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1/*
2 * mm/readahead.c - address_space-level file readahead.
3 *
4 * Copyright (C) 2002, Linus Torvalds
5 *
6 * 09Apr2002 Andrew Morton
7 * Initial version.
8 */
9
10#include <linux/kernel.h>
11#include <linux/dax.h>
12#include <linux/gfp.h>
13#include <linux/export.h>
14#include <linux/blkdev.h>
15#include <linux/backing-dev.h>
16#include <linux/task_io_accounting_ops.h>
17#include <linux/pagevec.h>
18#include <linux/pagemap.h>
19#include <linux/syscalls.h>
20#include <linux/file.h>
21#include <linux/mm_inline.h>
22
23#include "internal.h"
24
25/*
26 * Initialise a struct file's readahead state. Assumes that the caller has
27 * memset *ra to zero.
28 */
29void
30file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
31{
32 ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
33 ra->prev_pos = -1;
34}
35EXPORT_SYMBOL_GPL(file_ra_state_init);
36
37/*
38 * see if a page needs releasing upon read_cache_pages() failure
39 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
40 * before calling, such as the NFS fs marking pages that are cached locally
41 * on disk, thus we need to give the fs a chance to clean up in the event of
42 * an error
43 */
44static void read_cache_pages_invalidate_page(struct address_space *mapping,
45 struct page *page)
46{
47 if (page_has_private(page)) {
48 if (!trylock_page(page))
49 BUG();
50 page->mapping = mapping;
51 do_invalidatepage(page, 0, PAGE_SIZE);
52 page->mapping = NULL;
53 unlock_page(page);
54 }
55 put_page(page);
56}
57
58/*
59 * release a list of pages, invalidating them first if need be
60 */
61static void read_cache_pages_invalidate_pages(struct address_space *mapping,
62 struct list_head *pages)
63{
64 struct page *victim;
65
66 while (!list_empty(pages)) {
67 victim = lru_to_page(pages);
68 list_del(&victim->lru);
69 read_cache_pages_invalidate_page(mapping, victim);
70 }
71}
72
73/**
74 * read_cache_pages - populate an address space with some pages & start reads against them
75 * @mapping: the address_space
76 * @pages: The address of a list_head which contains the target pages. These
77 * pages have their ->index populated and are otherwise uninitialised.
78 * @filler: callback routine for filling a single page.
79 * @data: private data for the callback routine.
80 *
81 * Hides the details of the LRU cache etc from the filesystems.
82 */
83int read_cache_pages(struct address_space *mapping, struct list_head *pages,
84 int (*filler)(void *, struct page *), void *data)
85{
86 struct page *page;
87 int ret = 0;
88
89 while (!list_empty(pages)) {
90 page = lru_to_page(pages);
91 list_del(&page->lru);
92 if (add_to_page_cache_lru(page, mapping, page->index,
93 readahead_gfp_mask(mapping))) {
94 read_cache_pages_invalidate_page(mapping, page);
95 continue;
96 }
97 put_page(page);
98
99 ret = filler(data, page);
100 if (unlikely(ret)) {
101 read_cache_pages_invalidate_pages(mapping, pages);
102 break;
103 }
104 task_io_account_read(PAGE_SIZE);
105 }
106 return ret;
107}
108
109EXPORT_SYMBOL(read_cache_pages);
110
111static int read_pages(struct address_space *mapping, struct file *filp,
112 struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
113{
114 struct blk_plug plug;
115 unsigned page_idx;
116 int ret;
117
118 blk_start_plug(&plug);
119
120 if (mapping->a_ops->readpages) {
121 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
122 /* Clean up the remaining pages */
123 put_pages_list(pages);
124 goto out;
125 }
126
127 for (page_idx = 0; page_idx < nr_pages; page_idx++) {
128 struct page *page = lru_to_page(pages);
129 list_del(&page->lru);
130 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
131 mapping->a_ops->readpage(filp, page);
132 put_page(page);
133 }
134 ret = 0;
135
136out:
137 blk_finish_plug(&plug);
138
139 return ret;
140}
141
142/*
143 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all
144 * the pages first, then submits them all for I/O. This avoids the very bad
145 * behaviour which would occur if page allocations are causing VM writeback.
146 * We really don't want to intermingle reads and writes like that.
147 *
148 * Returns the number of pages requested, or the maximum amount of I/O allowed.
149 */
150int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
151 pgoff_t offset, unsigned long nr_to_read,
152 unsigned long lookahead_size)
153{
154 struct inode *inode = mapping->host;
155 struct page *page;
156 unsigned long end_index; /* The last page we want to read */
157 LIST_HEAD(page_pool);
158 int page_idx;
159 int ret = 0;
160 loff_t isize = i_size_read(inode);
161 gfp_t gfp_mask = readahead_gfp_mask(mapping);
162
163 if (isize == 0)
164 goto out;
165
166 end_index = ((isize - 1) >> PAGE_SHIFT);
167
168 /*
169 * Preallocate as many pages as we will need.
