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Linux/mm/readahead.c

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  1 // SPDX-License-Identifier: GPL-2.0-only
  2 /*
  3  * mm/readahead.c - address_space-level file readahead.
  4  *
  5  * Copyright (C) 2002, Linus Torvalds
  6  *
  7  * 09Apr2002    Andrew Morton
  8  *              Initial version.
  9  */
 10 
 11 #include <linux/kernel.h>
 12 #include <linux/dax.h>
 13 #include <linux/gfp.h>
 14 #include <linux/export.h>
 15 #include <linux/blkdev.h>
 16 #include <linux/backing-dev.h>
 17 #include <linux/task_io_accounting_ops.h>
 18 #include <linux/pagevec.h>
 19 #include <linux/pagemap.h>
 20 #include <linux/syscalls.h>
 21 #include <linux/file.h>
 22 #include <linux/mm_inline.h>
 23 #include <linux/blk-cgroup.h>
 24 #include <linux/fadvise.h>
 25 
 26 #include "internal.h"
 27 
 28 /*
 29  * Initialise a struct file's readahead state.  Assumes that the caller has
 30  * memset *ra to zero.
 31  */
 32 void
 33 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
 34 {
 35         ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
 36         ra->prev_pos = -1;
 37 }
 38 EXPORT_SYMBOL_GPL(file_ra_state_init);
 39 
 40 /*
 41  * see if a page needs releasing upon read_cache_pages() failure
 42  * - the caller of read_cache_pages() may have set PG_private or PG_fscache
 43  *   before calling, such as the NFS fs marking pages that are cached locally
 44  *   on disk, thus we need to give the fs a chance to clean up in the event of
 45  *   an error
 46  */
 47 static void read_cache_pages_invalidate_page(struct address_space *mapping,
 48                                              struct page *page)
 49 {
 50         if (page_has_private(page)) {
 51                 if (!trylock_page(page))
 52                         BUG();
 53                 page->mapping = mapping;
 54                 do_invalidatepage(page, 0, PAGE_SIZE);
 55                 page->mapping = NULL;
 56                 unlock_page(page);
 57         }
 58         put_page(page);
 59 }
 60 
 61 /*
 62  * release a list of pages, invalidating them first if need be
 63  */
 64 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
 65                                               struct list_head *pages)
 66 {
 67         struct page *victim;
 68 
 69         while (!list_empty(pages)) {
 70                 victim = lru_to_page(pages);
 71                 list_del(&victim->lru);
 72                 read_cache_pages_invalidate_page(mapping, victim);
 73         }
 74 }
 75 
 76 /**
 77  * read_cache_pages - populate an address space with some pages & start reads against them
 78  * @mapping: the address_space
 79  * @pages: The address of a list_head which contains the target pages.  These
 80  *   pages have their ->index populated and are otherwise uninitialised.
 81  * @filler: callback routine for filling a single page.
 82  * @data: private data for the callback routine.
 83  *
 84  * Hides the details of the LRU cache etc from the filesystems.
 85  *
 86  * Returns: %0 on success, error return by @filler otherwise
 87  */
 88 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
 89                         int (*filler)(void *, struct page *), void *data)
 90 {
 91         struct page *page;
 92         int ret = 0;
 93 
 94         while (!list_empty(pages)) {
 95                 page = lru_to_page(pages);
 96                 list_del(&page->lru);
 97                 if (add_to_page_cache_lru(page, mapping, page->index,
 98                                 readahead_gfp_mask(mapping))) {
 99                         read_cache_pages_invalidate_page(mapping, page);
100                         continue;
101                 }
102                 put_page(page);
103 
104                 ret = filler(data, page);
105                 if (unlikely(ret)) {
106                         read_cache_pages_invalidate_pages(mapping, pages);
107                         break;
108                 }
109                 task_io_account_read(PAGE_SIZE);
110         }
111         return ret;
112 }
113 
114 EXPORT_SYMBOL(read_cache_pages);
115 
116 static int read_pages(struct address_space *mapping, struct file *filp,
117                 struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
118 {
119         struct blk_plug plug;
120         unsigned page_idx;
121         int ret;
122 
123         blk_start_plug(&plug);
124 
125         if (mapping->a_ops->readpages) {
126                 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
127                 /* Clean up the remaining pages */
128                 put_pages_list(pages);
129                 goto out;
130         }
131 
132         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
133                 struct page *page = lru_to_page(pages);
134                 list_del(&page->lru);
135                 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
136                         mapping->a_ops->readpage(filp, page);
137                 put_page(page);
138         }
139         ret = 0;
140 
141 out:
142         blk_finish_plug(&plug);
143 
144         return ret;
145 }
146 
147 /*
148  * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates
149  * the pages first, then submits them for I/O. This avoids the very bad
150  * behaviour which would occur if page allocations are causing VM writeback.
