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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/gfp.h>
12#include <linux/export.h>
13#include <linux/blkdev.h>
14#include <linux/backing-dev.h>
15#include <linux/task_io_accounting_ops.h>
16#include <linux/pagevec.h>
17#include <linux/pagemap.h>
18#include <linux/syscalls.h>
19#include <linux/file.h>
20
21#include "internal.h"
22
23/*
24 * Initialise a struct file's readahead state. Assumes that the caller has
25 * memset *ra to zero.
26 */
27void
28file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
29{
30 ra->ra_pages = mapping->backing_dev_info->ra_pages;
31 ra->prev_pos = -1;
32}
33EXPORT_SYMBOL_GPL(file_ra_state_init);
34
35#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
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_CACHE_SIZE);
52 page->mapping = NULL;
53 unlock_page(page);
54 }
55 page_cache_release(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 = list_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 = list_to_page(pages);
91 list_del(&page->lru);
92 if (add_to_page_cache_lru(page, mapping,
93 page->index, GFP_KERNEL)) {
94 read_cache_pages_invalidate_page(mapping, page);
95 continue;
96 }
97 page_cache_release(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_CACHE_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 nr_pages)
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 = list_to_page(pages);
129 list_del(&page->lru);
130 if (!add_to_page_cache_lru(page, mapping,
131 page->index, GFP_KERNEL)) {
132 mapping->a_ops->readpage(filp, page);
133 }
134 page_cache_release(page);
135 }
136 ret = 0;
137
138out:
139 blk_finish_plug(&plug);
140
141 return ret;
142}
143
144/*
145 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all
146 * the pages first, then submits them all for I/O. This avoids the very bad
147 * behaviour which would occur if page allocations are causing VM writeback.
148 * We really don't want to intermingle reads and writes like that.
149 *
150 * Returns the number of pages requested, or the maximum amount of I/O allowed.
151 */
152int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
153 pgoff_t offset, unsigned long nr_to_read,
154 unsigned long lookahead_size)
155{
156 struct inode *inode = mapping->host;
157 struct page *page;
158 unsigned long end_index; /* The last page we want to read */
159 LIST_HEAD(page_pool);
160 int page_idx;
161 int ret = 0;
162 loff_t isize = i_size_read(inode);
163
164 if (isize == 0)
165 goto out;
166
167 end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
168
169 /*
170 * Preallocate as many pages as we will need.
171 */
172 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
173 pgoff_t page_offset = offset + page_idx;
174
175 if (page_offset > end_index)
176 break;
177
178 rcu_read_lock();
179 page = radix_tree_lookup(&mapping->page_tree, page_offset);
180 rcu_read_unlock();
181 if (page && !radix_tree_exceptional_entry(page))
182 continue;
183
184 page = page_cache_alloc_readahead(mapping);
185 if (!page)
186 break;
187 page->index = page_offset;
188 list_add(&page->lru, &page_pool);
189 if (page_idx == nr_to_read - lookahead_size)
190 SetPageReadahead(page);
191 ret++;
192 }
193
194 /*
195 * Now start the IO. We ignore I/O errors - if the page is not
196 * uptodate then the caller will launch readpage again, and
197 * will then handle the error.
198 */
199 if (ret)
200 read_pages(mapping, filp, &page_pool, ret);
201 BUG_ON(!list_empty(&page_pool));
202out:
203 return ret;
204}
205
206/*
207 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
208 * memory at once.
209 */
210int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
211 pgoff_t offset, unsigned long nr_to_read)
212{
213 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
214 return -EINVAL;
215
216 nr_to_read = max_sane_readahead(nr_to_read);
217 while (nr_to_read) {
218 int err;
219
220 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
221
222 if (this_chunk > nr_to_read)
223 this_chunk = nr_to_read;
224 err = __do_page_cache_readahead(mapping, filp,
225 offset, this_chunk, 0);
226 if (err < 0)
227 return err;
228
229 offset += this_chunk;
230 nr_to_read -= this_chunk;
231 }
232 return 0;
233}
234
235#define MAX_READAHEAD ((512*4096)/PAGE_CACHE_SIZE)
236/*
237 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
238 * sensible upper limit.