170 */
171 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
172 pgoff_t page_offset = offset + page_idx;
173
174 if (page_offset > end_index)
175 break;
176
177 rcu_read_lock();
178 page = radix_tree_lookup(&mapping->i_pages, page_offset);
179 rcu_read_unlock();
180 if (page && !radix_tree_exceptional_entry(page))
181 continue;
182
183 page = __page_cache_alloc(gfp_mask);
184 if (!page)
185 break;
186 page->index = page_offset;
187 list_add(&page->lru, &page_pool);
188 if (page_idx == nr_to_read - lookahead_size)
189 SetPageReadahead(page);
190 ret++;
191 }
192
193 /*
194 * Now start the IO. We ignore I/O errors - if the page is not
195 * uptodate then the caller will launch readpage again, and
196 * will then handle the error.
197 */
198 if (ret)
199 read_pages(mapping, filp, &page_pool, ret, gfp_mask);
200 BUG_ON(!list_empty(&page_pool));
201out:
202 return ret;
203}
204
205/*
206 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
207 * memory at once.
208 */
209int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
210 pgoff_t offset, unsigned long nr_to_read)
211{
212 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
213 struct file_ra_state *ra = &filp->f_ra;
214 unsigned long max_pages;
215
216 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
217 return -EINVAL;
218
219 /*
220 * If the request exceeds the readahead window, allow the read to
221 * be up to the optimal hardware IO size
222 */
223 max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
224 nr_to_read = min(nr_to_read, max_pages);
225 while (nr_to_read) {
226 int err;
227
228 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
229
230 if (this_chunk > nr_to_read)
231 this_chunk = nr_to_read;
232 err = __do_page_cache_readahead(mapping, filp,
233 offset, this_chunk, 0);
234 if (err < 0)
235 return err;
236
237 offset += this_chunk;
238 nr_to_read -= this_chunk;
239 }
240 return 0;
241}
242
243/*
244 * Set the initial window size, round to next power of 2 and square
245 * for small size, x 4 for medium, and x 2 for large
246 * for 128k (32 page) max ra
247 * 1-8 page = 32k initial, > 8 page = 128k initial
248 */
249static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
250{
251 unsigned long newsize = roundup_pow_of_two(size);
252
253 if (newsize <= max / 32)
254 newsize = newsize * 4;
255 else if (newsize <= max / 4)
256 newsize = newsize * 2;
257 else
258 newsize = max;
259
260 return newsize;
261}
262
263/*
264 * Get the previous window size, ramp it up, and
265 * return it as the new window size.
266 */
267static unsigned long get_next_ra_size(struct file_ra_state *ra,
268 unsigned long max)
269{
270 unsigned long cur = ra->size;
271 unsigned long newsize;
272
273 if (cur < max / 16)
274 newsize = 4 * cur;
275 else
276 newsize = 2 * cur;
277
278 return min(newsize, max);
279}
280
281/*
282 * On-demand readahead design.
283 *
284 * The fields in struct file_ra_state represent the most-recently-executed
285 * readahead attempt:
286 *
287 * |<----- async_size ---------|
288 * |------------------- size -------------------->|
289 * |==================#===========================|
290 * ^start ^page marked with PG_readahead
291 *
292 * To overlap application thinking time and disk I/O time, we do
293 * `readahead pipelining': Do not wait until the application consumed all
294 * readahead pages and stalled on the missing page at readahead_index;
295 * Instead, submit an asynchronous readahead I/O as soon as there are
296 * only async_size pages left in the readahead window. Normally async_size
297 * will be equal to size, for maximum pipelining.
298 *
299 * In interleaved sequential reads, concurrent streams on the same fd can
300 * be invalidating each other's readahead state. So we flag the new readahead
301 * page at (start+size-async_size) with PG_readahead, and use it as readahead
302 * indicator. The flag won't be set on already cached pages, to avoid the
303 * readahead-for-nothing fuss, saving pointless page cache lookups.
304 *
305 * prev_pos tracks the last visited byte in the _previous_ read request.
306 * It should be maintained by the caller, and will be used for detecting
307 * small random reads. Note that the readahead algorithm checks loosely
308 * for sequential patterns. Hence interleaved reads might be served as
309 * sequential ones.
310 *
311 * There is a special-case: if the first page which the application tries to
312 * read happens to be the first page of the file, it is assumed that a linear
313 * read is about to happen and the window is immediately set to the initial size
314 * based on I/O request size and the max_readahead.
315 *
316 * The code ramps up the readahead size aggressively at first, but slow down as
317 * it approaches max_readhead.
