Line data Source code
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/pagevec.h>
124 : #include <linux/pagemap.h>
125 : #include <linux/psi.h>
126 : #include <linux/syscalls.h>
127 : #include <linux/file.h>
128 : #include <linux/mm_inline.h>
129 : #include <linux/blk-cgroup.h>
130 : #include <linux/fadvise.h>
131 : #include <linux/sched/mm.h>
132 :
133 : #include "internal.h"
134 :
135 : /*
136 : * Initialise a struct file's readahead state. Assumes that the caller has
137 : * memset *ra to zero.
138 : */
139 : void
140 0 : file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
141 : {
142 0 : ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
143 0 : ra->prev_pos = -1;
144 0 : }
145 : EXPORT_SYMBOL_GPL(file_ra_state_init);
146 :
147 0 : static void read_pages(struct readahead_control *rac)
148 : {
149 0 : const struct address_space_operations *aops = rac->mapping->a_ops;
150 : struct folio *folio;
151 : struct blk_plug plug;
152 :
153 0 : if (!readahead_count(rac))
154 0 : return;
155 :
156 : if (unlikely(rac->_workingset))
157 : psi_memstall_enter(&rac->_pflags);
158 0 : blk_start_plug(&plug);
159 :
160 0 : if (aops->readahead) {
161 0 : aops->readahead(rac);
162 : /*
163 : * Clean up the remaining folios. The sizes in ->ra
164 : * may be used to size the next readahead, so make sure
165 : * they accurately reflect what happened.
166 : */
167 0 : while ((folio = readahead_folio(rac)) != NULL) {
168 0 : unsigned long nr = folio_nr_pages(folio);
169 :
170 0 : folio_get(folio);
171 0 : rac->ra->size -= nr;
172 0 : if (rac->ra->async_size >= nr) {
173 0 : rac->ra->async_size -= nr;
174 0 : filemap_remove_folio(folio);
175 : }
176 0 : folio_unlock(folio);
177 : folio_put(folio);
178 : }
179 : } else {
180 0 : while ((folio = readahead_folio(rac)) != NULL)
181 0 : aops->read_folio(rac->file, folio);
182 : }
183 :
184 0 : blk_finish_plug(&plug);
185 : if (unlikely(rac->_workingset))
186 : psi_memstall_leave(&rac->_pflags);
187 0 : rac->_workingset = false;
188 :
189 0 : BUG_ON(readahead_count(rac));
190 : }
191 :
192 : /**
193 : * page_cache_ra_unbounded - Start unchecked readahead.
194 : * @ractl: Readahead control.
195 : * @nr_to_read: The number of pages to read.
196 : * @lookahead_size: Where to start the next readahead.
197 : *
198 : * This function is for filesystems to call when they want to start
199 : * readahead beyond a file's stated i_size. This is almost certainly
200 : * not the function you want to call. Use page_cache_async_readahead()
201 : * or page_cache_sync_readahead() instead.
202 : *
203 : * Context: File is referenced by caller. Mutexes may be held by caller.
204 : * May sleep, but will not reenter filesystem to reclaim memory.
205 : */
206 0 : void page_cache_ra_unbounded(struct readahead_control *ractl,
207 : unsigned long nr_to_read, unsigned long lookahead_size)
208 : {
209 0 : struct address_space *mapping = ractl->mapping;
210 0 : unsigned long index = readahead_index(ractl);
211 0 : gfp_t gfp_mask = readahead_gfp_mask(mapping);
212 : unsigned long i;
213 :
214 : /*
215 : * Partway through the readahead operation, we will have added
216 : * locked pages to the page cache, but will not yet have submitted
217 : * them for I/O. Adding another page may need to allocate memory,
218 : * which can trigger memory reclaim. Telling the VM we're in
219 : * the middle of a filesystem operation will cause it to not
220 : * touch file-backed pages, preventing a deadlock. Most (all?)