239 */
240unsigned long max_sane_readahead(unsigned long nr)
241{
242 return min(nr, MAX_READAHEAD);
243}
244
245/*
246 * Set the initial window size, round to next power of 2 and square
247 * for small size, x 4 for medium, and x 2 for large
248 * for 128k (32 page) max ra
249 * 1-8 page = 32k initial, > 8 page = 128k initial
250 */
251static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
252{
253 unsigned long newsize = roundup_pow_of_two(size);
254
255 if (newsize <= max / 32)
256 newsize = newsize * 4;
257 else if (newsize <= max / 4)
258 newsize = newsize * 2;
259 else
260 newsize = max;
261
262 return newsize;
263}
264
265/*
266 * Get the previous window size, ramp it up, and
267 * return it as the new window size.
268 */
269static unsigned long get_next_ra_size(struct file_ra_state *ra,
270 unsigned long max)
271{
272 unsigned long cur = ra->size;
273 unsigned long newsize;
274
275 if (cur < max / 16)
276 newsize = 4 * cur;
277 else
278 newsize = 2 * cur;
279
280 return min(newsize, max);
281}
282
283/*
284 * On-demand readahead design.
285 *
286 * The fields in struct file_ra_state represent the most-recently-executed
287 * readahead attempt:
288 *
289 * |<----- async_size ---------|
290 * |------------------- size -------------------->|
291 * |==================#===========================|
292 * ^start ^page marked with PG_readahead
293 *
294 * To overlap application thinking time and disk I/O time, we do
295 * `readahead pipelining': Do not wait until the application consumed all
296 * readahead pages and stalled on the missing page at readahead_index;
297 * Instead, submit an asynchronous readahead I/O as soon as there are
298 * only async_size pages left in the readahead window. Normally async_size
299 * will be equal to size, for maximum pipelining.
300 *
301 * In interleaved sequential reads, concurrent streams on the same fd can
302 * be invalidating each other's readahead state. So we flag the new readahead
303 * page at (start+size-async_size) with PG_readahead, and use it as readahead
304 * indicator. The flag won't be set on already cached pages, to avoid the
305 * readahead-for-nothing fuss, saving pointless page cache lookups.
306 *
307 * prev_pos tracks the last visited byte in the _previous_ read request.
308 * It should be maintained by the caller, and will be used for detecting
309 * small random reads. Note that the readahead algorithm checks loosely
310 * for sequential patterns. Hence interleaved reads might be served as
311 * sequential ones.
312 *
313 * There is a special-case: if the first page which the application tries to
314 * read happens to be the first page of the file, it is assumed that a linear
315 * read is about to happen and the window is immediately set to the initial size
316 * based on I/O request size and the max_readahead.
317 *
318 * The code ramps up the readahead size aggressively at first, but slow down as
319 * it approaches max_readhead.
320 */
321
322/*
323 * Count contiguously cached pages from @offset-1 to @offset-@max,
324 * this count is a conservative estimation of
325 * - length of the sequential read sequence, or
326 * - thrashing threshold in memory tight systems
327 */
328static pgoff_t count_history_pages(struct address_space *mapping,
329 struct file_ra_state *ra,
330 pgoff_t offset, unsigned long max)
331{
332 pgoff_t head;
333
334 rcu_read_lock();
335 head = page_cache_prev_hole(mapping, offset - 1, max);
336 rcu_read_unlock();
337
338 return offset - 1 - head;
339}
340
341/*
342 * page cache context based read-ahead
343 */
344static int try_context_readahead(struct address_space *mapping,
345 struct file_ra_state *ra,
346 pgoff_t offset,
347 unsigned long req_size,
348 unsigned long max)
349{
350 pgoff_t size;
351
352 size = count_history_pages(mapping, ra, offset, max);
353
354 /*
355 * not enough history pages:
356 * it could be a random read
357 */
358 if (size <= req_size)
359 return 0;
360
361 /*
362 * starts from beginning of file:
363 * it is a strong indication of long-run stream (or whole-file-read)
364 */
365 if (size >= offset)
366 size *= 2;
367
368 ra->start = offset;
369 ra->size = min(size + req_size, max);
370 ra->async_size = 1;
371
372 return 1;
373}
374
375/*
376 * A minimal readahead algorithm for trivial sequential/random reads.