151  * We really don't want to intermingle reads and writes like that.
152  *
153  * Returns the number of pages requested, or the maximum amount of I/O allowed.
154  */
155 unsigned int __do_page_cache_readahead(struct address_space *mapping,
156                 struct file *filp, pgoff_t offset, unsigned long nr_to_read,
157                 unsigned long lookahead_size)
158 {
159         struct inode *inode = mapping->host;
160         struct page *page;
161         unsigned long end_index;        /* The last page we want to read */
162         LIST_HEAD(page_pool);
163         int page_idx;
164         unsigned int nr_pages = 0;
165         loff_t isize = i_size_read(inode);
166         gfp_t gfp_mask = readahead_gfp_mask(mapping);
167 
168         if (isize == 0)
169                 goto out;
170 
171         end_index = ((isize - 1) >> PAGE_SHIFT);
172 
173         /*
174          * Preallocate as many pages as we will need.
175          */
176         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
177                 pgoff_t page_offset = offset + page_idx;
178 
179                 if (page_offset > end_index)
180                         break;
181 
182                 page = xa_load(&mapping->i_pages, page_offset);
183                 if (page && !xa_is_value(page)) {
184                         /*
185                          * Page already present?  Kick off the current batch of
186                          * contiguous pages before continuing with the next
187                          * batch.
188                          */
189                         if (nr_pages)
190                                 read_pages(mapping, filp, &page_pool, nr_pages,
191                                                 gfp_mask);
192                         nr_pages = 0;
193                         continue;
194                 }
195 
196                 page = __page_cache_alloc(gfp_mask);
197                 if (!page)
198                         break;
199                 page->index = page_offset;
200                 list_add(&page->lru, &page_pool);
201                 if (page_idx == nr_to_read - lookahead_size)
202                         SetPageReadahead(page);
203                 nr_pages++;
204         }
205 
206         /*
207          * Now start the IO.  We ignore I/O errors - if the page is not
208          * uptodate then the caller will launch readpage again, and
209          * will then handle the error.
210          */
211         if (nr_pages)
212                 read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
213         BUG_ON(!list_empty(&page_pool));
214 out:
215         return nr_pages;
216 }
217 
218 /*
219  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
220  * memory at once.
221  */
222 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
223                                pgoff_t offset, unsigned long nr_to_read)
224 {
225         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
226         struct file_ra_state *ra = &filp->f_ra;
227         unsigned long max_pages;
228 
229         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
230                 return -EINVAL;
231 
232         /*
233          * If the request exceeds the readahead window, allow the read to
234          * be up to the optimal hardware IO size
235          */
236         max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
237         nr_to_read = min(nr_to_read, max_pages);
238         while (nr_to_read) {
239                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
240 
241                 if (this_chunk > nr_to_read)
242                         this_chunk = nr_to_read;
243                 __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
244 
245                 offset += this_chunk;
246                 nr_to_read -= this_chunk;
247         }
248         return 0;
249 }
250 
251 /*
252  * Set the initial window size, round to next power of 2 and square
253  * for small size, x 4 for medium, and x 2 for large
254  * for 128k (32 page) max ra
255  * 1-8 page = 32k initial, > 8 page = 128k initial
256  */
257 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
258 {
259         unsigned long newsize = roundup_pow_of_two(size);
260 
261         if (newsize <= max / 32)
262                 newsize = newsize * 4;
263         else if (newsize <= max / 4)
264                 newsize = newsize * 2;
265         else
266                 newsize = max;
267 
268         return newsize;
269 }
270 
271 /*
272  *  Get the previous window size, ramp it up, and
273  *  return it as the new window size.
274  */
275 static unsigned long get_next_ra_size(struct file_ra_state *ra,
276                                       unsigned long max)
277 {
278         unsigned long cur = ra->size;
279 
280         if (cur < max / 16)
281                 return 4 * cur;
282         if (cur <= max / 2)
283                 return 2 * cur;
284         return max;
285 }
286 
287 /*
288  * On-demand readahead design.
289  *
290  * The fields in struct file_ra_state represent the most-recently-executed
291  * readahead attempt:
292  *
293  *                        |<----- async_size ---------|
294  *     |------------------- size -------------------->|
295  *     |==================#===========================|
296  *     ^start             ^page marked with PG_readahead
297  *
298  * To overlap application thinking time and disk I/O time, we do
299  * `readahead pipelining': Do not wait until the application consumed all
300  * readahead pages and stalled on the missing page at readahead_index;
301  * Instead, submit an asynchronous readahead I/O as soon as there are
302  * only async_size pages left in the readahead window. Normally async_size
303  * will be equal to size, for maximum pipelining.