221 : * filesystems already specify __GFP_NOFS in their mapping's
222 : * gfp_mask, but let's be explicit here.
223 : */
224 0 : unsigned int nofs = memalloc_nofs_save();
225 :
226 0 : filemap_invalidate_lock_shared(mapping);
227 : /*
228 : * Preallocate as many pages as we will need.
229 : */
230 0 : for (i = 0; i < nr_to_read; i++) {
231 0 : struct folio *folio = xa_load(&mapping->i_pages, index + i);
232 :
233 0 : if (folio && !xa_is_value(folio)) {
234 : /*
235 : * Page already present? Kick off the current batch
236 : * of contiguous pages before continuing with the
237 : * next batch. This page may be the one we would
238 : * have intended to mark as Readahead, but we don't
239 : * have a stable reference to this page, and it's
240 : * not worth getting one just for that.
241 : */
242 0 : read_pages(ractl);
243 0 : ractl->_index++;
244 0 : i = ractl->_index + ractl->_nr_pages - index - 1;
245 0 : continue;
246 : }
247 :
248 0 : folio = filemap_alloc_folio(gfp_mask, 0);
249 0 : if (!folio)
250 : break;
251 0 : if (filemap_add_folio(mapping, folio, index + i,
252 : gfp_mask) < 0) {
253 0 : folio_put(folio);
254 0 : read_pages(ractl);
255 0 : ractl->_index++;
256 0 : i = ractl->_index + ractl->_nr_pages - index - 1;
257 0 : continue;
258 : }
259 0 : if (i == nr_to_read - lookahead_size)
260 : folio_set_readahead(folio);
261 0 : ractl->_workingset |= folio_test_workingset(folio);
262 0 : ractl->_nr_pages++;
263 : }
264 :
265 : /*
266 : * Now start the IO. We ignore I/O errors - if the folio is not
267 : * uptodate then the caller will launch read_folio again, and
268 : * will then handle the error.
269 : */
270 0 : read_pages(ractl);
271 0 : filemap_invalidate_unlock_shared(mapping);
272 0 : memalloc_nofs_restore(nofs);
273 0 : }
274 : EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);
275 :
276 : /*
277 : * do_page_cache_ra() actually reads a chunk of disk. It allocates
278 : * the pages first, then submits them for I/O. This avoids the very bad
279 : * behaviour which would occur if page allocations are causing VM writeback.
280 : * We really don't want to intermingle reads and writes like that.
281 : */
282 0 : static void do_page_cache_ra(struct readahead_control *ractl,
283 : unsigned long nr_to_read, unsigned long lookahead_size)
284 : {
285 0 : struct inode *inode = ractl->mapping->host;
286 0 : unsigned long index = readahead_index(ractl);
287 0 : loff_t isize = i_size_read(inode);
288 : pgoff_t end_index; /* The last page we want to read */
289 :
290 0 : if (isize == 0)
291 : return;
292 :
293 0 : end_index = (isize - 1) >> PAGE_SHIFT;
294 0 : if (index > end_index)
295 : return;
296 : /* Don't read past the page containing the last byte of the file */
297 0 : if (nr_to_read > end_index - index)
298 0 : nr_to_read = end_index - index + 1;
299 :
300 0 : page_cache_ra_unbounded(ractl, nr_to_read, lookahead_size);
301 : }
302 :
303 : /*
304 : * Chunk the readahead into 2 megabyte units, so that we don't pin too much
305 : * memory at once.