377 */
378static unsigned long
379ondemand_readahead(struct address_space *mapping,
380 struct file_ra_state *ra, struct file *filp,
381 bool hit_readahead_marker, pgoff_t offset,
382 unsigned long req_size)
383{
384 unsigned long max = max_sane_readahead(ra->ra_pages);
385 pgoff_t prev_offset;
386
387 /*
388 * start of file
389 */
390 if (!offset)
391 goto initial_readahead;
392
393 /*
394 * It's the expected callback offset, assume sequential access.
395 * Ramp up sizes, and push forward the readahead window.
396 */
397 if ((offset == (ra->start + ra->size - ra->async_size) ||
398 offset == (ra->start + ra->size))) {
399 ra->start += ra->size;
400 ra->size = get_next_ra_size(ra, max);
401 ra->async_size = ra->size;
402 goto readit;
403 }
404
405 /*
406 * Hit a marked page without valid readahead state.
407 * E.g. interleaved reads.
408 * Query the pagecache for async_size, which normally equals to
409 * readahead size. Ramp it up and use it as the new readahead size.
410 */
411 if (hit_readahead_marker) {
412 pgoff_t start;
413
414 rcu_read_lock();
415 start = page_cache_next_hole(mapping, offset + 1, max);
416 rcu_read_unlock();
417
418 if (!start || start - offset > max)
419 return 0;
420
421 ra->start = start;
422 ra->size = start - offset; /* old async_size */
423 ra->size += req_size;
424 ra->size = get_next_ra_size(ra, max);
425 ra->async_size = ra->size;
426 goto readit;
427 }
428
429 /*
430 * oversize read
431 */
432 if (req_size > max)
433 goto initial_readahead;
434
435 /*
436 * sequential cache miss
437 * trivial case: (offset - prev_offset) == 1
438 * unaligned reads: (offset - prev_offset) == 0
439 */
440 prev_offset = (unsigned long long)ra->prev_pos >> PAGE_CACHE_SHIFT;
441 if (offset - prev_offset <= 1UL)
442 goto initial_readahead;
443
444 /*
445 * Query the page cache and look for the traces(cached history pages)
446 * that a sequential stream would leave behind.
447 */
448 if (try_context_readahead(mapping, ra, offset, req_size, max))
449 goto readit;
450
451 /*
452 * standalone, small random read
453 * Read as is, and do not pollute the readahead state.
454 */
455 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
456
457initial_readahead:
458 ra->start = offset;
459 ra->size = get_init_ra_size(req_size, max);
460 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
461
462readit:
463 /*
464 * Will this read hit the readahead marker made by itself?
465 * If so, trigger the readahead marker hit now, and merge
466 * the resulted next readahead window into the current one.
467 */
468 if (offset == ra->start && ra->size == ra->async_size) {
469 ra->async_size = get_next_ra_size(ra, max);
470 ra->size += ra->async_size;
471 }
472
473 return ra_submit(ra, mapping, filp);
474}
475
476/**
477 * page_cache_sync_readahead - generic file readahead
478 * @mapping: address_space which holds the pagecache and I/O vectors
479 * @ra: file_ra_state which holds the readahead state
480 * @filp: passed on to ->readpage() and ->readpages()
481 * @offset: start offset into @mapping, in pagecache page-sized units
482 * @req_size: hint: total size of the read which the caller is performing in
483 * pagecache pages
484 *
485 * page_cache_sync_readahead() should be called when a cache miss happened:
486 * it will submit the read. The readahead logic may decide to piggyback more
487 * pages onto the read request if access patterns suggest it will improve
488 * performance.