304  *
305  * In interleaved sequential reads, concurrent streams on the same fd can
306  * be invalidating each other's readahead state. So we flag the new readahead
307  * page at (start+size-async_size) with PG_readahead, and use it as readahead
308  * indicator. The flag won't be set on already cached pages, to avoid the
309  * readahead-for-nothing fuss, saving pointless page cache lookups.
310  *
311  * prev_pos tracks the last visited byte in the _previous_ read request.
312  * It should be maintained by the caller, and will be used for detecting
313  * small random reads. Note that the readahead algorithm checks loosely
314  * for sequential patterns. Hence interleaved reads might be served as
315  * sequential ones.
316  *
317  * There is a special-case: if the first page which the application tries to
318  * read happens to be the first page of the file, it is assumed that a linear
319  * read is about to happen and the window is immediately set to the initial size
320  * based on I/O request size and the max_readahead.
321  *
322  * The code ramps up the readahead size aggressively at first, but slow down as
323  * it approaches max_readhead.
324  */
325 
326 /*
327  * Count contiguously cached pages from @offset-1 to @offset-@max,
328  * this count is a conservative estimation of
329  *      - length of the sequential read sequence, or
330  *      - thrashing threshold in memory tight systems
331  */
332 static pgoff_t count_history_pages(struct address_space *mapping,
333                                    pgoff_t offset, unsigned long max)
334 {
335         pgoff_t head;
336 
337         rcu_read_lock();
338         head = page_cache_prev_miss(mapping, offset - 1, max);
339         rcu_read_unlock();
340 
341         return offset - 1 - head;
342 }
343 
344 /*
345  * page cache context based read-ahead
346  */
347 static int try_context_readahead(struct address_space *mapping,
348                                  struct file_ra_state *ra,
349                                  pgoff_t offset,
350                                  unsigned long req_size,
351                                  unsigned long max)
352 {
353         pgoff_t size;
354 
355         size = count_history_pages(mapping, offset, max);
356 
357         /*
358          * not enough history pages:
359          * it could be a random read
360          */
361         if (size <= req_size)
362                 return 0;
363 
364         /*
365          * starts from beginning of file:
366          * it is a strong indication of long-run stream (or whole-file-read)
367          */
368         if (size >= offset)
369                 size *= 2;
370 
371         ra->start = offset;
372         ra->size = min(size + req_size, max);
373         ra->async_size = 1;
374 
375         return 1;
376 }
377 
378 /*
379  * A minimal readahead algorithm for trivial sequential/random reads.
380  */
381 static unsigned long
382 ondemand_readahead(struct address_space *mapping,
383                    struct file_ra_state *ra, struct file *filp,
384                    bool hit_readahead_marker, pgoff_t offset,
385                    unsigned long req_size)
386 {
387         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
388         unsigned long max_pages = ra->ra_pages;
389         unsigned long add_pages;
390         pgoff_t prev_offset;
391 
392         /*
393          * If the request exceeds the readahead window, allow the read to
394          * be up to the optimal hardware IO size
395          */
396         if (req_size > max_pages && bdi->io_pages > max_pages)
397                 max_pages = min(req_size, bdi->io_pages);
398 
399         /*
400          * start of file
401          */
402         if (!offset)
403                 goto initial_readahead;
404 
405         /*
406          * It's the expected callback offset, assume sequential access.
407          * Ramp up sizes, and push forward the readahead window.
408          */
409         if ((offset == (ra->start + ra->size - ra->async_size) ||
410              offset == (ra->start + ra->size))) {
411                 ra->start += ra->size;
412                 ra->size = get_next_ra_size(ra, max_pages);
413                 ra->async_size = ra->size;
414                 goto readit;
415         }
416 
417         /*
418          * Hit a marked page without valid readahead state.
419          * E.g. interleaved reads.
420          * Query the pagecache for async_size, which normally equals to
421          * readahead size. Ramp it up and use it as the new readahead size.
422          */
423         if (hit_readahead_marker) {
424                 pgoff_t start;
425 
426                 rcu_read_lock();
427                 start = page_cache_next_miss(mapping, offset + 1, max_pages);
428                 rcu_read_unlock();
429 
430                 if (!start || start - offset > max_pages)
431                         return 0;
432 
433                 ra->start = start;
434                 ra->size = start - offset;      /* old async_size */
435                 ra->size += req_size;
436                 ra->size = get_next_ra_size(ra, max_pages);
437                 ra->async_size = ra->size;
438                 goto readit;
439         }
440 
441         /*
442          * oversize read
443          */
444         if (req_size > max_pages)
445                 goto initial_readahead;
446 
447         /*
448          * sequential cache miss
449          * trivial case: (offset - prev_offset) == 1
450          * unaligned reads: (offset - prev_offset) == 0
451          */
452         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
453         if (offset - prev_offset <= 1UL)
454                 goto initial_readahead;
455 
456         /*
457          * Query the page cache and look for the traces(cached history pages)
458          * that a sequential stream would leave behind.
