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/**
12 * DOC: Readahead Overview
13 *
14 * Readahead is used to read content into the page cache before it is
15 * explicitly requested by the application. Readahead only ever
16 * attempts to read folios that are not yet in the page cache. If a
17 * folio is present but not up-to-date, readahead will not try to read
18 * it. In that case a simple ->read_folio() will be requested.
19 *
20 * Readahead is triggered when an application read request (whether a
21 * system call or a page fault) finds that the requested folio is not in
22 * the page cache, or that it is in the page cache and has the
23 * readahead flag set. This flag indicates that the folio was read
24 * as part of a previous readahead request and now that it has been
25 * accessed, it is time for the next readahead.
26 *
27 * Each readahead request is partly synchronous read, and partly async
28 * readahead. This is reflected in the struct file_ra_state which
29 * contains ->size being the total number of pages, and ->async_size
30 * which is the number of pages in the async section. The readahead
31 * flag will be set on the first folio in this async section to trigger
32 * a subsequent readahead. Once a series of sequential reads has been
33 * established, there should be no need for a synchronous component and
34 * all readahead request will be fully asynchronous.
35 *
36 * When either of the triggers causes a readahead, three numbers need
37 * to be determined: the start of the region to read, the size of the
38 * region, and the size of the async tail.
39 *
40 * The start of the region is simply the first page address at or after
41 * the accessed address, which is not currently populated in the page
42 * cache. This is found with a simple search in the page cache.
43 *
44 * The size of the async tail is determined by subtracting the size that
45 * was explicitly requested from the determined request size, unless
46 * this would be less than zero - then zero is used. NOTE THIS
47 * CALCULATION IS WRONG WHEN THE START OF THE REGION IS NOT THE ACCESSED
48 * PAGE. ALSO THIS CALCULATION IS NOT USED CONSISTENTLY.
49 *
50 * The size of the region is normally determined from the size of the
51 * previous readahead which loaded the preceding pages. This may be
52 * discovered from the struct file_ra_state for simple sequential reads,
53 * or from examining the state of the page cache when multiple
54 * sequential reads are interleaved. Specifically: where the readahead
55 * was triggered by the readahead flag, the size of the previous
56 * readahead is assumed to be the number of pages from the triggering
57 * page to the start of the new readahead. In these cases, the size of
58 * the previous readahead is scaled, often doubled, for the new
59 * readahead, though see get_next_ra_size() for details.
60 *
61 * If the size of the previous read cannot be determined, the number of
62 * preceding pages in the page cache is used to estimate the size of
63 * a previous read. This estimate could easily be misled by random
64 * reads being coincidentally adjacent, so it is ignored unless it is
65 * larger than the current request, and it is not scaled up, unless it
66 * is at the start of file.
67 *
68 * In general readahead is accelerated at the start of the file, as
69 * reads from there are often sequential. There are other minor
70 * adjustments to the readahead size in various special cases and these
71 * are best discovered by reading the code.
72 *
73 * The above calculation, based on the previous readahead size,
74 * determines the size of the readahead, to which any requested read
75 * size may be added.
76 *
77 * Readahead requests are sent to the filesystem using the ->readahead()
78 * address space operation, for which mpage_readahead() is a canonical
79 * implementation. ->readahead() should normally initiate reads on all
80 * folios, but may fail to read any or all folios without causing an I/O
81 * error. The page cache reading code will issue a ->read_folio() request
82 * for any folio which ->readahead() did not read, and only an error
83 * from this will be final.
84 *
85 * ->readahead() will generally call readahead_folio() repeatedly to get
86 * each folio from those prepared for readahead. It may fail to read a
87 * folio by:
88 *
89 * * not calling readahead_folio() sufficiently many times, effectively
90 * ignoring some folios, as might be appropriate if the path to
91 * storage is congested.
92 *
93 * * failing to actually submit a read request for a given folio,
94 * possibly due to insufficient resources, or
95 *
96 * * getting an error during subsequent processing of a request.
97 *
98 * In the last two cases, the folio should be unlocked by the filesystem
99 * to indicate that the read attempt has failed. In the first case the
100 * folio will be unlocked by the VFS.
