box has..
  */
 void __init page_writeback_init(void)
 {
         writeback_set_ratelimit();
         register_cpu_notifier(&ratelimit_nb);
 
         fprop_global_init(&writeout_completions, GFP_KERNEL);
 }
 
 /**
  * tag_pages_for_writeback - tag pages to be written by write_cache_pages
  * @mapping: address space structure to write
  * @start: starting page index
  * @end: ending page index (inclusive)
  *
  * This function scans the page range from @start to @end (inclusive) and tags
  * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
  * that write_cache_pages (or whoever calls this function) will then use
  * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
  * used to avoid livelocking of writeback by a process steadily creating new
  * dirty pages in the file (thus it is important for this function to be quick
  * so that it can tag pages faster than a dirtying process can create them).
  */
 /*
  * We tag pages in batches of WRITEBACK_TAG_BATCH to reduce tree_lock latency.
  */
 void tag_pages_for_writeback(struct address_space *mapping,
                              pgoff_t start, pgoff_t end)
 {
 #define WRITEBACK_TAG_BATCH 4096
         unsigned long tagged;
 
         do {
                 spin_lock_irq(&mapping->tree_lock);
                 tagged = radix_tree_range_tag_if_tagged(&mapping->page_tree,
                                 &start, end, WRITEBACK_TAG_BATCH,
                                 PAGECACHE_TAG_DIRTY, PAGECACHE_TAG_TOWRITE);
                 spin_unlock_irq(&mapping->tree_lock);
                 WARN_ON_ONCE(tagged > WRITEBACK_TAG_BATCH);
                 cond_resched();
                 /* We check 'start' to handle wrapping when end == ~0UL */
         } while (tagged >= WRITEBACK_TAG_BATCH && start);
 }
 EXPORT_SYMBOL(tag_pages_for_writeback);
 
 /**
  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
  * @writepage: function called for each page
  * @data: data passed to writepage function
  *
  * If a page is already under I/O, write_cache_pages() skips it, even
  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
  * and msync() need to guarantee that all the data which was dirty at the time
  * the call was made get new I/O started against them.  If wbc->sync_mode is
  * WB_SYNC_ALL then we were called for data integrity and we must wait for
  * existing IO to complete.
  *
  * To avoid livelocks (when other process dirties new pages), we first tag
  * pages which should be written back with TOWRITE tag and only then start
  * writing them. For data-integrity sync we have to be careful so that we do
  * not miss some pages (e.g., because some other process has cleared TOWRITE
  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
  * by the process clearing the DIRTY tag (and submitting the page for IO).
  */
 int write_cache_pages(struct address_space *mapping,
                       struct writeback_control *wbc, writepage_t writepage,
                       void *data)
 {
         int ret = 0;
         int done = 0;
         struct pagevec pvec;
         int nr_pages;
         pgoff_t uninitialized_var(writeback_index);
         pgoff_t index;
         pgoff_t end;            /* Inclusive */
         pgoff_t done_index;
         int cycled;
         int range_whole = 0;
         int tag;
 
         pagevec_init(&pvec, 0);
         if (wbc->range_cyclic) {
                 writeback_index = mapping->writeback_index; /* prev offset */
                 index = writeback_index;
                 if (index == 0)
                         cycled = 1;
                 else
                         cycled = 0;
                 end = -1;
         } else {
                 index = wbc->range_start >> PAGE_CACHE_SHIFT;
                 end = wbc->range_end >> PAGE_CACHE_SHIFT;
                 if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
                         range_whole = 1;
                 cycled = 1; /* ignore range_cyclic tests */
         }
         if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
                 tag = PAGECACHE_TAG_TOWRITE;
         else
                 tag = PAGECACHE_TAG_DIRTY;
 retry:
         if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
                 tag_pages_for_writeback(mapping, index, end);
         done_index = index;
         while (!done && (index <= end)) {
                 int i;
 
                 nr_pages = pagevec_lookup_tag(&pvec, mapping, &index, tag,
                               min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1);
                 if (nr_pages == 0)
                         break;
 
                 for (i = 0; i < nr_pages; i++) {
                         struct page *page = pvec.pages[i];
 
                         /*
                          * At this point, the page may be truncated or
                          * invalidated (changing page->mapping to NULL), or
                          * even swizzled back from swapper_space to tmpfs file
                          * mapping. However, page->index will not change
                          * because we have a reference on the page.
                          */
                         if (page->index > end) {
                                 /*
                                  * can't be range_cyclic (1st pass) because
                                  * end == -1 in that case.
                                  */
                                 done = 1;
                                 break;
                         }
 