306 : */
307 0 : void force_page_cache_ra(struct readahead_control *ractl,
308 : unsigned long nr_to_read)
309 : {
310 0 : struct address_space *mapping = ractl->mapping;
311 0 : struct file_ra_state *ra = ractl->ra;
312 0 : struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
313 : unsigned long max_pages, index;
314 :
315 0 : if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
316 : return;
317 :
318 : /*
319 : * If the request exceeds the readahead window, allow the read to
320 : * be up to the optimal hardware IO size
321 : */
322 0 : index = readahead_index(ractl);
323 0 : max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
324 0 : nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
325 0 : while (nr_to_read) {
326 0 : unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
327 :
328 0 : if (this_chunk > nr_to_read)
329 0 : this_chunk = nr_to_read;
330 0 : ractl->_index = index;
331 0 : do_page_cache_ra(ractl, this_chunk, 0);
332 :
333 0 : index += this_chunk;
334 0 : nr_to_read -= this_chunk;
335 : }
336 : }
337 :
338 : /*
339 : * Set the initial window size, round to next power of 2 and square
340 : * for small size, x 4 for medium, and x 2 for large
341 : * for 128k (32 page) max ra
342 : * 1-2 page = 16k, 3-4 page 32k, 5-8 page = 64k, > 8 page = 128k initial
343 : */
344 0 : static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
345 : {
346 0 : unsigned long newsize = roundup_pow_of_two(size);
347 :
348 0 : if (newsize <= max / 32)
349 0 : newsize = newsize * 4;
350 0 : else if (newsize <= max / 4)
351 0 : newsize = newsize * 2;
352 : else
353 : newsize = max;
354 :
355 0 : return newsize;
356 : }
357 :
358 : /*
359 : * Get the previous window size, ramp it up, and
360 : * return it as the new window size.
361 : */
362 : static unsigned long get_next_ra_size(struct file_ra_state *ra,
363 : unsigned long max)
364 : {
365 0 : unsigned long cur = ra->size;
366 :
367 0 : if (cur < max / 16)
368 0 : return 4 * cur;
369 0 : if (cur <= max / 2)
370 0 : return 2 * cur;
371 : return max;
372 : }
373 :
374 : /*
375 : * On-demand readahead design.
376 : *
377 : * The fields in struct file_ra_state represent the most-recently-executed
378 : * readahead attempt:
379 : *
380 : * |<----- async_size ---------|
381 : * |------------------- size -------------------->|
382 : * |==================#===========================|
383 : * ^start ^page marked with PG_readahead
384 : *
385 : * To overlap application thinking time and disk I/O time, we do
386 : * `readahead pipelining': Do not wait until the application consumed all
387 : * readahead pages and stalled on the missing page at readahead_index;
388 : * Instead, submit an asynchronous readahead I/O as soon as there are
389 : * only async_size pages left in the readahead window. Normally async_size
390 : * will be equal to size, for maximum pipelining.
391 : *
392 : * In interleaved sequential reads, concurrent streams on the same fd can
393 : * be invalidating each other's readahead state. So we flag the new readahead
394 : * page at (start+size-async_size) with PG_readahead, and use it as readahead
395 : * indicator. The flag won't be set on already cached pages, to avoid the
396 : * readahead-for-nothing fuss, saving pointless page cache lookups.
397 : *
398 : * prev_pos tracks the last visited byte in the _previous_ read request.
399 : * It should be maintained by the caller, and will be used for detecting
400 : * small random reads. Note that the readahead algorithm checks loosely
401 : * for sequential patterns. Hence interleaved reads might be served as
402 : * sequential ones.
403 : *
404 : * There is a special-case: if the first page which the application tries to
405 : * read happens to be the first page of the file, it is assumed that a linear
406 : * read is about to happen and the window is immediately set to the initial size
407 : * based on I/O request size and the max_readahead.
408 : *
409 : * The code ramps up the readahead size aggressively at first, but slow down as
410 : * it approaches max_readhead.