101 *
102 * Those folios not in the final ``async_size`` of the request should be
103 * considered to be important and ->readahead() should not fail them due
104 * to congestion or temporary resource unavailability, but should wait
105 * for necessary resources (e.g. memory or indexing information) to
106 * become available. Folios in the final ``async_size`` may be
107 * considered less urgent and failure to read them is more acceptable.
108 * In this case it is best to use filemap_remove_folio() to remove the
109 * folios from the page cache as is automatically done for folios that
110 * were not fetched with readahead_folio(). This will allow a
111 * subsequent synchronous readahead request to try them again. If they
112 * are left in the page cache, then they will be read individually using
113 * ->read_folio() which may be less efficient.
114 */
115
116#include <linux/blkdev.h>
117#include <linux/kernel.h>
118#include <linux/dax.h>
119#include <linux/gfp.h>
120#include <linux/export.h>
121#include <linux/backing-dev.h>
122#include <linux/task_io_accounting_ops.h>
123#include <linux/pagemap.h>
124#include <linux/psi.h>
125#include <linux/syscalls.h>
126#include <linux/file.h>
127#include <linux/mm_inline.h>
128#include <linux/blk-cgroup.h>
129#include <linux/fadvise.h>
130#include <linux/sched/mm.h>
131
132#include "internal.h"
133
134/*
135 * Initialise a struct file's readahead state. Assumes that the caller has
136 * memset *ra to zero.
137 */
138void
139file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
140{
141 ra->ra_pages = inode_to_bdi(inode: mapping->host)->ra_pages;
142 ra->prev_pos = -1;
143}
144EXPORT_SYMBOL_GPL(file_ra_state_init);
145
146static void read_pages(struct readahead_control *rac)
147{
148 const struct address_space_operations *aops = rac->mapping->a_ops;
149 struct folio *folio;
150 struct blk_plug plug;
151
152 if (!readahead_count(rac))
153 return;
154
155 if (unlikely(rac->_workingset))
156 psi_memstall_enter(flags: &rac->_pflags);
157 blk_start_plug(&plug);
158
159 if (aops->readahead) {
160 aops->readahead(rac);
161 /*
162 * Clean up the remaining folios. The sizes in ->ra
163 * may be used to size the next readahead, so make sure
164 * they accurately reflect what happened.
165 */
166 while ((folio = readahead_folio(ractl: rac)) != NULL) {
167 unsigned long nr = folio_nr_pages(folio);
168
169 folio_get(folio);
170 rac->ra->size -= nr;
171 if (rac->ra->async_size >= nr) {
172 rac->ra->async_size -= nr;
173 filemap_remove_folio(folio);
174 }
175 folio_unlock(folio);
176 folio_put(folio);
177 }
178 } else {
179 while ((folio = readahead_folio(ractl: rac)) != NULL)
180 aops->read_folio(rac->file, folio);
181 }
182
183 blk_finish_plug(&plug);
184 if (unlikely(rac->_workingset))
185 psi_memstall_leave(flags: &rac->_pflags);
186 rac->_workingset = false;
187
188 BUG_ON(readahead_count(rac));
189}
190
191/**
192 * page_cache_ra_unbounded - Start unchecked readahead.
193 * @ractl: Readahead control.
194 * @nr_to_read: The number of pages to read.
195 * @lookahead_size: Where to start the next readahead.
196 *
197 * This function is for filesystems to call when they want to start
198 * readahead beyond a file's stated i_size. This is almost certainly
199 * not the function you want to call. Use page_cache_async_readahead()
200 * or page_cache_sync_readahead() instead.
201 *
202 * Context: File is referenced by caller. Mutexes may be held by caller.
203 * May sleep, but will not reenter filesystem to reclaim memory.
204 */
205void page_cache_ra_unbounded(struct readahead_control *ractl,
206 unsigned long nr_to_read, unsigned long lookahead_size)
207{
208 struct address_space *mapping = ractl->mapping;
209 unsigned long index = readahead_index(rac: ractl);
210 gfp_t gfp_mask = readahead_gfp_mask(x: mapping);
211 unsigned long i;
212
213 /*
214 * Partway through the readahead operation, we will have added
215 * locked pages to the page cache, but will not yet have submitted
216 * them for I/O. Adding another page may need to allocate memory,
217 * which can trigger memory reclaim. Telling the VM we're in
218 * the middle of a filesystem operation will cause it to not
219 * touch file-backed pages, preventing a deadlock. Most (all?)