                         done_index = page->index;
 
                         lock_page(page);
 
                         /*
                          * Page truncated or invalidated. We can freely skip it
                          * then, even for data integrity operations: the page
                          * has disappeared concurrently, so there could be no
                          * real expectation of this data interity operation
                          * even if there is now a new, dirty page at the same
                          * pagecache address.
                          */
                         if (unlikely(page->mapping != mapping)) {
 continue_unlock:
                                 unlock_page(page);
                                 continue;
                         }
 
                         if (!PageDirty(page)) {
                                 /* someone wrote it for us */
                                 goto continue_unlock;
                         }
 
                         if (PageWriteback(page)) {
                                 if (wbc->sync_mode != WB_SYNC_NONE)
                                         wait_on_page_writeback(page);
                                 else
                                         goto continue_unlock;
                         }
 
                         BUG_ON(PageWriteback(page));
                         if (!clear_page_dirty_for_io(page))
                                 goto continue_unlock;
 
                         trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
                         ret = (*writepage)(page, wbc, data);
                         if (unlikely(ret)) {
                                 if (ret == AOP_WRITEPAGE_ACTIVATE) {
                                         unlock_page(page);
                                         ret = 0;
                                 } else {
                                         /*
                                          * done_index is set past this page,
                                          * so media errors will not choke
                                          * background writeout for the entire
                                          * file. This has consequences for
                                          * range_cyclic semantics (ie. it may
                                          * not be suitable for data integrity
                                          * writeout).
                                          */
                                         done_index = page->index + 1;
                                         done = 1;
                                         break;
                                 }
                         }
 
                         /*
                          * We stop writing back only if we are not doing
                          * integrity sync. In case of integrity sync we have to
                          * keep going until we have written all the pages
                          * we tagged for writeback prior to entering this loop.
                          */
                         if (--wbc->nr_to_write <= 0 &&
                             wbc->sync_mode == WB_SYNC_NONE) {
                                 done = 1;
                                 break;
                         }
                 }
                 pagevec_release(&pvec);
                 cond_resched();
         }
         if (!cycled && !done) {
                 /*
                  * range_cyclic:
                  * We hit the last page and there is more work to be done: wrap
                  * back to the start of the file
                  */
                 cycled = 1;
                 index = 0;
                 end = writeback_index - 1;
                 goto retry;
         }
         if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
                 mapping->writeback_index = done_index;
 
         return ret;
 }
 EXPORT_SYMBOL(write_cache_pages);
 
 /*
  * Function used by generic_writepages to call the real writepage
  * function and set the mapping flags on error
  */
 static int __writepage(struct page *page, struct writeback_control *wbc,
                        void *data)
 {
         struct address_space *mapping = data;
         int ret = mapping->a_ops->writepage(page, wbc);
         mapping_set_error(mapping, ret);
         return ret;
 }
 
 /**
  * generic_writepages - walk the list of dirty pages of the given address space and writepage() all of them.
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
  *
  * This is a library function, which implements the writepages()
  * address_space_operation.
  */
 int generic_writepages(struct address_space *mapping,
                        struct writeback_control *wbc)
 {
         struct blk_plug plug;
         int ret;
 
         /* deal with chardevs and other special file */
         if (!mapping->a_ops->writepage)
                 return 0;
 
         blk_start_plug(&plug);
         ret = write_cache_pages(mapping, wbc, __writepage, mapping);
         blk_finish_plug(&plug);
         return ret;
 }
 
 EXPORT_SYMBOL(generic_writepages);
 
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
         int ret;
 
         if (wbc->nr_to_write <= 0)
                 return 0;
         if (mapping->a_ops->writepages)
                 ret = mapping->a_ops->writepages(mapping, wbc);
         else
                 ret = generic_writepages(mapping, wbc);
         return ret;
 }
 
 /**
  * write_one_page - write out a single page and optionally wait on I/O
  * @page: the page to write
  * @wait: if true, wait on writeout
  *
  * The page must be locked by the caller and will be unlocked upon return.
  *
  * write_one_page() returns a negative error code if I/O failed.
  */
 int write_one_page(struct page *page, int wait)
 {
         struct address_space *mapping = page->mapping;
         int ret = 0;
         struct writeback_control wbc = {
                 .sync_mode = WB_SYNC_ALL,
                 .nr_to_write = 1,
         };
 
