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  */
138 void
file_ra_state_init(struct file_ra_state * ra,struct address_space * mapping)139 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
140 {
141 	ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
142 	ra->prev_pos = -1;
143 }
144 EXPORT_SYMBOL_GPL(file_ra_state_init);
145 
read_pages(struct readahead_control * rac)146 static 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(&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(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(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(&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  */
page_cache_ra_unbounded(struct readahead_control * ractl,unsigned long nr_to_read,unsigned long lookahead_size)205 void 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(ractl);
210 	gfp_t gfp_mask = readahead_gfp_mask(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 + i);
231 
232 		if (folio && !xa_is_value(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(ractl);
242 			ractl->_index++;
243 			i = ractl->_index + ractl->_nr_pages - index - 1;
244 			continue;
245 		}
246 
247 		folio = filemap_alloc_folio(gfp_mask, 0);
248 		if (!folio)
249 			break;
250 		if (filemap_add_folio(mapping, folio, index + i,
251 					gfp_mask) < 0) {
252 			folio_put(folio);
253 			read_pages(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(ractl);
270 	filemap_invalidate_unlock_shared(mapping);
271 	memalloc_nofs_restore(nofs);
272 }
273 EXPORT_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  */
do_page_cache_ra(struct readahead_control * ractl,unsigned long nr_to_read,unsigned long lookahead_size)281 static 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(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  */
force_page_cache_ra(struct readahead_control * ractl,unsigned long nr_to_read)306 void 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(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(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, this_chunk, 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  */
get_init_ra_size(unsigned long size,unsigned long max)343 static 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  */
get_next_ra_size(struct file_ra_state * ra,unsigned long max)361 static 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  */
count_history_pages(struct address_space * mapping,pgoff_t index,unsigned long max)418 static 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 - 1, max);
425 	rcu_read_unlock();
426 
427 	return index - 1 - head;
428 }
429 
430 /*
431  * page cache context based readahead
432  */
try_context_readahead(struct address_space * mapping,struct file_ra_state * ra,pgoff_t index,unsigned long req_size,unsigned long max)433 static 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 
ra_alloc_folio(struct readahead_control * ractl,pgoff_t index,pgoff_t mark,unsigned int order,gfp_t gfp)464 static 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(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 
page_cache_ra_order(struct readahead_control * ractl,struct file_ra_state * ra,unsigned int new_order)486 void 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(ractl);
491 	pgoff_t limit = (i_size_read(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(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(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;
545 fallback:
546 	do_page_cache_ra(ractl, ra->size, ra->async_size);
547 }
548 
549 /*
550  * A minimal readahead algorithm for trivial sequential/random reads.
551  */
ondemand_readahead(struct readahead_control * ractl,struct folio * folio,unsigned long req_size)552 static 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(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(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_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(ractl->mapping, index + 1,
600 				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_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(ractl->mapping, ra, index, req_size,
634 			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, req_size, 0);
642 	return;
643 
644 initial_readahead:
645 	ra->start = index;
646 	ra->size = get_init_ra_size(req_size, max_pages);
647 	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
648 
649 readit:
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_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, order);
669 }
670 
page_cache_sync_ra(struct readahead_control * ractl,unsigned long req_count)671 void 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, req_count);
692 		return;
693 	}
694 
695 	ondemand_readahead(ractl, NULL, req_count);
696 }
697 EXPORT_SYMBOL_GPL(page_cache_sync_ra);
698 
page_cache_async_ra(struct readahead_control * ractl,struct folio * folio,unsigned long req_count)699 void 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_count);
718 }
719 EXPORT_SYMBOL_GPL(page_cache_async_ra);
720 
ksys_readahead(int fd,loff_t offset,size_t count)721 ssize_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(f.file, offset, count, POSIX_FADV_WILLNEED);
743 out:
744 	fdput(f);
745 	return ret;
746 }
747 
SYSCALL_DEFINE3(readahead,int,fd,loff_t,offset,size_t,count)748 SYSCALL_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)
COMPAT_SYSCALL_DEFINE4(readahead,int,fd,compat_arg_u64_dual (offset),size_t,count)754 COMPAT_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  */
readahead_expand(struct readahead_control * ractl,loff_t new_start,size_t new_len)778 void 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(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(folio))
794 			return; /* Folio apparently present */
795 
796 		folio = filemap_alloc_folio(gfp_mask, 0);
797 		if (!folio)
798 			return;
799 		if (filemap_add_folio(mapping, folio, index, 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(&ractl->_pflags);
807 		}
808 		ractl->_nr_pages++;
809 		ractl->_index = folio->index;
810 	}
811 
812 	new_len += new_start - readahead_pos(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(folio))
821 			return; /* Folio apparently present */
822 
823 		folio = filemap_alloc_folio(gfp_mask, 0);
824 		if (!folio)
825 			return;
826 		if (filemap_add_folio(mapping, folio, index, 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(&ractl->_pflags);
834 		}
835 		ractl->_nr_pages++;
836 		if (ra) {
837 			ra->size++;
838 			ra->async_size++;
839 		}
840 	}
841 }
842 EXPORT_SYMBOL(readahead_expand);
843