411 : */
412 :
413 : /*
414 : * Count contiguously cached pages from @index-1 to @index-@max,
415 : * this count is a conservative estimation of
416 : * - length of the sequential read sequence, or
417 : * - thrashing threshold in memory tight systems
418 : */
419 : static pgoff_t count_history_pages(struct address_space *mapping,
420 : pgoff_t index, unsigned long max)
421 : {
422 : pgoff_t head;
423 :
424 : rcu_read_lock();
425 0 : head = page_cache_prev_miss(mapping, index - 1, max);
426 : rcu_read_unlock();
427 :
428 0 : return index - 1 - head;
429 : }
430 :
431 : /*
432 : * page cache context based readahead
433 : */
434 0 : static int try_context_readahead(struct address_space *mapping,
435 : struct file_ra_state *ra,
436 : pgoff_t index,
437 : unsigned long req_size,
438 : unsigned long max)
439 : {
440 : pgoff_t size;
441 :
442 0 : size = count_history_pages(mapping, index, max);
443 :
444 : /*
445 : * not enough history pages:
446 : * it could be a random read
447 : */
448 0 : if (size <= req_size)
449 : return 0;
450 :
451 : /*
452 : * starts from beginning of file:
453 : * it is a strong indication of long-run stream (or whole-file-read)
454 : */
455 0 : if (size >= index)
456 0 : size *= 2;
457 :
458 0 : ra->start = index;
459 0 : ra->size = min(size + req_size, max);
460 0 : ra->async_size = 1;
461 :
462 0 : return 1;
463 : }
464 :
465 : /*
466 : * There are some parts of the kernel which assume that PMD entries
467 : * are exactly HPAGE_PMD_ORDER. Those should be fixed, but until then,
468 : * limit the maximum allocation order to PMD size. I'm not aware of any
469 : * assumptions about maximum order if THP are disabled, but 8 seems like
470 : * a good order (that's 1MB if you're using 4kB pages)
471 : */
472 : #ifdef CONFIG_TRANSPARENT_HUGEPAGE
473 : #define MAX_PAGECACHE_ORDER HPAGE_PMD_ORDER
474 : #else
475 : #define MAX_PAGECACHE_ORDER 8
476 : #endif
477 :
478 : static inline int ra_alloc_folio(struct readahead_control *ractl, pgoff_t index,
479 : pgoff_t mark, unsigned int order, gfp_t gfp)
480 : {
481 : int err;
482 : struct folio *folio = filemap_alloc_folio(gfp, order);
483 :
484 : if (!folio)
485 : return -ENOMEM;
486 : mark = round_up(mark, 1UL << order);
487 : if (index == mark)
488 : folio_set_readahead(folio);
489 : err = filemap_add_folio(ractl->mapping, folio, index, gfp);
490 : if (err) {
491 : folio_put(folio);
492 : return err;
493 : }
494 :
495 : ractl->_nr_pages += 1UL << order;
496 : ractl->_workingset |= folio_test_workingset(folio);
497 : return 0;
498 : }
499 :
500 0 : void page_cache_ra_order(struct readahead_control *ractl,
501 : struct file_ra_state *ra, unsigned int new_order)
502 : {
503 0 : struct address_space *mapping = ractl->mapping;
504 0 : pgoff_t index = readahead_index(ractl);
505 0 : pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
506 0 : pgoff_t mark = index + ra->size - ra->async_size;
507 0 : int err = 0;
508 0 : gfp_t gfp = readahead_gfp_mask(mapping);
509 :
510 0 : if (!mapping_large_folio_support(mapping) || ra->size < 4)
511 : goto fallback;
512 :
513 : limit = min(limit, index + ra->size - 1);
514 :
515 : if (new_order < MAX_PAGECACHE_ORDER) {
516 : new_order += 2;
517 : if (new_order > MAX_PAGECACHE_ORDER)
518 : new_order = MAX_PAGECACHE_ORDER;
519 : while ((1 << new_order) > ra->size)
520 : new_order--;
521 : }
522 :
523 : filemap_invalidate_lock_shared(mapping);
524 : while (index <= limit) {
525 : unsigned int order = new_order;
526 :
527 : /* Align with smaller pages if needed */
528 : if (index & ((1UL << order) - 1)) {
529 : order = __ffs(index);
530 : if (order == 1)
531 : order = 0;
532 : }
533 : /* Don't allocate pages past EOF */
534 : while (index + (1UL << order) - 1 > limit) {
535 : if (--order == 1)
536 : order = 0;
537 : }
538 : err = ra_alloc_folio(ractl, index, mark, order, gfp);
539 : if (err)
540 : break;
541 : index += 1UL << order;
542 : }
543 :
544 : if (index > limit) {
545 : ra->size += index - limit - 1;
546 : ra->async_size += index - limit - 1;
547 : }
548 :
549 : read_pages(ractl);
550 : filemap_invalidate_unlock_shared(mapping);
551 :
552 : /*
553 : * If there were already pages in the page cache, then we may have
554 : * left some gaps. Let the regular readahead code take care of this
555 : * situation.