         BUG_ON(!PageLocked(page));
 
         if (wait)
                 wait_on_page_writeback(page);
 
         if (clear_page_dirty_for_io(page)) {
                 page_cache_get(page);
                 ret = mapping->a_ops->writepage(page, &wbc);
                 if (ret == 0 && wait) {
                         wait_on_page_writeback(page);
                         if (PageError(page))
                                 ret = -EIO;
                 }
                 page_cache_release(page);
         } else {
                 unlock_page(page);
         }
         return ret;
 }
 EXPORT_SYMBOL(write_one_page);
 
 /*
  * For address_spaces which do not use buffers nor write back.
  */
 int __set_page_dirty_no_writeback(struct page *page)
 {
         if (!PageDirty(page))
                 return !TestSetPageDirty(page);
         return 0;
 }
 
 /*
  * Helper function for set_page_dirty family.
  * NOTE: This relies on being atomic wrt interrupts.
  */
 void account_page_dirtied(struct page *page, struct address_space *mapping)
 {
         trace_writeback_dirty_page(page, mapping);
 
         if (mapping_cap_account_dirty(mapping)) {
                 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 
                 __inc_zone_page_state(page, NR_FILE_DIRTY);
                 __inc_zone_page_state(page, NR_DIRTIED);
                 __inc_bdi_stat(bdi, BDI_RECLAIMABLE);
                 __inc_bdi_stat(bdi, BDI_DIRTIED);
                 task_io_account_write(PAGE_CACHE_SIZE);
                 current->nr_dirtied++;
                 this_cpu_inc(bdp_ratelimits);
         }
 }
 EXPORT_SYMBOL(account_page_dirtied);
 
 /*
  * Helper function for deaccounting dirty page without writeback.
  *
  * Doing this should *normally* only ever be done when a page
  * is truncated, and is not actually mapped anywhere at all. However,
  * fs/buffer.c does this when it notices that somebody has cleaned
  * out all the buffers on a page without actually doing it through
  * the VM. Can you say "ext3 is horribly ugly"? Thought you could.
  */
 void account_page_cleaned(struct page *page, struct address_space *mapping)
 {
         if (mapping_cap_account_dirty(mapping)) {
                 dec_zone_page_state(page, NR_FILE_DIRTY);
                 dec_bdi_stat(inode_to_bdi(mapping->host), BDI_RECLAIMABLE);
                 task_io_account_cancelled_write(PAGE_CACHE_SIZE);
         }
 }
 EXPORT_SYMBOL(account_page_cleaned);
 
 /*
  * For address_spaces which do not use buffers.  Just tag the page as dirty in
  * its radix tree.
  *
  * This is also used when a single buffer is being dirtied: we want to set the
  * page dirty in that case, but not all the buffers.  This is a "bottom-up"
  * dirtying, whereas __set_page_dirty_buffers() is a "top-down" dirtying.
  *
  * The caller must ensure this doesn't race with truncation.  Most will simply
  * hold the page lock, but e.g. zap_pte_range() calls with the page mapped and
  * the pte lock held, which also locks out truncation.
  */
 int __set_page_dirty_nobuffers(struct page *page)
 {
         if (!TestSetPageDirty(page)) {
                 struct address_space *mapping = page_mapping(page);
                 unsigned long flags;
 
                 if (!mapping)
                         return 1;
 
                 spin_lock_irqsave(&mapping->tree_lock, flags);
                 BUG_ON(page_mapping(page) != mapping);
                 WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
                 account_page_dirtied(page, mapping);
                 radix_tree_tag_set(&mapping->page_tree, page_index(page),
                                    PAGECACHE_TAG_DIRTY);
                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
                 if (mapping->host) {
                         /* !PageAnon && !swapper_space */
                         __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
                 }
                 return 1;
         }
         return 0;
 }
 EXPORT_SYMBOL(__set_page_dirty_nobuffers);
 
 /*
  * Call this whenever redirtying a page, to de-account the dirty counters
  * (NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied), so that they match the written
  * counters (NR_WRITTEN, BDI_WRITTEN) in long term. The mismatches will lead to
  * systematic errors in balanced_dirty_ratelimit and the dirty pages position
  * control.
  */
 void account_page_redirty(struct page *page)
 {
         struct address_space *mapping = page->mapping;
         if (mapping && mapping_cap_account_dirty(mapping)) {
                 current->nr_dirtied--;
                 dec_zone_page_state(page, NR_DIRTIED);
                 dec_bdi_stat(inode_to_bdi(mapping->host), BDI_DIRTIED);
         }
 }
 EXPORT_SYMBOL(account_page_redirty);
 