318 */
319
320/*
321 * Count contiguously cached pages from @offset-1 to @offset-@max,
322 * this count is a conservative estimation of
323 * - length of the sequential read sequence, or
324 * - thrashing threshold in memory tight systems
325 */
326static pgoff_t count_history_pages(struct address_space *mapping,
327 pgoff_t offset, unsigned long max)
328{
329 pgoff_t head;
330
331 rcu_read_lock();
332 head = page_cache_prev_hole(mapping, offset - 1, max);
333 rcu_read_unlock();
334
335 return offset - 1 - head;
336}
337
338/*
339 * page cache context based read-ahead
340 */
341static int try_context_readahead(struct address_space *mapping,
342 struct file_ra_state *ra,
343 pgoff_t offset,
344 unsigned long req_size,
345 unsigned long max)
346{
347 pgoff_t size;
348
349 size = count_history_pages(mapping, offset, max);
350
351 /*
352 * not enough history pages:
353 * it could be a random read
354 */
355 if (size <= req_size)
356 return 0;
357
358 /*
359 * starts from beginning of file:
360 * it is a strong indication of long-run stream (or whole-file-read)
361 */
362 if (size >= offset)
363 size *= 2;
364
365 ra->start = offset;
366 ra->size = min(size + req_size, max);
367 ra->async_size = 1;
368
369 return 1;
370}
371
372/*
373 * A minimal readahead algorithm for trivial sequential/random reads.
374 */
375static unsigned long
376ondemand_readahead(struct address_space *mapping,
377 struct file_ra_state *ra, struct file *filp,
378 bool hit_readahead_marker, pgoff_t offset,
379 unsigned long req_size)
380{
381 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
382 unsigned long max_pages = ra->ra_pages;
383 pgoff_t prev_offset;
384
385 /*
386 * If the request exceeds the readahead window, allow the read to
387 * be up to the optimal hardware IO size
388 */
389 if (req_size > max_pages && bdi->io_pages > max_pages)
390 max_pages = min(req_size, bdi->io_pages);
391
392 /*
393 * start of file
394 */
395 if (!offset)
396 goto initial_readahead;
397
398 /*
399 * It's the expected callback offset, assume sequential access.
400 * Ramp up sizes, and push forward the readahead window.
401 */
402 if ((offset == (ra->start + ra->size - ra->async_size) ||
403 offset == (ra->start + ra->size))) {
404 ra->start += ra->size;
405 ra->size = get_next_ra_size(ra, max_pages);
406 ra->async_size = ra->size;
407 goto readit;
408 }
409
410 /*
411 * Hit a marked page without valid readahead state.
412 * E.g. interleaved reads.
413 * Query the pagecache for async_size, which normally equals to
414 * readahead size. Ramp it up and use it as the new readahead size.
415 */
416 if (hit_readahead_marker) {
417 pgoff_t start;
418
419 rcu_read_lock();
420 start = page_cache_next_hole(mapping, offset + 1, max_pages);
421 rcu_read_unlock();
422
423 if (!start || start - offset > max_pages)
424 return 0;
425
426 ra->start = start;
427 ra->size = start - offset; /* old async_size */
428 ra->size += req_size;
429 ra->size = get_next_ra_size(ra, max_pages);
430 ra->async_size = ra->size;
431 goto readit;
432 }
433
434 /*
435 * oversize read
436 */
437 if (req_size > max_pages)
438 goto initial_readahead;
439
440 /*
441 * sequential cache miss
442 * trivial case: (offset - prev_offset) == 1
443 * unaligned reads: (offset - prev_offset) == 0
444 */
445 prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
446 if (offset - prev_offset <= 1UL)
447 goto initial_readahead;
448
449 /*
450 * Query the page cache and look for the traces(cached history pages)
451 * that a sequential stream would leave behind.
452 */
453 if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
454 goto readit;
455
456 /*
457 * standalone, small random read
458 * Read as is, and do not pollute the readahead state.
459 */
460 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
461
462initial_readahead:
463 ra->start = offset;
464 ra->size = get_init_ra_size(req_size, max_pages);
465 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
466
467readit:
468 /*
469 * Will this read hit the readahead marker made by itself?
470 * If so, trigger the readahead marker hit now, and merge
471 * the resulted next readahead window into the current one.
472 */
473 if (offset == ra->start && ra->size == ra->async_size) {
474 ra->async_size = get_next_ra_size(ra, max_pages);
475 ra->size += ra->async_size;
476 }
477
478 return ra_submit(ra, mapping, filp);
479}
480
481/**
482 * page_cache_sync_readahead - generic file readahead
483 * @mapping: address_space which holds the pagecache and I/O vectors
484 * @ra: file_ra_state which holds the readahead state
485 * @filp: passed on to ->readpage() and ->readpages()
486 * @offset: start offset into @mapping, in pagecache page-sized units
487 * @req_size: hint: total size of the read which the caller is performing in
488 * pagecache pages
489 *
490 * page_cache_sync_readahead() should be called when a cache miss happened:
491 * it will submit the read. The readahead logic may decide to piggyback more
492 * pages onto the read request if access patterns suggest it will improve
493 * performance.