489 */
490void page_cache_sync_readahead(struct address_space *mapping,
491 struct file_ra_state *ra, struct file *filp,
492 pgoff_t offset, unsigned long req_size)
493{
494 /* no read-ahead */
495 if (!ra->ra_pages)
496 return;
497
498 /* be dumb */
499 if (filp && (filp->f_mode & FMODE_RANDOM)) {
500 force_page_cache_readahead(mapping, filp, offset, req_size);
501 return;
502 }
503
504 /* do read-ahead */
505 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
506}
507EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
508
509/**
510 * page_cache_async_readahead - file readahead for marked pages
511 * @mapping: address_space which holds the pagecache and I/O vectors
512 * @ra: file_ra_state which holds the readahead state
513 * @filp: passed on to ->readpage() and ->readpages()
514 * @page: the page at @offset which has the PG_readahead flag set
515 * @offset: start offset into @mapping, in pagecache page-sized units
516 * @req_size: hint: total size of the read which the caller is performing in
517 * pagecache pages
518 *
519 * page_cache_async_readahead() should be called when a page is used which
520 * has the PG_readahead flag; this is a marker to suggest that the application
521 * has used up enough of the readahead window that we should start pulling in
522 * more pages.
523 */
524void
525page_cache_async_readahead(struct address_space *mapping,
526 struct file_ra_state *ra, struct file *filp,
527 struct page *page, pgoff_t offset,
528 unsigned long req_size)
529{
530 /* no read-ahead */
531 if (!ra->ra_pages)
532 return;
533
534 /*
535 * Same bit is used for PG_readahead and PG_reclaim.
536 */
537 if (PageWriteback(page))
538 return;
539
540 ClearPageReadahead(page);
541
542 /*
543 * Defer asynchronous read-ahead on IO congestion.
544 */
545 if (bdi_read_congested(mapping->backing_dev_info))
546 return;
547
548 /* do read-ahead */
549 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
550}
551EXPORT_SYMBOL_GPL(page_cache_async_readahead);
552
553static ssize_t
554do_readahead(struct address_space *mapping, struct file *filp,
555 pgoff_t index, unsigned long nr)
556{
557 if (!mapping || !mapping->a_ops)
558 return -EINVAL;
559
560 return force_page_cache_readahead(mapping, filp, index, nr);
561}
562
563SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
564{
565 ssize_t ret;
566 struct fd f;
567
568 ret = -EBADF;
569 f = fdget(fd);
570 if (f.file) {
571 if (f.file->f_mode & FMODE_READ) {
572 struct address_space *mapping = f.file->f_mapping;
573 pgoff_t start = offset >> PAGE_CACHE_SHIFT;
574 pgoff_t end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
575 unsigned long len = end - start + 1;
576 ret = do_readahead(mapping, f.file, start, len);
577 }
578 fdput(f);
579 }
580 return ret;
581}
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
26#include "internal.h"
27
28/*
29 * Initialise a struct file's readahead state. Assumes that the caller has
30 * memset *ra to zero.
31 */
32void
33file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
34{
35 ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
36 ra->prev_pos = -1;
37}
38EXPORT_SYMBOL_GPL(file_ra_state_init);
39
40/*
41 * see if a page needs releasing upon read_cache_pages() failure
42 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
43 * before calling, such as the NFS fs marking pages that are cached locally
44 * on disk, thus we need to give the fs a chance to clean up in the event of
45 * an error
46 */
47static void read_cache_pages_invalidate_page(struct address_space *mapping,
48 struct page *page)
49{
50 if (page_has_private(page)) {
51 if (!trylock_page(page))
52 BUG();
53 page->mapping = mapping;
54 do_invalidatepage(page, 0, PAGE_SIZE);
55 page->mapping = NULL;
56 unlock_page(page);
57 }
58 put_page(page);
59}
60
61/*
62 * release a list of pages, invalidating them first if need be
63 */
64static void read_cache_pages_invalidate_pages(struct address_space *mapping,
65 struct list_head *pages)
66{
67 struct page *victim;
68
69 while (!list_empty(pages)) {
70 victim = lru_to_page(pages);
71 list_del(&victim->lru);
72 read_cache_pages_invalidate_page(mapping, victim);
73 }
74}
75
76/**
77 * read_cache_pages - populate an address space with some pages & start reads against them
78 * @mapping: the address_space
79 * @pages: The address of a list_head which contains the target pages. These
80 * pages have their ->index populated and are otherwise uninitialised.