220 * filesystems already specify __GFP_NOFS in their mapping's
221 * gfp_mask, but let's be explicit here.
222 */
223 unsigned int nofs = memalloc_nofs_save();
224
225 filemap_invalidate_lock_shared(mapping);
226 /*
227 * Preallocate as many pages as we will need.
228 */
229 for (i = 0; i < nr_to_read; i++) {
230 struct folio *folio = xa_load(&mapping->i_pages, index: index + i);
231
232 if (folio && !xa_is_value(entry: folio)) {
233 /*
234 * Page already present? Kick off the current batch
235 * of contiguous pages before continuing with the
236 * next batch. This page may be the one we would
237 * have intended to mark as Readahead, but we don't
238 * have a stable reference to this page, and it's
239 * not worth getting one just for that.
240 */
241 read_pages(rac: ractl);
242 ractl->_index++;
243 i = ractl->_index + ractl->_nr_pages - index - 1;
244 continue;
245 }
246
247 folio = filemap_alloc_folio(gfp: gfp_mask, order: 0);
248 if (!folio)
249 break;
250 if (filemap_add_folio(mapping, folio, index: index + i,
251 gfp: gfp_mask) < 0) {
252 folio_put(folio);
253 read_pages(rac: ractl);
254 ractl->_index++;
255 i = ractl->_index + ractl->_nr_pages - index - 1;
256 continue;
257 }
258 if (i == nr_to_read - lookahead_size)
259 folio_set_readahead(folio);
260 ractl->_workingset |= folio_test_workingset(folio);
261 ractl->_nr_pages++;
262 }
263
264 /*
265 * Now start the IO. We ignore I/O errors - if the folio is not
266 * uptodate then the caller will launch read_folio again, and
267 * will then handle the error.
268 */
269 read_pages(rac: ractl);
270 filemap_invalidate_unlock_shared(mapping);
271 memalloc_nofs_restore(flags: nofs);
272}
273EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);
274
275/*
276 * do_page_cache_ra() actually reads a chunk of disk. It allocates
277 * the pages first, then submits them for I/O. This avoids the very bad
278 * behaviour which would occur if page allocations are causing VM writeback.
279 * We really don't want to intermingle reads and writes like that.
280 */
281static void do_page_cache_ra(struct readahead_control *ractl,
282 unsigned long nr_to_read, unsigned long lookahead_size)
283{
284 struct inode *inode = ractl->mapping->host;
285 unsigned long index = readahead_index(rac: ractl);
286 loff_t isize = i_size_read(inode);
287 pgoff_t end_index; /* The last page we want to read */
288
289 if (isize == 0)
290 return;
291
292 end_index = (isize - 1) >> PAGE_SHIFT;
293 if (index > end_index)
294 return;
295 /* Don't read past the page containing the last byte of the file */
296 if (nr_to_read > end_index - index)
297 nr_to_read = end_index - index + 1;
298
299 page_cache_ra_unbounded(ractl, nr_to_read, lookahead_size);
300}
301
302/*
303 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
304 * memory at once.
305 */
306void force_page_cache_ra(struct readahead_control *ractl,
307 unsigned long nr_to_read)
308{
309 struct address_space *mapping = ractl->mapping;
310 struct file_ra_state *ra = ractl->ra;
311 struct backing_dev_info *bdi = inode_to_bdi(inode: mapping->host);
312 unsigned long max_pages, index;
313
314 if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
315 return;
316
317 /*
318 * If the request exceeds the readahead window, allow the read to
319 * be up to the optimal hardware IO size
320 */
321 index = readahead_index(rac: ractl);
322 max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
323 nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
324 while (nr_to_read) {
325 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
326
327 if (this_chunk > nr_to_read)
328 this_chunk = nr_to_read;
329 ractl->_index = index;
330 do_page_cache_ra(ractl, nr_to_read: this_chunk, lookahead_size: 0);
331
332 index += this_chunk;
333 nr_to_read -= this_chunk;
334 }
335}
336
337/*
338 * Set the initial window size, round to next power of 2 and square
339 * for small size, x 4 for medium, and x 2 for large
340 * for 128k (32 page) max ra
341 * 1-2 page = 16k, 3-4 page 32k, 5-8 page = 64k, > 8 page = 128k initial
342 */
343static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
344{
345 unsigned long newsize = roundup_pow_of_two(size);
346
347 if (newsize <= max / 32)
348 newsize = newsize * 4;
349 else if (newsize <= max / 4)
350 newsize = newsize * 2;
351 else
352 newsize = max;
353
354 return newsize;
355}
356
357/*
358 * Get the previous window size, ramp it up, and
359 * return it as the new window size.