494 */
495void page_cache_sync_readahead(struct address_space *mapping,
496 struct file_ra_state *ra, struct file *filp,
497 pgoff_t offset, unsigned long req_size)
498{
499 /* no read-ahead */
500 if (!ra->ra_pages)
501 return;
502
503 /* be dumb */
504 if (filp && (filp->f_mode & FMODE_RANDOM)) {
505 force_page_cache_readahead(mapping, filp, offset, req_size);
506 return;
507 }
508
509 /* do read-ahead */
510 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
511}
512EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
513
514/**
515 * page_cache_async_readahead - file readahead for marked pages
516 * @mapping: address_space which holds the pagecache and I/O vectors
517 * @ra: file_ra_state which holds the readahead state
518 * @filp: passed on to ->readpage() and ->readpages()
519 * @page: the page at @offset which has the PG_readahead flag set
520 * @offset: start offset into @mapping, in pagecache page-sized units
521 * @req_size: hint: total size of the read which the caller is performing in
522 * pagecache pages
523 *
524 * page_cache_async_readahead() should be called when a page is used which
525 * has the PG_readahead flag; this is a marker to suggest that the application
526 * has used up enough of the readahead window that we should start pulling in
527 * more pages.
528 */
529void
530page_cache_async_readahead(struct address_space *mapping,
531 struct file_ra_state *ra, struct file *filp,
532 struct page *page, pgoff_t offset,
533 unsigned long req_size)
534{
535 /* no read-ahead */
536 if (!ra->ra_pages)
537 return;
538
539 /*
540 * Same bit is used for PG_readahead and PG_reclaim.
541 */
542 if (PageWriteback(page))
543 return;
544
545 ClearPageReadahead(page);
546
547 /*
548 * Defer asynchronous read-ahead on IO congestion.
549 */
550 if (inode_read_congested(mapping->host))
551 return;
552
553 /* do read-ahead */
554 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
555}
556EXPORT_SYMBOL_GPL(page_cache_async_readahead);
557
558static ssize_t
559do_readahead(struct address_space *mapping, struct file *filp,
560 pgoff_t index, unsigned long nr)
561{
562 if (!mapping || !mapping->a_ops)
563 return -EINVAL;
564
565 /*
566 * Readahead doesn't make sense for DAX inodes, but we don't want it
567 * to report a failure either. Instead, we just return success and
568 * don't do any work.
569 */
570 if (dax_mapping(mapping))
571 return 0;
572
573 return force_page_cache_readahead(mapping, filp, index, nr);
574}
575
576ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
577{
578 ssize_t ret;
579 struct fd f;
580
581 ret = -EBADF;
582 f = fdget(fd);
583 if (f.file) {
584 if (f.file->f_mode & FMODE_READ) {
585 struct address_space *mapping = f.file->f_mapping;
586 pgoff_t start = offset >> PAGE_SHIFT;
587 pgoff_t end = (offset + count - 1) >> PAGE_SHIFT;
588 unsigned long len = end - start + 1;
589 ret = do_readahead(mapping, f.file, start, len);
590 }
591 fdput(f);
592 }
593 return ret;
594}
595
596SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
597{
598 return ksys_readahead(fd, offset, count);
599}
1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * mm/readahead.c - address_space-level file readahead.
4 *
5 * Copyright (C) 2002, Linus Torvalds
6 *
7 * 09Apr2002 Andrew Morton
8 * Initial version.
9 */
10
11#include <linux/kernel.h>
12#include <linux/dax.h>
13#include <linux/gfp.h>
14#include <linux/export.h>
15#include <linux/blkdev.h>
16#include <linux/backing-dev.h>
17#include <linux/task_io_accounting_ops.h>
18#include <linux/pagevec.h>
19#include <linux/pagemap.h>
20#include <linux/syscalls.h>
21#include <linux/file.h>
22#include <linux/mm_inline.h>
23#include <linux/blk-cgroup.h>
24#include <linux/fadvise.h>
25#include <linux/sched/mm.h>
26
27#include "internal.h"
28
29/*
30 * Initialise a struct file's readahead state. Assumes that the caller has
31 * memset *ra to zero.