81 * @filler: callback routine for filling a single page.
82 * @data: private data for the callback routine.
83 *
84 * Hides the details of the LRU cache etc from the filesystems.
85 *
86 * Returns: %0 on success, error return by @filler otherwise
87 */
88int read_cache_pages(struct address_space *mapping, struct list_head *pages,
89 int (*filler)(void *, struct page *), void *data)
90{
91 struct page *page;
92 int ret = 0;
93
94 while (!list_empty(pages)) {
95 page = lru_to_page(pages);
96 list_del(&page->lru);
97 if (add_to_page_cache_lru(page, mapping, page->index,
98 readahead_gfp_mask(mapping))) {
99 read_cache_pages_invalidate_page(mapping, page);
100 continue;
101 }
102 put_page(page);
103
104 ret = filler(data, page);
105 if (unlikely(ret)) {
106 read_cache_pages_invalidate_pages(mapping, pages);
107 break;
108 }
109 task_io_account_read(PAGE_SIZE);
110 }
111 return ret;
112}
113
114EXPORT_SYMBOL(read_cache_pages);
115
116static int read_pages(struct address_space *mapping, struct file *filp,
117 struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
118{
119 struct blk_plug plug;
120 unsigned page_idx;
121 int ret;
122
123 blk_start_plug(&plug);
124
125 if (mapping->a_ops->readpages) {
126 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
127 /* Clean up the remaining pages */
128 put_pages_list(pages);
129 goto out;
130 }
131
132 for (page_idx = 0; page_idx < nr_pages; page_idx++) {
133 struct page *page = lru_to_page(pages);
134 list_del(&page->lru);
135 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
136 mapping->a_ops->readpage(filp, page);
137 put_page(page);
138 }
139 ret = 0;
140
141out:
142 blk_finish_plug(&plug);
143
144 return ret;
145}
146
147/*
148 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates
149 * the pages first, then submits them for I/O. This avoids the very bad
150 * behaviour which would occur if page allocations are causing VM writeback.
151 * We really don't want to intermingle reads and writes like that.
152 *
153 * Returns the number of pages requested, or the maximum amount of I/O allowed.
154 */
155unsigned int __do_page_cache_readahead(struct address_space *mapping,
156 struct file *filp, pgoff_t offset, unsigned long nr_to_read,
157 unsigned long lookahead_size)
158{
159 struct inode *inode = mapping->host;
160 struct page *page;
161 unsigned long end_index; /* The last page we want to read */
162 LIST_HEAD(page_pool);
163 int page_idx;
164 unsigned int nr_pages = 0;
165 loff_t isize = i_size_read(inode);
166 gfp_t gfp_mask = readahead_gfp_mask(mapping);
167
168 if (isize == 0)
169 goto out;
170
171 end_index = ((isize - 1) >> PAGE_SHIFT);
172
173 /*
174 * Preallocate as many pages as we will need.
175 */
176 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
177 pgoff_t page_offset = offset + page_idx;
178
179 if (page_offset > end_index)
180 break;
181
182 page = xa_load(&mapping->i_pages, page_offset);
183 if (page && !xa_is_value(page)) {
184 /*
185 * Page already present? Kick off the current batch of
186 * contiguous pages before continuing with the next
187 * batch.
188 */
189 if (nr_pages)
190 read_pages(mapping, filp, &page_pool, nr_pages,
191 gfp_mask);
192 nr_pages = 0;
193 continue;
194 }
195
196 page = __page_cache_alloc(gfp_mask);
197 if (!page)
198 break;
199 page->index = page_offset;
200 list_add(&page->lru, &page_pool);
201 if (page_idx == nr_to_read - lookahead_size)
202 SetPageReadahead(page);
203 nr_pages++;
204 }
205
206 /*
207 * Now start the IO. We ignore I/O errors - if the page is not
208 * uptodate then the caller will launch readpage again, and
209 * will then handle the error.