556 : */
557 : if (!err)
558 : return;
559 : fallback:
560 0 : do_page_cache_ra(ractl, ra->size, ra->async_size);
561 : }
562 :
563 : /*
564 : * A minimal readahead algorithm for trivial sequential/random reads.
565 : */
566 0 : static void ondemand_readahead(struct readahead_control *ractl,
567 : struct folio *folio, unsigned long req_size)
568 : {
569 0 : struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
570 0 : struct file_ra_state *ra = ractl->ra;
571 0 : unsigned long max_pages = ra->ra_pages;
572 : unsigned long add_pages;
573 0 : pgoff_t index = readahead_index(ractl);
574 : pgoff_t expected, prev_index;
575 0 : unsigned int order = folio ? folio_order(folio) : 0;
576 :
577 : /*
578 : * If the request exceeds the readahead window, allow the read to
579 : * be up to the optimal hardware IO size
580 : */
581 0 : if (req_size > max_pages && bdi->io_pages > max_pages)
582 0 : max_pages = min(req_size, bdi->io_pages);
583 :
584 : /*
585 : * start of file
586 : */
587 0 : if (!index)
588 : goto initial_readahead;
589 :
590 : /*
591 : * It's the expected callback index, assume sequential access.
592 : * Ramp up sizes, and push forward the readahead window.
593 : */
594 0 : expected = round_up(ra->start + ra->size - ra->async_size,
595 : 1UL << order);
596 0 : if (index == expected || index == (ra->start + ra->size)) {
597 0 : ra->start += ra->size;
598 0 : ra->size = get_next_ra_size(ra, max_pages);
599 0 : ra->async_size = ra->size;
600 0 : goto readit;
601 : }
602 :
603 : /*
604 : * Hit a marked folio without valid readahead state.
605 : * E.g. interleaved reads.
606 : * Query the pagecache for async_size, which normally equals to
607 : * readahead size. Ramp it up and use it as the new readahead size.
608 : */
609 0 : if (folio) {
610 : pgoff_t start;
611 :
612 : rcu_read_lock();
613 0 : start = page_cache_next_miss(ractl->mapping, index + 1,
614 : max_pages);
615 : rcu_read_unlock();
616 :
617 0 : if (!start || start - index > max_pages)
618 : return;
619 :
620 0 : ra->start = start;
621 0 : ra->size = start - index; /* old async_size */
622 0 : ra->size += req_size;
623 0 : ra->size = get_next_ra_size(ra, max_pages);
624 0 : ra->async_size = ra->size;
625 0 : goto readit;
626 : }
627 :
628 : /*
629 : * oversize read
630 : */
631 0 : if (req_size > max_pages)
632 : goto initial_readahead;
633 :
634 : /*
635 : * sequential cache miss
636 : * trivial case: (index - prev_index) == 1
637 : * unaligned reads: (index - prev_index) == 0
638 : */
639 0 : prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
640 0 : if (index - prev_index <= 1UL)
641 : goto initial_readahead;
642 :
643 : /*
644 : * Query the page cache and look for the traces(cached history pages)
645 : * that a sequential stream would leave behind.