32 */
33void
34file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
35{
36 ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
37 ra->prev_pos = -1;
38}
39EXPORT_SYMBOL_GPL(file_ra_state_init);
40
41/*
42 * see if a page needs releasing upon read_cache_pages() failure
43 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
44 * before calling, such as the NFS fs marking pages that are cached locally
45 * on disk, thus we need to give the fs a chance to clean up in the event of
46 * an error
47 */
48static void read_cache_pages_invalidate_page(struct address_space *mapping,
49 struct page *page)
50{
51 if (page_has_private(page)) {
52 if (!trylock_page(page))
53 BUG();
54 page->mapping = mapping;
55 do_invalidatepage(page, 0, PAGE_SIZE);
56 page->mapping = NULL;
57 unlock_page(page);
58 }
59 put_page(page);
60}
61
62/*
63 * release a list of pages, invalidating them first if need be
64 */
65static void read_cache_pages_invalidate_pages(struct address_space *mapping,
66 struct list_head *pages)
67{
68 struct page *victim;
69
70 while (!list_empty(pages)) {
71 victim = lru_to_page(pages);
72 list_del(&victim->lru);
73 read_cache_pages_invalidate_page(mapping, victim);
74 }
75}
76
77/**
78 * read_cache_pages - populate an address space with some pages & start reads against them
79 * @mapping: the address_space
80 * @pages: The address of a list_head which contains the target pages. These
81 * pages have their ->index populated and are otherwise uninitialised.
82 * @filler: callback routine for filling a single page.
83 * @data: private data for the callback routine.
84 *
85 * Hides the details of the LRU cache etc from the filesystems.
86 *
87 * Returns: %0 on success, error return by @filler otherwise
88 */
89int read_cache_pages(struct address_space *mapping, struct list_head *pages,
90 int (*filler)(void *, struct page *), void *data)
91{
92 struct page *page;
93 int ret = 0;
94
95 while (!list_empty(pages)) {
96 page = lru_to_page(pages);
97 list_del(&page->lru);
98 if (add_to_page_cache_lru(page, mapping, page->index,
99 readahead_gfp_mask(mapping))) {
100 read_cache_pages_invalidate_page(mapping, page);
101 continue;
102 }
103 put_page(page);
104
105 ret = filler(data, page);
106 if (unlikely(ret)) {
107 read_cache_pages_invalidate_pages(mapping, pages);
108 break;
109 }
110 task_io_account_read(PAGE_SIZE);
111 }
112 return ret;
113}
114
115EXPORT_SYMBOL(read_cache_pages);
116
117static void read_pages(struct readahead_control *rac, struct list_head *pages,
118 bool skip_page)
119{
120 const struct address_space_operations *aops = rac->mapping->a_ops;
121 struct page *page;
122 struct blk_plug plug;
123
124 if (!readahead_count(rac))
125 goto out;
126
127 blk_start_plug(&plug);
128
129 if (aops->readahead) {
130 aops->readahead(rac);
131 /* Clean up the remaining pages */
132 while ((page = readahead_page(rac))) {
133 unlock_page(page);
134 put_page(page);
135 }
136 } else if (aops->readpages) {
137 aops->readpages(rac->file, rac->mapping, pages,
138 readahead_count(rac));
139 /* Clean up the remaining pages */
140 put_pages_list(pages);
141 rac->_index += rac->_nr_pages;
142 rac->_nr_pages = 0;
143 } else {
144 while ((page = readahead_page(rac))) {
145 aops->readpage(rac->file, page);
146 put_page(page);
147 }
148 }
149
150 blk_finish_plug(&plug);
151
152 BUG_ON(!list_empty(pages));
153 BUG_ON(readahead_count(rac));
154
155out:
156 if (skip_page)
157 rac->_index++;
158}
159
160/**
161 * page_cache_readahead_unbounded - Start unchecked readahead.
162 * @mapping: File address space.
163 * @file: This instance of the open file; used for authentication.
164 * @index: First page index to read.
165 * @nr_to_read: The number of pages to read.
166 * @lookahead_size: Where to start the next readahead.
167 *
168 * This function is for filesystems to call when they want to start
169 * readahead beyond a file's stated i_size. This is almost certainly
170 * not the function you want to call. Use page_cache_async_readahead()
171 * or page_cache_sync_readahead() instead.
172 *
173 * Context: File is referenced by caller. Mutexes may be held by caller.
174 * May sleep, but will not reenter filesystem to reclaim memory.
175 */
176void page_cache_readahead_unbounded(struct address_space *mapping,
177 struct file *file, pgoff_t index, unsigned long nr_to_read,
178 unsigned long lookahead_size)
179{
180 LIST_HEAD(page_pool);
181 gfp_t gfp_mask = readahead_gfp_mask(mapping);
182 struct readahead_control rac = {
183 .mapping = mapping,
184 .file = file,
185 ._index = index,
186 };
187 unsigned long i;
188
189 /*
190 * Partway through the readahead operation, we will have added
191 * locked pages to the page cache, but will not yet have submitted
192 * them for I/O. Adding another page may need to allocate memory,
193 * which can trigger memory reclaim. Telling the VM we're in
194 * the middle of a filesystem operation will cause it to not
195 * touch file-backed pages, preventing a deadlock. Most (all?)