360 */
361static unsigned long get_next_ra_size(struct file_ra_state *ra,
362 unsigned long max)
363{
364 unsigned long cur = ra->size;
365
366 if (cur < max / 16)
367 return 4 * cur;
368 if (cur <= max / 2)
369 return 2 * cur;
370 return max;
371}
372
373/*
374 * On-demand readahead design.
375 *
376 * The fields in struct file_ra_state represent the most-recently-executed
377 * readahead attempt:
378 *
379 * |<----- async_size ---------|
380 * |------------------- size -------------------->|
381 * |==================#===========================|
382 * ^start ^page marked with PG_readahead
383 *
384 * To overlap application thinking time and disk I/O time, we do
385 * `readahead pipelining': Do not wait until the application consumed all
386 * readahead pages and stalled on the missing page at readahead_index;
387 * Instead, submit an asynchronous readahead I/O as soon as there are
388 * only async_size pages left in the readahead window. Normally async_size
389 * will be equal to size, for maximum pipelining.
390 *
391 * In interleaved sequential reads, concurrent streams on the same fd can
392 * be invalidating each other's readahead state. So we flag the new readahead
393 * page at (start+size-async_size) with PG_readahead, and use it as readahead
394 * indicator. The flag won't be set on already cached pages, to avoid the
395 * readahead-for-nothing fuss, saving pointless page cache lookups.
396 *
397 * prev_pos tracks the last visited byte in the _previous_ read request.
398 * It should be maintained by the caller, and will be used for detecting
399 * small random reads. Note that the readahead algorithm checks loosely
400 * for sequential patterns. Hence interleaved reads might be served as
401 * sequential ones.
402 *
403 * There is a special-case: if the first page which the application tries to
404 * read happens to be the first page of the file, it is assumed that a linear
405 * read is about to happen and the window is immediately set to the initial size
406 * based on I/O request size and the max_readahead.
407 *
408 * The code ramps up the readahead size aggressively at first, but slow down as
409 * it approaches max_readhead.
410 */
411
412/*
413 * Count contiguously cached pages from @index-1 to @index-@max,
414 * this count is a conservative estimation of
415 * - length of the sequential read sequence, or
416 * - thrashing threshold in memory tight systems
417 */
418static pgoff_t count_history_pages(struct address_space *mapping,
419 pgoff_t index, unsigned long max)
420{
421 pgoff_t head;
422
423 rcu_read_lock();
424 head = page_cache_prev_miss(mapping, index: index - 1, max_scan: max);
425 rcu_read_unlock();
426
427 return index - 1 - head;
428}
429
430/*
431 * page cache context based readahead
432 */
433static int try_context_readahead(struct address_space *mapping,
434 struct file_ra_state *ra,
435 pgoff_t index,
436 unsigned long req_size,
437 unsigned long max)
438{
439 pgoff_t size;
440
441 size = count_history_pages(mapping, index, max);
442
443 /*
444 * not enough history pages:
445 * it could be a random read
446 */
447 if (size <= req_size)
448 return 0;
449
450 /*
451 * starts from beginning of file:
452 * it is a strong indication of long-run stream (or whole-file-read)
453 */
454 if (size >= index)
455 size *= 2;
456
457 ra->start = index;
458 ra->size = min(size + req_size, max);
459 ra->async_size = 1;
460
461 return 1;
462}
463
464static inline int ra_alloc_folio(struct readahead_control *ractl, pgoff_t index,
465 pgoff_t mark, unsigned int order, gfp_t gfp)
466{
467 int err;