 /*
  * When a writepage implementation decides that it doesn't want to write this
  * page for some reason, it should redirty the locked page via
  * redirty_page_for_writepage() and it should then unlock the page and return 0
  */
 int redirty_page_for_writepage(struct writeback_control *wbc, struct page *page)
 {
         int ret;
 
         wbc->pages_skipped++;
         ret = __set_page_dirty_nobuffers(page);
         account_page_redirty(page);
         return ret;
 }
 EXPORT_SYMBOL(redirty_page_for_writepage);
 
 /*
  * Dirty a page.
  *
  * For pages with a mapping this should be done under the page lock
  * for the benefit of asynchronous memory errors who prefer a consistent
  * dirty state. This rule can be broken in some special cases,
  * but should be better not to.
  *
  * If the mapping doesn't provide a set_page_dirty a_op, then
  * just fall through and assume that it wants buffer_heads.
  */
 int set_page_dirty(struct page *page)
 {
         struct address_space *mapping = page_mapping(page);
 
         if (likely(mapping)) {
                 int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
                 /*
                  * readahead/lru_deactivate_page could remain
                  * PG_readahead/PG_reclaim due to race with end_page_writeback
                  * About readahead, if the page is written, the flags would be
                  * reset. So no problem.
                  * About lru_deactivate_page, if the page is redirty, the flag
                  * will be reset. So no problem. but if the page is used by readahead
                  * it will confuse readahead and make it restart the size rampup
                  * process. But it's a trivial problem.
                  */
                 if (PageReclaim(page))
                         ClearPageReclaim(page);
 #ifdef CONFIG_BLOCK
                 if (!spd)
                         spd = __set_page_dirty_buffers;
 #endif
                 return (*spd)(page);
         }
         if (!PageDirty(page)) {
                 if (!TestSetPageDirty(page))
                         return 1;
         }
         return 0;
 }
 EXPORT_SYMBOL(set_page_dirty);
 
 /*
  * set_page_dirty() is racy if the caller has no reference against
  * page->mapping->host, and if the page is unlocked.  This is because another
  * CPU could truncate the page off the mapping and then free the mapping.
  *
  * Usually, the page _is_ locked, or the caller is a user-space process which
  * holds a reference on the inode by having an open file.
  *
  * In other cases, the page should be locked before running set_page_dirty().
  */
 int set_page_dirty_lock(struct page *page)
 {
         int ret;
 
         lock_page(page);
         ret = set_page_dirty(page);
         unlock_page(page);
         return ret;
 }
 EXPORT_SYMBOL(set_page_dirty_lock);
 
 /*
  * Clear a page's dirty flag, while caring for dirty memory accounting.
  * Returns true if the page was previously dirty.
  *
  * This is for preparing to put the page under writeout.  We leave the page
  * tagged as dirty in the radix tree so that a concurrent write-for-sync
  * can discover it via a PAGECACHE_TAG_DIRTY walk.  The ->writepage
  * implementation will run either set_page_writeback() or set_page_dirty(),
  * at which stage we bring the page's dirty flag and radix-tree dirty tag
  * back into sync.
  *
  * This incoherency between the page's dirty flag and radix-tree tag is
  * unfortunate, but it only exists while the page is locked.
  */
 int clear_page_dirty_for_io(struct page *page)
 {
         struct address_space *mapping = page_mapping(page);
 
         BUG_ON(!PageLocked(page));
 
         if (mapping && mapping_cap_account_dirty(mapping)) {
                 /*
                  * Yes, Virginia, this is indeed insane.
                  *
                  * We use this sequence to make sure that
                  *  (a) we account for dirty stats properly
                  *  (b) we tell the low-level filesystem to
                  *      mark the whole page dirty if it was
                  *      dirty in a pagetable. Only to then
                  *  (c) clean the page again and return 1 to
                  *      cause the writeback.
                  *
                  * This way we avoid all nasty races with the
                  * dirty bit in multiple places and clearing
                  * them concurrently from different threads.
                  *
                  * Note! Normally the "set_page_dirty(page)"
                  * has no effect on the actual dirty bit - since
                  * that will already usually be set. But we
                  * need the side effects, and it can help us
                  * avoid races.
                  *
                  * We basically use the page "master dirty bit"
                  * as a serialization point for all the different
                  * threads doing their things.
                  */
                 if (page_mkclean(page))
                         set_page_dirty(page);
                 /*
                  * We carefully synchronise fault handlers against
                  * installing a dirty pte and marking the page dirty
                  * at this point.  We do this by having them hold the
                  * page lock while dirtying the page, and pages are
                  * always locked coming in here, so we get the desired
                  * exclusion.
                  */
                 if (TestClearPageDirty(page)) {
                         dec_zone_page_state(page, NR_FILE_DIRTY);
                         dec_bdi_stat(inode_to_bdi(mapping->host),
                                         BDI_RECLAIMABLE);
                         return 1;
                 }
                 return 0;
         }
         return TestClearPageDirty(page);
 }
 EXPORT_SYMBOL(clear_page_dirty_for_io);
 