196 * filesystems already specify __GFP_NOFS in their mapping's
197 * gfp_mask, but let's be explicit here.
198 */
199 unsigned int nofs = memalloc_nofs_save();
200
201 /*
202 * Preallocate as many pages as we will need.
203 */
204 for (i = 0; i < nr_to_read; i++) {
205 struct page *page = xa_load(&mapping->i_pages, index + i);
206
207 BUG_ON(index + i != rac._index + rac._nr_pages);
208
209 if (page && !xa_is_value(page)) {
210 /*
211 * Page already present? Kick off the current batch
212 * of contiguous pages before continuing with the
213 * next batch. This page may be the one we would
214 * have intended to mark as Readahead, but we don't
215 * have a stable reference to this page, and it's
216 * not worth getting one just for that.
217 */
218 read_pages(&rac, &page_pool, true);
219 continue;
220 }
221
222 page = __page_cache_alloc(gfp_mask);
223 if (!page)
224 break;
225 if (mapping->a_ops->readpages) {
226 page->index = index + i;
227 list_add(&page->lru, &page_pool);
228 } else if (add_to_page_cache_lru(page, mapping, index + i,
229 gfp_mask) < 0) {
230 put_page(page);
231 read_pages(&rac, &page_pool, true);
232 continue;
233 }
234 if (i == nr_to_read - lookahead_size)
235 SetPageReadahead(page);
236 rac._nr_pages++;
237 }
238
239 /*
240 * Now start the IO. We ignore I/O errors - if the page is not
241 * uptodate then the caller will launch readpage again, and
242 * will then handle the error.
243 */
244 read_pages(&rac, &page_pool, false);
245 memalloc_nofs_restore(nofs);
246}
247EXPORT_SYMBOL_GPL(page_cache_readahead_unbounded);
248
249/*
250 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates
251 * the pages first, then submits them for I/O. This avoids the very bad
252 * behaviour which would occur if page allocations are causing VM writeback.
253 * We really don't want to intermingle reads and writes like that.
254 */
255void __do_page_cache_readahead(struct address_space *mapping,
256 struct file *file, pgoff_t index, unsigned long nr_to_read,
257 unsigned long lookahead_size)
258{
259 struct inode *inode = mapping->host;
260 loff_t isize = i_size_read(inode);
261 pgoff_t end_index; /* The last page we want to read */
262
263 if (isize == 0)
264 return;
265
266 end_index = (isize - 1) >> PAGE_SHIFT;
267 if (index > end_index)
268 return;
269 /* Don't read past the page containing the last byte of the file */
270 if (nr_to_read > end_index - index)
271 nr_to_read = end_index - index + 1;
272
273 page_cache_readahead_unbounded(mapping, file, index, nr_to_read,
274 lookahead_size);
275}
276
277/*
278 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
279 * memory at once.
280 */
281void force_page_cache_readahead(struct address_space *mapping,
282 struct file *filp, pgoff_t index, unsigned long nr_to_read)
283{
284 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
285 struct file_ra_state *ra = &filp->f_ra;
286 unsigned long max_pages;
287
288 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages &&
289 !mapping->a_ops->readahead))
290 return;
291
292 /*
293 * If the request exceeds the readahead window, allow the read to
294 * be up to the optimal hardware IO size
295 */
296 max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
297 nr_to_read = min(nr_to_read, max_pages);
298 while (nr_to_read) {
299 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
300
301 if (this_chunk > nr_to_read)
302 this_chunk = nr_to_read;
303 __do_page_cache_readahead(mapping, filp, index, this_chunk, 0);
304
305 index += this_chunk;
306 nr_to_read -= this_chunk;
307 }
308}
309
310/*
311 * Set the initial window size, round to next power of 2 and square
312 * for small size, x 4 for medium, and x 2 for large
313 * for 128k (32 page) max ra
314 * 1-8 page = 32k initial, > 8 page = 128k initial
315 */
316static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
317{
318 unsigned long newsize = roundup_pow_of_two(size);
319
320 if (newsize <= max / 32)
321 newsize = newsize * 4;
322 else if (newsize <= max / 4)
323 newsize = newsize * 2;
324 else
325 newsize = max;
326
327 return newsize;
328}
329
330/*
331 * Get the previous window size, ramp it up, and
332 * return it as the new window size.
333 */
334static unsigned long get_next_ra_size(struct file_ra_state *ra,
335 unsigned long max)
336{
337 unsigned long cur = ra->size;
338
339 if (cur < max / 16)
340 return 4 * cur;
341 if (cur <= max / 2)
342 return 2 * cur;
343 return max;
344}
345
346/*
347 * On-demand readahead design.
