xref: /openbmc/linux/mm/readahead.c (revision b866371f)
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
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  
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   */
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   */
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   */
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   */
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   */
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   */
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   */
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  
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  
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   */
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  
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  
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  
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  
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)
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   */
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