468 struct folio *folio = filemap_alloc_folio(gfp, order);
469
470 if (!folio)
471 return -ENOMEM;
472 mark = round_up(mark, 1UL << order);
473 if (index == mark)
474 folio_set_readahead(folio);
475 err = filemap_add_folio(mapping: ractl->mapping, folio, index, gfp);
476 if (err) {
477 folio_put(folio);
478 return err;
479 }
480
481 ractl->_nr_pages += 1UL << order;
482 ractl->_workingset |= folio_test_workingset(folio);
483 return 0;
484}
485
486void page_cache_ra_order(struct readahead_control *ractl,
487 struct file_ra_state *ra, unsigned int new_order)
488{
489 struct address_space *mapping = ractl->mapping;
490 pgoff_t index = readahead_index(rac: ractl);
491 pgoff_t limit = (i_size_read(inode: mapping->host) - 1) >> PAGE_SHIFT;
492 pgoff_t mark = index + ra->size - ra->async_size;
493 int err = 0;
494 gfp_t gfp = readahead_gfp_mask(x: mapping);
495
496 if (!mapping_large_folio_support(mapping) || ra->size < 4)
497 goto fallback;
498
499 limit = min(limit, index + ra->size - 1);
500
501 if (new_order < MAX_PAGECACHE_ORDER) {
502 new_order += 2;
503 if (new_order > MAX_PAGECACHE_ORDER)
504 new_order = MAX_PAGECACHE_ORDER;
505 while ((1 << new_order) > ra->size)
506 new_order--;
507 }
508
509 filemap_invalidate_lock_shared(mapping);
510 while (index <= limit) {
511 unsigned int order = new_order;
512
513 /* Align with smaller pages if needed */
514 if (index & ((1UL << order) - 1)) {
515 order = __ffs(index);
516 if (order == 1)
517 order = 0;
518 }
519 /* Don't allocate pages past EOF */
520 while (index + (1UL << order) - 1 > limit) {
521 if (--order == 1)
522 order = 0;
523 }
524 err = ra_alloc_folio(ractl, index, mark, order, gfp);
525 if (err)
526 break;
527 index += 1UL << order;
528 }
529
530 if (index > limit) {
531 ra->size += index - limit - 1;
532 ra->async_size += index - limit - 1;
533 }
534
535 read_pages(rac: ractl);
536 filemap_invalidate_unlock_shared(mapping);
537
538 /*
539 * If there were already pages in the page cache, then we may have
540 * left some gaps. Let the regular readahead code take care of this
541 * situation.
542 */
543 if (!err)
544 return;
545fallback:
546 do_page_cache_ra(ractl, nr_to_read: ra->size, lookahead_size: ra->async_size);
547}
548
549/*
550 * A minimal readahead algorithm for trivial sequential/random reads.
551 */
552static void ondemand_readahead(struct readahead_control *ractl,
553 struct folio *folio, unsigned long req_size)
554{
555 struct backing_dev_info *bdi = inode_to_bdi(inode: ractl->mapping->host);
556 struct file_ra_state *ra = ractl->ra;
557 unsigned long max_pages = ra->ra_pages;
558 unsigned long add_pages;
559 pgoff_t index = readahead_index(rac: ractl);
560 pgoff_t expected, prev_index;
561 unsigned int order = folio ? folio_order(folio) : 0;
562
563 /*
564 * If the request exceeds the readahead window, allow the read to
565 * be up to the optimal hardware IO size
566 */
567 if (req_size > max_pages && bdi->io_pages > max_pages)
568 max_pages = min(req_size, bdi->io_pages);
569
570 /*
571 * start of file
572 */
573 if (!index)
574 goto initial_readahead;
575
576 /*
577 * It's the expected callback index, assume sequential access.
578 * Ramp up sizes, and push forward the readahead window.
579 */
580 expected = round_up(ra->start + ra->size - ra->async_size,
581 1UL << order);
582 if (index == expected || index == (ra->start + ra->size)) {
583 ra->start += ra->size;
584 ra->size = get_next_ra_size(ra, max: max_pages);
585 ra->async_size = ra->size;
586 goto readit;
587 }
588
589 /*
590 * Hit a marked folio without valid readahead state.