210 */
211 if (nr_pages)
212 read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
213 BUG_ON(!list_empty(&page_pool));
214out:
215 return nr_pages;
216}
217
218/*
219 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
220 * memory at once.
221 */
222int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
223 pgoff_t offset, unsigned long nr_to_read)
224{
225 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
226 struct file_ra_state *ra = &filp->f_ra;
227 unsigned long max_pages;
228
229 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
230 return -EINVAL;
231
232 /*
233 * If the request exceeds the readahead window, allow the read to
234 * be up to the optimal hardware IO size
235 */
236 max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
237 nr_to_read = min(nr_to_read, max_pages);
238 while (nr_to_read) {
239 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
240
241 if (this_chunk > nr_to_read)
242 this_chunk = nr_to_read;
243 __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
244
245 offset += this_chunk;
246 nr_to_read -= this_chunk;
247 }
248 return 0;
249}
250
251/*
252 * Set the initial window size, round to next power of 2 and square
253 * for small size, x 4 for medium, and x 2 for large
254 * for 128k (32 page) max ra
255 * 1-8 page = 32k initial, > 8 page = 128k initial
256 */
257static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
258{
259 unsigned long newsize = roundup_pow_of_two(size);
260
261 if (newsize <= max / 32)
262 newsize = newsize * 4;
263 else if (newsize <= max / 4)
264 newsize = newsize * 2;
265 else
266 newsize = max;
267
268 return newsize;
269}
270
271/*
272 * Get the previous window size, ramp it up, and
273 * return it as the new window size.
274 */
275static unsigned long get_next_ra_size(struct file_ra_state *ra,
276 unsigned long max)
277{
278 unsigned long cur = ra->size;
279
280 if (cur < max / 16)
281 return 4 * cur;
282 if (cur <= max / 2)
283 return 2 * cur;
284 return max;
285}
286
287/*
288 * On-demand readahead design.
289 *
290 * The fields in struct file_ra_state represent the most-recently-executed
291 * readahead attempt:
292 *
293 * |<----- async_size ---------|
294 * |------------------- size -------------------->|
295 * |==================#===========================|
296 * ^start ^page marked with PG_readahead
297 *
298 * To overlap application thinking time and disk I/O time, we do
299 * `readahead pipelining': Do not wait until the application consumed all
300 * readahead pages and stalled on the missing page at readahead_index;
301 * Instead, submit an asynchronous readahead I/O as soon as there are
302 * only async_size pages left in the readahead window. Normally async_size
303 * will be equal to size, for maximum pipelining.
304 *
305 * In interleaved sequential reads, concurrent streams on the same fd can
306 * be invalidating each other's readahead state. So we flag the new readahead
307 * page at (start+size-async_size) with PG_readahead, and use it as readahead
308 * indicator. The flag won't be set on already cached pages, to avoid the
309 * readahead-for-nothing fuss, saving pointless page cache lookups.
310 *
311 * prev_pos tracks the last visited byte in the _previous_ read request.
312 * It should be maintained by the caller, and will be used for detecting
313 * small random reads. Note that the readahead algorithm checks loosely
314 * for sequential patterns. Hence interleaved reads might be served as
315 * sequential ones.
316 *
317 * There is a special-case: if the first page which the application tries to
318 * read happens to be the first page of the file, it is assumed that a linear
319 * read is about to happen and the window is immediately set to the initial size
320 * based on I/O request size and the max_readahead.
321 *
322 * The code ramps up the readahead size aggressively at first, but slow down as
323 * it approaches max_readhead.
