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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/*
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}