591 * E.g. interleaved reads.
592 * Query the pagecache for async_size, which normally equals to
593 * readahead size. Ramp it up and use it as the new readahead size.
594 */
595 if (folio) {
596 pgoff_t start;
597
598 rcu_read_lock();
599 start = page_cache_next_miss(mapping: ractl->mapping, index: index + 1,
600 max_scan: max_pages);
601 rcu_read_unlock();
602
603 if (!start || start - index > max_pages)
604 return;
605
606 ra->start = start;
607 ra->size = start - index; /* old async_size */
608 ra->size += req_size;
609 ra->size = get_next_ra_size(ra, max: max_pages);
610 ra->async_size = ra->size;
611 goto readit;
612 }
613
614 /*
615 * oversize read
616 */
617 if (req_size > max_pages)
618 goto initial_readahead;
619
620 /*
621 * sequential cache miss
622 * trivial case: (index - prev_index) == 1
623 * unaligned reads: (index - prev_index) == 0
624 */
625 prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
626 if (index - prev_index <= 1UL)
627 goto initial_readahead;
628
629 /*
630 * Query the page cache and look for the traces(cached history pages)
631 * that a sequential stream would leave behind.
632 */
633 if (try_context_readahead(mapping: ractl->mapping, ra, index, req_size,
634 max: max_pages))
635 goto readit;
636
637 /*
638 * standalone, small random read
639 * Read as is, and do not pollute the readahead state.
640 */
641 do_page_cache_ra(ractl, nr_to_read: req_size, lookahead_size: 0);
642 return;
643
644initial_readahead:
645 ra->start = index;
646 ra->size = get_init_ra_size(size: req_size, max: max_pages);
647 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
648
649readit:
650 /*
651 * Will this read hit the readahead marker made by itself?
652 * If so, trigger the readahead marker hit now, and merge
653 * the resulted next readahead window into the current one.
654 * Take care of maximum IO pages as above.
655 */
656 if (index == ra->start && ra->size == ra->async_size) {
657 add_pages = get_next_ra_size(ra, max: max_pages);
658 if (ra->size + add_pages <= max_pages) {
659 ra->async_size = add_pages;
660 ra->size += add_pages;
661 } else {
662 ra->size = max_pages;
663 ra->async_size = max_pages >> 1;
664 }
665 }
666
667 ractl->_index = ra->start;
668 page_cache_ra_order(ractl, ra, new_order: order);
669}
670
671void page_cache_sync_ra(struct readahead_control *ractl,
672 unsigned long req_count)
673{
674 bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
675
676 /*
677 * Even if readahead is disabled, issue this request as readahead
678 * as we'll need it to satisfy the requested range. The forced
679 * readahead will do the right thing and limit the read to just the
680 * requested range, which we'll set to 1 page for this case.
681 */
682 if (!ractl->ra->ra_pages || blk_cgroup_congested()) {
683 if (!ractl->file)
684 return;
685 req_count = 1;
686 do_forced_ra = true;
687 }
688
689 /* be dumb */
690 if (do_forced_ra) {
691 force_page_cache_ra(ractl, nr_to_read: req_count);
692 return;
693 }
694
695 ondemand_readahead(ractl, NULL, req_size: req_count);
696}
697EXPORT_SYMBOL_GPL(page_cache_sync_ra);
698
699void page_cache_async_ra(struct readahead_control *ractl,
700 struct folio *folio, unsigned long req_count)
701{
702 /* no readahead */
703 if (!ractl->ra->ra_pages)
704 return;
705
706 /*
707 * Same bit is used for PG_readahead and PG_reclaim.
708 */
709 if (folio_test_writeback(folio))
710 return;
711
712 folio_clear_readahead(folio);
713
714 if (blk_cgroup_congested())
715 return;
716
717 ondemand_readahead(ractl, folio, req_size: req_count);
718}
719EXPORT_SYMBOL_GPL(page_cache_async_ra);
720
721ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
722{
723 ssize_t ret;
724 struct fd f;
725
726 ret = -EBADF;
727 f = fdget(fd);
728 if (!f.file || !(f.file->f_mode & FMODE_READ))
729 goto out;
730
731 /*
732 * The readahead() syscall is intended to run only on files
733 * that can execute readahead. If readahead is not possible
734 * on this file, then we must return -EINVAL.