324 */
325
326/*
327 * Count contiguously cached pages from @offset-1 to @offset-@max,
328 * this count is a conservative estimation of
329 * - length of the sequential read sequence, or
330 * - thrashing threshold in memory tight systems
331 */
332static pgoff_t count_history_pages(struct address_space *mapping,
333 pgoff_t offset, unsigned long max)
334{
335 pgoff_t head;
336
337 rcu_read_lock();
338 head = page_cache_prev_miss(mapping, offset - 1, max);
339 rcu_read_unlock();
340
341 return offset - 1 - head;
342}
343
344/*
345 * page cache context based read-ahead
346 */
347static int try_context_readahead(struct address_space *mapping,
348 struct file_ra_state *ra,
349 pgoff_t offset,
350 unsigned long req_size,
351 unsigned long max)
352{
353 pgoff_t size;
354
355 size = count_history_pages(mapping, offset, max);
356
357 /*
358 * not enough history pages:
359 * it could be a random read
360 */
361 if (size <= req_size)
362 return 0;
363
364 /*
365 * starts from beginning of file:
366 * it is a strong indication of long-run stream (or whole-file-read)
367 */
368 if (size >= offset)
369 size *= 2;
370
371 ra->start = offset;
372 ra->size = min(size + req_size, max);
373 ra->async_size = 1;
374
375 return 1;
376}
377
378/*
379 * A minimal readahead algorithm for trivial sequential/random reads.
380 */
381static unsigned long
382ondemand_readahead(struct address_space *mapping,
383 struct file_ra_state *ra, struct file *filp,
384 bool hit_readahead_marker, pgoff_t offset,
385 unsigned long req_size)
386{
387 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
388 unsigned long max_pages = ra->ra_pages;
389 unsigned long add_pages;
390 pgoff_t prev_offset;
391
392 /*
393 * If the request exceeds the readahead window, allow the read to
394 * be up to the optimal hardware IO size
395 */
396 if (req_size > max_pages && bdi->io_pages > max_pages)
397 max_pages = min(req_size, bdi->io_pages);
398
399 /*
400 * start of file
401 */
402 if (!offset)
403 goto initial_readahead;
404
405 /*
406 * It's the expected callback offset, assume sequential access.
407 * Ramp up sizes, and push forward the readahead window.
408 */
409 if ((offset == (ra->start + ra->size - ra->async_size) ||
410 offset == (ra->start + ra->size))) {
411 ra->start += ra->size;
412 ra->size = get_next_ra_size(ra, max_pages);
413 ra->async_size = ra->size;
414 goto readit;
415 }
416
417 /*
418 * Hit a marked page without valid readahead state.
419 * E.g. interleaved reads.
420 * Query the pagecache for async_size, which normally equals to
421 * readahead size. Ramp it up and use it as the new readahead size.
422 */
423 if (hit_readahead_marker) {
424 pgoff_t start;
425
426 rcu_read_lock();
427 start = page_cache_next_miss(mapping, offset + 1, max_pages);
428 rcu_read_unlock();
429
430 if (!start || start - offset > max_pages)
431 return 0;
432
433 ra->start = start;
434 ra->size = start - offset; /* old async_size */
435 ra->size += req_size;
436 ra->size = get_next_ra_size(ra, max_pages);
437 ra->async_size = ra->size;
438 goto readit;
439 }
440
441 /*
442 * oversize read
443 */
444 if (req_size > max_pages)
445 goto initial_readahead;
446
447 /*
448 * sequential cache miss
449 * trivial case: (offset - prev_offset) == 1
450 * unaligned reads: (offset - prev_offset) == 0
451 */
452 prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
453 if (offset - prev_offset <= 1UL)
454 goto initial_readahead;
455
456 /*
457 * Query the page cache and look for the traces(cached history pages)
458 * that a sequential stream would leave behind.
459 */
460 if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
461 goto readit;
462
463 /*
464 * standalone, small random read
465 * Read as is, and do not pollute the readahead state.
466 */
467 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
468
469initial_readahead:
470 ra->start = offset;
471 ra->size = get_init_ra_size(req_size, max_pages);
472 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
473
474readit:
475 /*
476 * Will this read hit the readahead marker made by itself?