348 *
349 * The fields in struct file_ra_state represent the most-recently-executed
350 * readahead attempt:
351 *
352 * |<----- async_size ---------|
353 * |------------------- size -------------------->|
354 * |==================#===========================|
355 * ^start ^page marked with PG_readahead
356 *
357 * To overlap application thinking time and disk I/O time, we do
358 * `readahead pipelining': Do not wait until the application consumed all
359 * readahead pages and stalled on the missing page at readahead_index;
360 * Instead, submit an asynchronous readahead I/O as soon as there are
361 * only async_size pages left in the readahead window. Normally async_size
362 * will be equal to size, for maximum pipelining.
363 *
364 * In interleaved sequential reads, concurrent streams on the same fd can
365 * be invalidating each other's readahead state. So we flag the new readahead
366 * page at (start+size-async_size) with PG_readahead, and use it as readahead
367 * indicator. The flag won't be set on already cached pages, to avoid the
368 * readahead-for-nothing fuss, saving pointless page cache lookups.
369 *
370 * prev_pos tracks the last visited byte in the _previous_ read request.
371 * It should be maintained by the caller, and will be used for detecting
372 * small random reads. Note that the readahead algorithm checks loosely
373 * for sequential patterns. Hence interleaved reads might be served as
374 * sequential ones.
375 *
376 * There is a special-case: if the first page which the application tries to
377 * read happens to be the first page of the file, it is assumed that a linear
378 * read is about to happen and the window is immediately set to the initial size
379 * based on I/O request size and the max_readahead.
380 *
381 * The code ramps up the readahead size aggressively at first, but slow down as
382 * it approaches max_readhead.
383 */
384
385/*
386 * Count contiguously cached pages from @index-1 to @index-@max,
387 * this count is a conservative estimation of
388 * - length of the sequential read sequence, or
389 * - thrashing threshold in memory tight systems
390 */
391static pgoff_t count_history_pages(struct address_space *mapping,
392 pgoff_t index, unsigned long max)
393{
394 pgoff_t head;
395
396 rcu_read_lock();
397 head = page_cache_prev_miss(mapping, index - 1, max);
398 rcu_read_unlock();
399
400 return index - 1 - head;
401}
402
403/*
404 * page cache context based read-ahead
405 */
406static int try_context_readahead(struct address_space *mapping,
407 struct file_ra_state *ra,
408 pgoff_t index,
409 unsigned long req_size,
410 unsigned long max)
411{
412 pgoff_t size;
413
414 size = count_history_pages(mapping, index, max);
415
416 /*
417 * not enough history pages:
418 * it could be a random read
419 */
420 if (size <= req_size)
421 return 0;
422
423 /*
424 * starts from beginning of file:
425 * it is a strong indication of long-run stream (or whole-file-read)
426 */
427 if (size >= index)
428 size *= 2;
429
430 ra->start = index;
431 ra->size = min(size + req_size, max);
432 ra->async_size = 1;
433
434 return 1;
435}
436
437/*
438 * A minimal readahead algorithm for trivial sequential/random reads.
439 */
440static void ondemand_readahead(struct address_space *mapping,
441 struct file_ra_state *ra, struct file *filp,
442 bool hit_readahead_marker, pgoff_t index,
443 unsigned long req_size)
444{
445 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
446 unsigned long max_pages = ra->ra_pages;
447 unsigned long add_pages;
448 pgoff_t prev_index;
449
450 /*
451 * If the request exceeds the readahead window, allow the read to
452 * be up to the optimal hardware IO size
453 */
454 if (req_size > max_pages && bdi->io_pages > max_pages)
455 max_pages = min(req_size, bdi->io_pages);
456
457 /*
458 * start of file
459 */
460 if (!index)
461 goto initial_readahead;
462
463 /*
464 * It's the expected callback index, assume sequential access.
465 * Ramp up sizes, and push forward the readahead window.
466 */
467 if ((index == (ra->start + ra->size - ra->async_size) ||
468 index == (ra->start + ra->size))) {
469 ra->start += ra->size;
470 ra->size = get_next_ra_size(ra, max_pages);
471 ra->async_size = ra->size;
472 goto readit;
473 }
474
475 /*
476 * Hit a marked page without valid readahead state.
477 * E.g. interleaved reads.
478 * Query the pagecache for async_size, which normally equals to
479 * readahead size. Ramp it up and use it as the new readahead size.
480 */
481 if (hit_readahead_marker) {
482 pgoff_t start;
483
484 rcu_read_lock();
485 start = page_cache_next_miss(mapping, index + 1, max_pages);
486 rcu_read_unlock();
487
488 if (!start || start - index > max_pages)
489 return;
490
491 ra->start = start;
492 ra->size = start - index; /* old async_size */
493 ra->size += req_size;
494 ra->size = get_next_ra_size(ra, max_pages);
495 ra->async_size = ra->size;
496 goto readit;
497 }
498
499 /*
500 * oversize read
501 */
502 if (req_size > max_pages)
503 goto initial_readahead;
504
505 /*
506 * sequential cache miss
507 * trivial case: (index - prev_index) == 1
508 * unaligned reads: (index - prev_index) == 0
509 */
510 prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
511 if (index - prev_index <= 1UL)
512 goto initial_readahead;
513
514 /*
515 * Query the page cache and look for the traces(cached history pages)
516 * that a sequential stream would leave behind.