 int test_clear_page_writeback(struct page *page)
 {
         struct address_space *mapping = page_mapping(page);
         struct mem_cgroup *memcg;
         int ret;
 
         memcg = mem_cgroup_begin_page_stat(page);
         if (mapping) {
                 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
                 unsigned long flags;
 
                 spin_lock_irqsave(&mapping->tree_lock, flags);
                 ret = TestClearPageWriteback(page);
                 if (ret) {
                         radix_tree_tag_clear(&mapping->page_tree,
                                                 page_index(page),
                                                 PAGECACHE_TAG_WRITEBACK);
                         if (bdi_cap_account_writeback(bdi)) {
                                 __dec_bdi_stat(bdi, BDI_WRITEBACK);
                                 __bdi_writeout_inc(bdi);
                         }
                 }
                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
         } else {
                 ret = TestClearPageWriteback(page);
         }
         if (ret) {
                 mem_cgroup_dec_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
                 dec_zone_page_state(page, NR_WRITEBACK);
                 inc_zone_page_state(page, NR_WRITTEN);
         }
         mem_cgroup_end_page_stat(memcg);
         return ret;
 }
 
 int __test_set_page_writeback(struct page *page, bool keep_write)
 {
         struct address_space *mapping = page_mapping(page);
         struct mem_cgroup *memcg;
         int ret;
 
         memcg = mem_cgroup_begin_page_stat(page);
         if (mapping) {
                 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
                 unsigned long flags;
 
                 spin_lock_irqsave(&mapping->tree_lock, flags);
                 ret = TestSetPageWriteback(page);
                 if (!ret) {
                         radix_tree_tag_set(&mapping->page_tree,
                                                 page_index(page),
                                                 PAGECACHE_TAG_WRITEBACK);
                         if (bdi_cap_account_writeback(bdi))
                                 __inc_bdi_stat(bdi, BDI_WRITEBACK);
                 }
                 if (!PageDirty(page))
                         radix_tree_tag_clear(&mapping->page_tree,
                                                 page_index(page),
                                                 PAGECACHE_TAG_DIRTY);
                 if (!keep_write)
                         radix_tree_tag_clear(&mapping->page_tree,
                                                 page_index(page),
                                                 PAGECACHE_TAG_TOWRITE);
                 spin_unlock_irqrestore(&mapping->tree_lock, flags);
         } else {
                 ret = TestSetPageWriteback(page);
         }
         if (!ret) {
                 mem_cgroup_inc_page_stat(memcg, MEM_CGROUP_STAT_WRITEBACK);
                 inc_zone_page_state(page, NR_WRITEBACK);
         }
         mem_cgroup_end_page_stat(memcg);
         return ret;
 
 }
 EXPORT_SYMBOL(__test_set_page_writeback);
 
 /*
  * Return true if any of the pages in the mapping are marked with the
  * passed tag.
  */
 int mapping_tagged(struct address_space *mapping, int tag)
 {
         return radix_tree_tagged(&mapping->page_tree, tag);
 }
 EXPORT_SYMBOL(mapping_tagged);
 
 /**
  * wait_for_stable_page() - wait for writeback to finish, if necessary.
  * @page:       The page to wait on.
  *
  * This function determines if the given page is related to a backing device
  * that requires page contents to be held stable during writeback.  If so, then
  * it will wait for any pending writeback to complete.
  */
 void wait_for_stable_page(struct page *page)
 {
         if (bdi_cap_stable_pages_required(inode_to_bdi(page->mapping->host)))
                 wait_on_page_writeback(page);
 }
 EXPORT_SYMBOL_GPL(wait_for_stable_page);
 

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