477 * If so, trigger the readahead marker hit now, and merge
478 * the resulted next readahead window into the current one.
479 * Take care of maximum IO pages as above.
480 */
481 if (offset == ra->start && ra->size == ra->async_size) {
482 add_pages = get_next_ra_size(ra, max_pages);
483 if (ra->size + add_pages <= max_pages) {
484 ra->async_size = add_pages;
485 ra->size += add_pages;
486 } else {
487 ra->size = max_pages;
488 ra->async_size = max_pages >> 1;
489 }
490 }
491
492 return ra_submit(ra, mapping, filp);
493}
494
495/**
496 * page_cache_sync_readahead - generic file readahead
497 * @mapping: address_space which holds the pagecache and I/O vectors
498 * @ra: file_ra_state which holds the readahead state
499 * @filp: passed on to ->readpage() and ->readpages()
500 * @offset: start offset into @mapping, in pagecache page-sized units
501 * @req_size: hint: total size of the read which the caller is performing in
502 * pagecache pages
503 *
504 * page_cache_sync_readahead() should be called when a cache miss happened:
505 * it will submit the read. The readahead logic may decide to piggyback more
506 * pages onto the read request if access patterns suggest it will improve
507 * performance.
508 */
509void page_cache_sync_readahead(struct address_space *mapping,
510 struct file_ra_state *ra, struct file *filp,
511 pgoff_t offset, unsigned long req_size)
512{
513 /* no read-ahead */
514 if (!ra->ra_pages)
515 return;
516
517 if (blk_cgroup_congested())
518 return;
519
520 /* be dumb */
521 if (filp && (filp->f_mode & FMODE_RANDOM)) {
522 force_page_cache_readahead(mapping, filp, offset, req_size);
523 return;
524 }
525
526 /* do read-ahead */
527 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
528}
529EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
530
531/**
532 * page_cache_async_readahead - file readahead for marked pages
533 * @mapping: address_space which holds the pagecache and I/O vectors
534 * @ra: file_ra_state which holds the readahead state
535 * @filp: passed on to ->readpage() and ->readpages()
536 * @page: the page at @offset which has the PG_readahead flag set
537 * @offset: start offset into @mapping, in pagecache page-sized units
538 * @req_size: hint: total size of the read which the caller is performing in
539 * pagecache pages
540 *
541 * page_cache_async_readahead() should be called when a page is used which
542 * has the PG_readahead flag; this is a marker to suggest that the application
543 * has used up enough of the readahead window that we should start pulling in
544 * more pages.
545 */
546void
547page_cache_async_readahead(struct address_space *mapping,
548 struct file_ra_state *ra, struct file *filp,
549 struct page *page, pgoff_t offset,
550 unsigned long req_size)
551{
552 /* no read-ahead */
553 if (!ra->ra_pages)
554 return;
555
556 /*
557 * Same bit is used for PG_readahead and PG_reclaim.
558 */
559 if (PageWriteback(page))
560 return;
561
562 ClearPageReadahead(page);
563
564 /*
565 * Defer asynchronous read-ahead on IO congestion.
566 */
567 if (inode_read_congested(mapping->host))
568 return;
569
570 if (blk_cgroup_congested())
571 return;
572
573 /* do read-ahead */
574 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
575}
576EXPORT_SYMBOL_GPL(page_cache_async_readahead);
577
578ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
579{
580 ssize_t ret;
581 struct fd f;
582
583 ret = -EBADF;
584 f = fdget(fd);
585 if (!f.file || !(f.file->f_mode & FMODE_READ))
586 goto out;
587
588 /*
589 * The readahead() syscall is intended to run only on files
590 * that can execute readahead. If readahead is not possible
591 * on this file, then we must return -EINVAL.
592 */
593 ret = -EINVAL;
594 if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
595 !S_ISREG(file_inode(f.file)->i_mode))
596 goto out;
597
598 ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
599out:
600 fdput(f);
601 return ret;
602}
603
604SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
605{
606 return ksys_readahead(fd, offset, count);
607}