517 */
518 if (try_context_readahead(mapping, ra, index, req_size, max_pages))
519 goto readit;
520
521 /*
522 * standalone, small random read
523 * Read as is, and do not pollute the readahead state.
524 */
525 __do_page_cache_readahead(mapping, filp, index, req_size, 0);
526 return;
527
528initial_readahead:
529 ra->start = index;
530 ra->size = get_init_ra_size(req_size, max_pages);
531 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
532
533readit:
534 /*
535 * Will this read hit the readahead marker made by itself?
536 * If so, trigger the readahead marker hit now, and merge
537 * the resulted next readahead window into the current one.
538 * Take care of maximum IO pages as above.
539 */
540 if (index == ra->start && ra->size == ra->async_size) {
541 add_pages = get_next_ra_size(ra, max_pages);
542 if (ra->size + add_pages <= max_pages) {
543 ra->async_size = add_pages;
544 ra->size += add_pages;
545 } else {
546 ra->size = max_pages;
547 ra->async_size = max_pages >> 1;
548 }
549 }
550
551 ra_submit(ra, mapping, filp);
552}
553
554/**
555 * page_cache_sync_readahead - generic file readahead
556 * @mapping: address_space which holds the pagecache and I/O vectors
557 * @ra: file_ra_state which holds the readahead state
558 * @filp: passed on to ->readpage() and ->readpages()
559 * @index: Index of first page to be read.
560 * @req_count: Total number of pages being read by the caller.
561 *
562 * page_cache_sync_readahead() should be called when a cache miss happened:
563 * it will submit the read. The readahead logic may decide to piggyback more
564 * pages onto the read request if access patterns suggest it will improve
565 * performance.
566 */
567void page_cache_sync_readahead(struct address_space *mapping,
568 struct file_ra_state *ra, struct file *filp,
569 pgoff_t index, unsigned long req_count)
570{
571 /* no read-ahead */
572 if (!ra->ra_pages)
573 return;
574
575 if (blk_cgroup_congested())
576 return;
577
578 /* be dumb */
579 if (filp && (filp->f_mode & FMODE_RANDOM)) {
580 force_page_cache_readahead(mapping, filp, index, req_count);
581 return;
582 }
583
584 /* do read-ahead */
585 ondemand_readahead(mapping, ra, filp, false, index, req_count);
586}
587EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
588
589/**
590 * page_cache_async_readahead - file readahead for marked pages
591 * @mapping: address_space which holds the pagecache and I/O vectors
592 * @ra: file_ra_state which holds the readahead state
593 * @filp: passed on to ->readpage() and ->readpages()
594 * @page: The page at @index which triggered the readahead call.
595 * @index: Index of first page to be read.
596 * @req_count: Total number of pages being read by the caller.
597 *
598 * page_cache_async_readahead() should be called when a page is used which
599 * is marked as PageReadahead; this is a marker to suggest that the application
600 * has used up enough of the readahead window that we should start pulling in
601 * more pages.
602 */
603void
604page_cache_async_readahead(struct address_space *mapping,
605 struct file_ra_state *ra, struct file *filp,
606 struct page *page, pgoff_t index,
607 unsigned long req_count)
608{
609 /* no read-ahead */
610 if (!ra->ra_pages)
611 return;
612
613 /*
614 * Same bit is used for PG_readahead and PG_reclaim.
615 */
616 if (PageWriteback(page))
617 return;
618
619 ClearPageReadahead(page);
620
621 /*
622 * Defer asynchronous read-ahead on IO congestion.
623 */
624 if (inode_read_congested(mapping->host))
625 return;
626
627 if (blk_cgroup_congested())
628 return;
629
630 /* do read-ahead */
631 ondemand_readahead(mapping, ra, filp, true, index, req_count);
632}
633EXPORT_SYMBOL_GPL(page_cache_async_readahead);
634
635ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
636{
637 ssize_t ret;
638 struct fd f;
639
640 ret = -EBADF;
641 f = fdget(fd);
642 if (!f.file || !(f.file->f_mode & FMODE_READ))
643 goto out;
644
645 /*
646 * The readahead() syscall is intended to run only on files
647 * that can execute readahead. If readahead is not possible
648 * on this file, then we must return -EINVAL.
649 */
650 ret = -EINVAL;
651 if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
652 !S_ISREG(file_inode(f.file)->i_mode))
653 goto out;
654
655 ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
656out:
657 fdput(f);
658 return ret;
659}
660
661SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
662{
663 return ksys_readahead(fd, offset, count);
664}