646 : */
647 0 : if (try_context_readahead(ractl->mapping, ra, index, req_size,
648 : max_pages))
649 : goto readit;
650 :
651 : /*
652 : * standalone, small random read
653 : * Read as is, and do not pollute the readahead state.
654 : */
655 0 : do_page_cache_ra(ractl, req_size, 0);
656 0 : return;
657 :
658 : initial_readahead:
659 0 : ra->start = index;
660 0 : ra->size = get_init_ra_size(req_size, max_pages);
661 0 : ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
662 :
663 : readit:
664 : /*
665 : * Will this read hit the readahead marker made by itself?
666 : * If so, trigger the readahead marker hit now, and merge
667 : * the resulted next readahead window into the current one.
668 : * Take care of maximum IO pages as above.
669 : */
670 0 : if (index == ra->start && ra->size == ra->async_size) {
671 0 : add_pages = get_next_ra_size(ra, max_pages);
672 0 : if (ra->size + add_pages <= max_pages) {
673 0 : ra->async_size = add_pages;
674 0 : ra->size += add_pages;
675 : } else {
676 0 : ra->size = max_pages;
677 0 : ra->async_size = max_pages >> 1;
678 : }
679 : }
680 :
681 0 : ractl->_index = ra->start;
682 : page_cache_ra_order(ractl, ra, order);
683 : }
684 :
685 0 : void page_cache_sync_ra(struct readahead_control *ractl,
686 : unsigned long req_count)
687 : {
688 0 : bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
689 :
690 : /*
691 : * Even if readahead is disabled, issue this request as readahead
692 : * as we'll need it to satisfy the requested range. The forced
693 : * readahead will do the right thing and limit the read to just the
694 : * requested range, which we'll set to 1 page for this case.
695 : */
696 0 : if (!ractl->ra->ra_pages || blk_cgroup_congested()) {
697 0 : if (!ractl->file)
698 : return;
699 : req_count = 1;
700 : do_forced_ra = true;
701 : }
702 :
703 : /* be dumb */
704 0 : if (do_forced_ra) {
705 0 : force_page_cache_ra(ractl, req_count);
706 0 : return;
707 : }
708 :
709 0 : ondemand_readahead(ractl, NULL, req_count);
710 : }
711 : EXPORT_SYMBOL_GPL(page_cache_sync_ra);
712 :
713 0 : void page_cache_async_ra(struct readahead_control *ractl,
714 : struct folio *folio, unsigned long req_count)
715 : {
716 : /* no readahead */
717 0 : if (!ractl->ra->ra_pages)
718 : return;
719 :
720 : /*
721 : * Same bit is used for PG_readahead and PG_reclaim.
722 : */
723 0 : if (folio_test_writeback(folio))
724 : return;
725 :
726 0 : folio_clear_readahead(folio);
727 :
728 : if (blk_cgroup_congested())
729 : return;
730 :
731 0 : ondemand_readahead(ractl, folio, req_count);
732 : }
733 : EXPORT_SYMBOL_GPL(page_cache_async_ra);
734 :
735 0 : ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
736 : {
737 : ssize_t ret;
738 : struct fd f;
739 :
740 0 : ret = -EBADF;
741 0 : f = fdget(fd);
742 0 : if (!f.file || !(f.file->f_mode & FMODE_READ))
743 : goto out;
744 :
745 : /*
746 : * The readahead() syscall is intended to run only on files
747 : * that can execute readahead. If readahead is not possible
748 : * on this file, then we must return -EINVAL.