735 */
736 ret = -EINVAL;
737 if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
738 (!S_ISREG(file_inode(f.file)->i_mode) &&
739 !S_ISBLK(file_inode(f.file)->i_mode)))
740 goto out;
741
742 ret = vfs_fadvise(file: f.file, offset, len: count, POSIX_FADV_WILLNEED);
743out:
744 fdput(fd: f);
745 return ret;
746}
747
748SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
749{
750 return ksys_readahead(fd, offset, count);
751}
752
753#if defined(CONFIG_COMPAT) && defined(__ARCH_WANT_COMPAT_READAHEAD)
754COMPAT_SYSCALL_DEFINE4(readahead, int, fd, compat_arg_u64_dual(offset), size_t, count)
755{
756 return ksys_readahead(fd, compat_arg_u64_glue(offset), count);
757}
758#endif
759
760/**
761 * readahead_expand - Expand a readahead request
762 * @ractl: The request to be expanded
763 * @new_start: The revised start
764 * @new_len: The revised size of the request
765 *
766 * Attempt to expand a readahead request outwards from the current size to the
767 * specified size by inserting locked pages before and after the current window
768 * to increase the size to the new window. This may involve the insertion of
769 * THPs, in which case the window may get expanded even beyond what was
770 * requested.
771 *
772 * The algorithm will stop if it encounters a conflicting page already in the
773 * pagecache and leave a smaller expansion than requested.
774 *
775 * The caller must check for this by examining the revised @ractl object for a
776 * different expansion than was requested.
777 */
778void readahead_expand(struct readahead_control *ractl,
779 loff_t new_start, size_t new_len)
780{
781 struct address_space *mapping = ractl->mapping;
782 struct file_ra_state *ra = ractl->ra;
783 pgoff_t new_index, new_nr_pages;
784 gfp_t gfp_mask = readahead_gfp_mask(x: mapping);
785
786 new_index = new_start / PAGE_SIZE;
787
788 /* Expand the leading edge downwards */
789 while (ractl->_index > new_index) {
790 unsigned long index = ractl->_index - 1;
791 struct folio *folio = xa_load(&mapping->i_pages, index);
792
793 if (folio && !xa_is_value(entry: folio))
794 return; /* Folio apparently present */
795
796 folio = filemap_alloc_folio(gfp: gfp_mask, order: 0);
797 if (!folio)
798 return;
799 if (filemap_add_folio(mapping, folio, index, gfp: gfp_mask) < 0) {
800 folio_put(folio);
801 return;
802 }
803 if (unlikely(folio_test_workingset(folio)) &&
804 !ractl->_workingset) {
805 ractl->_workingset = true;
806 psi_memstall_enter(flags: &ractl->_pflags);
807 }
808 ractl->_nr_pages++;
809 ractl->_index = folio->index;
810 }
811
812 new_len += new_start - readahead_pos(rac: ractl);
813 new_nr_pages = DIV_ROUND_UP(new_len, PAGE_SIZE);
814
815 /* Expand the trailing edge upwards */
816 while (ractl->_nr_pages < new_nr_pages) {
817 unsigned long index = ractl->_index + ractl->_nr_pages;
818 struct folio *folio = xa_load(&mapping->i_pages, index);
819
820 if (folio && !xa_is_value(entry: folio))
821 return; /* Folio apparently present */
822
823 folio = filemap_alloc_folio(gfp: gfp_mask, order: 0);
824 if (!folio)
825 return;
826 if (filemap_add_folio(mapping, folio, index, gfp: gfp_mask) < 0) {
827 folio_put(folio);
828 return;
829 }
830 if (unlikely(folio_test_workingset(folio)) &&
831 !ractl->_workingset) {
832 ractl->_workingset = true;
833 psi_memstall_enter(flags: &ractl->_pflags);
834 }
835 ractl->_nr_pages++;
836 if (ra) {
837 ra->size++;
838 ra->async_size++;
839 }
840 }
841}
842EXPORT_SYMBOL(readahead_expand);
843

source code of linux/mm/readahead.c