459          */
460         if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
461                 goto readit;
462 
463         /*
464          * standalone, small random read
465          * Read as is, and do not pollute the readahead state.
466          */
467         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
468 
469 initial_readahead:
470         ra->start = offset;
471         ra->size = get_init_ra_size(req_size, max_pages);
472         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
473 
474 readit:
475         /*
476          * Will this read hit the readahead marker made by itself?
477          * If so, trigger the readahead marker hit now, and merge
478          * the resulted next readahead window into the current one.
479          * Take care of maximum IO pages as above.
480          */
481         if (offset == ra->start && ra->size == ra->async_size) {
482                 add_pages = get_next_ra_size(ra, max_pages);
483                 if (ra->size + add_pages <= max_pages) {
484                         ra->async_size = add_pages;
485                         ra->size += add_pages;
486                 } else {
487                         ra->size = max_pages;
488                         ra->async_size = max_pages >> 1;
489                 }
490         }
491 
492         return ra_submit(ra, mapping, filp);
493 }
494 
495 /**
496  * page_cache_sync_readahead - generic file readahead
497  * @mapping: address_space which holds the pagecache and I/O vectors
498  * @ra: file_ra_state which holds the readahead state
499  * @filp: passed on to ->readpage() and ->readpages()
500  * @offset: start offset into @mapping, in pagecache page-sized units
501  * @req_size: hint: total size of the read which the caller is performing in
502  *            pagecache pages
503  *
504  * page_cache_sync_readahead() should be called when a cache miss happened:
505  * it will submit the read.  The readahead logic may decide to piggyback more
506  * pages onto the read request if access patterns suggest it will improve
507  * performance.
508  */
509 void page_cache_sync_readahead(struct address_space *mapping,
510                                struct file_ra_state *ra, struct file *filp,
511                                pgoff_t offset, unsigned long req_size)
512 {
513         /* no read-ahead */
514         if (!ra->ra_pages)
515                 return;
516 
517         if (blk_cgroup_congested())
518                 return;
519 
520         /* be dumb */
521         if (filp && (filp->f_mode & FMODE_RANDOM)) {
522                 force_page_cache_readahead(mapping, filp, offset, req_size);
523                 return;
524         }
525 
526         /* do read-ahead */
527         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
528 }
529 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
530 
531 /**
532  * page_cache_async_readahead - file readahead for marked pages
533  * @mapping: address_space which holds the pagecache and I/O vectors
534  * @ra: file_ra_state which holds the readahead state
535  * @filp: passed on to ->readpage() and ->readpages()
536  * @page: the page at @offset which has the PG_readahead flag set
537  * @offset: start offset into @mapping, in pagecache page-sized units
538  * @req_size: hint: total size of the read which the caller is performing in
539  *            pagecache pages
540  *
541  * page_cache_async_readahead() should be called when a page is used which
542  * has the PG_readahead flag; this is a marker to suggest that the application
543  * has used up enough of the readahead window that we should start pulling in
544  * more pages.
545  */
546 void
547 page_cache_async_readahead(struct address_space *mapping,
548                            struct file_ra_state *ra, struct file *filp,
549                            struct page *page, pgoff_t offset,
550                            unsigned long req_size)
551 {
552         /* no read-ahead */
553         if (!ra->ra_pages)
554                 return;
555 
556         /*
557          * Same bit is used for PG_readahead and PG_reclaim.
558          */
559         if (PageWriteback(page))
560                 return;
561 
562         ClearPageReadahead(page);
563 
564         /*
565          * Defer asynchronous read-ahead on IO congestion.
566          */
567         if (inode_read_congested(mapping->host))
568                 return;
569 
570         if (blk_cgroup_congested())
571                 return;
572 
573         /* do read-ahead */
574         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
575 }
576 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
577 
578 ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
579 {
580         ssize_t ret;
581         struct fd f;
582 
583         ret = -EBADF;
584         f = fdget(fd);
585         if (!f.file || !(f.file->f_mode & FMODE_READ))
586                 goto out;
587 
588         /*
589          * The readahead() syscall is intended to run only on files
590          * that can execute readahead. If readahead is not possible
591          * on this file, then we must return -EINVAL.
592          */
593         ret = -EINVAL;
594         if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
595             !S_ISREG(file_inode(f.file)->i_mode))
596                 goto out;
597 
598         ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
599 out:
600         fdput(f);
601         return ret;
602 }
603 
604 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
605 {
606         return ksys_readahead(fd, offset, count);
607 }
608 

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