749 : */
750 0 : ret = -EINVAL;
751 0 : if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
752 0 : !S_ISREG(file_inode(f.file)->i_mode))
753 : goto out;
754 :
755 0 : ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
756 : out:
757 0 : fdput(f);
758 0 : return ret;
759 : }
760 :
761 0 : SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
762 : {
763 0 : return ksys_readahead(fd, offset, count);
764 : }
765 :
766 : #if defined(CONFIG_COMPAT) && defined(__ARCH_WANT_COMPAT_READAHEAD)
767 : COMPAT_SYSCALL_DEFINE4(readahead, int, fd, compat_arg_u64_dual(offset), size_t, count)
768 : {
769 : return ksys_readahead(fd, compat_arg_u64_glue(offset), count);
770 : }
771 : #endif
772 :
773 : /**
774 : * readahead_expand - Expand a readahead request
775 : * @ractl: The request to be expanded
776 : * @new_start: The revised start
777 : * @new_len: The revised size of the request
778 : *
779 : * Attempt to expand a readahead request outwards from the current size to the
780 : * specified size by inserting locked pages before and after the current window
781 : * to increase the size to the new window. This may involve the insertion of
782 : * THPs, in which case the window may get expanded even beyond what was
783 : * requested.
784 : *
785 : * The algorithm will stop if it encounters a conflicting page already in the
786 : * pagecache and leave a smaller expansion than requested.
787 : *
788 : * The caller must check for this by examining the revised @ractl object for a
789 : * different expansion than was requested.
790 : */
791 0 : void readahead_expand(struct readahead_control *ractl,
792 : loff_t new_start, size_t new_len)
793 : {
794 0 : struct address_space *mapping = ractl->mapping;
795 0 : struct file_ra_state *ra = ractl->ra;
796 : pgoff_t new_index, new_nr_pages;
797 0 : gfp_t gfp_mask = readahead_gfp_mask(mapping);
798 :
799 0 : new_index = new_start / PAGE_SIZE;
800 :
801 : /* Expand the leading edge downwards */
802 0 : while (ractl->_index > new_index) {
803 0 : unsigned long index = ractl->_index - 1;
804 0 : struct folio *folio = xa_load(&mapping->i_pages, index);
805 :
806 0 : if (folio && !xa_is_value(folio))
807 : return; /* Folio apparently present */
808 :
809 0 : folio = filemap_alloc_folio(gfp_mask, 0);
810 0 : if (!folio)
811 : return;
812 0 : if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
813 : folio_put(folio);
814 : return;
815 : }
816 0 : if (unlikely(folio_test_workingset(folio)) &&
817 0 : !ractl->_workingset) {
818 0 : ractl->_workingset = true;
819 0 : psi_memstall_enter(&ractl->_pflags);
820 : }
821 0 : ractl->_nr_pages++;
822 0 : ractl->_index = folio->index;
823 : }
824 :
825 0 : new_len += new_start - readahead_pos(ractl);
826 0 : new_nr_pages = DIV_ROUND_UP(new_len, PAGE_SIZE);
827 :
828 : /* Expand the trailing edge upwards */
829 0 : while (ractl->_nr_pages < new_nr_pages) {
830 0 : unsigned long index = ractl->_index + ractl->_nr_pages;
831 0 : struct folio *folio = xa_load(&mapping->i_pages, index);
832 :
833 0 : if (folio && !xa_is_value(folio))
834 : return; /* Folio apparently present */
835 :
836 0 : folio = filemap_alloc_folio(gfp_mask, 0);
837 0 : if (!folio)
838 : return;
839 0 : if (filemap_add_folio(mapping, folio, index, gfp_mask) < 0) {
840 : folio_put(folio);
841 : return;
842 : }
843 0 : if (unlikely(folio_test_workingset(folio)) &&
844 0 : !ractl->_workingset) {
845 0 : ractl->_workingset = true;
846 0 : psi_memstall_enter(&ractl->_pflags);
847 : }
848 0 : ractl->_nr_pages++;
849 0 : if (ra) {
850 0 : ra->size++;
851 0 : ra->async_size++;
852 : }
853 : }
854 : }
855 : EXPORT_SYMBOL(readahead_expand);
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