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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/fs.h>
12#include <linux/gfp.h>
13#include <linux/mm.h>
14#include <linux/module.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
21/*
22 * Initialise a struct file's readahead state. Assumes that the caller has
23 * memset *ra to zero.
24 */
25void
26file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
27{
28 ra->ra_pages = mapping->backing_dev_info->ra_pages;
29 ra->prev_pos = -1;
30}
31EXPORT_SYMBOL_GPL(file_ra_state_init);
32
33#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
34
35/*
36 * see if a page needs releasing upon read_cache_pages() failure
37 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
38 * before calling, such as the NFS fs marking pages that are cached locally
39 * on disk, thus we need to give the fs a chance to clean up in the event of
40 * an error
41 */
42static void read_cache_pages_invalidate_page(struct address_space *mapping,
43 struct page *page)
44{
45 if (page_has_private(page)) {
46 if (!trylock_page(page))
47 BUG();
48 page->mapping = mapping;
49 do_invalidatepage(page, 0);
50 page->mapping = NULL;
51 unlock_page(page);
52 }
53 page_cache_release(page);
54}
55
56/*
57 * release a list of pages, invalidating them first if need be
58 */
59static void read_cache_pages_invalidate_pages(struct address_space *mapping,
60 struct list_head *pages)
61{
62 struct page *victim;
63
64 while (!list_empty(pages)) {
65 victim = list_to_page(pages);
66 list_del(&victim->lru);
67 read_cache_pages_invalidate_page(mapping, victim);
68 }
69}
70
71/**
72 * read_cache_pages - populate an address space with some pages & start reads against them
73 * @mapping: the address_space
74 * @pages: The address of a list_head which contains the target pages. These
75 * pages have their ->index populated and are otherwise uninitialised.
76 * @filler: callback routine for filling a single page.
77 * @data: private data for the callback routine.
78 *
79 * Hides the details of the LRU cache etc from the filesystems.
80 */
81int read_cache_pages(struct address_space *mapping, struct list_head *pages,
82 int (*filler)(void *, struct page *), void *data)
83{
84 struct page *page;
85 int ret = 0;
86
87 while (!list_empty(pages)) {
88 page = list_to_page(pages);
89 list_del(&page->lru);
90 if (add_to_page_cache_lru(page, mapping,
91 page->index, GFP_KERNEL)) {
92 read_cache_pages_invalidate_page(mapping, page);
93 continue;
94 }
95 page_cache_release(page);
96
97 ret = filler(data, page);
98 if (unlikely(ret)) {
99 read_cache_pages_invalidate_pages(mapping, pages);
100 break;
101 }
102 task_io_account_read(PAGE_CACHE_SIZE);
103 }
104 return ret;
105}
106
107EXPORT_SYMBOL(read_cache_pages);
108
109static int read_pages(struct address_space *mapping, struct file *filp,
110 struct list_head *pages, unsigned nr_pages)
111{
112 struct blk_plug plug;
113 unsigned page_idx;
114 int ret;
115
116 blk_start_plug(&plug);
117
118 if (mapping->a_ops->readpages) {
119 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
120 /* Clean up the remaining pages */
121 put_pages_list(pages);
122 goto out;
123 }
124
125 for (page_idx = 0; page_idx < nr_pages; page_idx++) {
126 struct page *page = list_to_page(pages);
127 list_del(&page->lru);
128 if (!add_to_page_cache_lru(page, mapping,
129 page->index, GFP_KERNEL)) {
130 mapping->a_ops->readpage(filp, page);
131 }
132 page_cache_release(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 */
150static int
151__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
152 pgoff_t offset, unsigned long nr_to_read,
153 unsigned long lookahead_size)
154{
155 struct inode *inode = mapping->host;
156 struct page *page;
157 unsigned long end_index; /* The last page we want to read */
158 LIST_HEAD(page_pool);
159 int page_idx;
160 int ret = 0;
161 loff_t isize = i_size_read(inode);
162
163 if (isize == 0)
164 goto out;
165
166 end_index = ((isize - 1) >> PAGE_CACHE_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->page_tree, page_offset);
179 rcu_read_unlock();
180 if (page)
181 continue;
182
183 page = page_cache_alloc_readahead(mapping);
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);
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 int ret = 0;
213
214 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
215 return -EINVAL;
216
217 nr_to_read = max_sane_readahead(nr_to_read);
218 while (nr_to_read) {
219 int err;
220
221 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
222
223 if (this_chunk > nr_to_read)
224 this_chunk = nr_to_read;
225 err = __do_page_cache_readahead(mapping, filp,
226 offset, this_chunk, 0);
227 if (err < 0) {
228 ret = err;
229 break;
230 }
231 ret += err;
232 offset += this_chunk;
233 nr_to_read -= this_chunk;
234 }
235 return ret;
236}
237
238/*
239 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
240 * sensible upper limit.
241 */
242unsigned long max_sane_readahead(unsigned long nr)
243{
244 return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
245 + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
246}
247
248/*
249 * Submit IO for the read-ahead request in file_ra_state.
250 */
251unsigned long ra_submit(struct file_ra_state *ra,
252 struct address_space *mapping, struct file *filp)
253{
254 int actual;
255
256 actual = __do_page_cache_readahead(mapping, filp,
257 ra->start, ra->size, ra->async_size);
258
259 return actual;
260}
261
262/*
263 * Set the initial window size, round to next power of 2 and square
264 * for small size, x 4 for medium, and x 2 for large
265 * for 128k (32 page) max ra
266 * 1-8 page = 32k initial, > 8 page = 128k initial
267 */
268static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
269{
270 unsigned long newsize = roundup_pow_of_two(size);
271
272 if (newsize <= max / 32)
273 newsize = newsize * 4;
274 else if (newsize <= max / 4)
275 newsize = newsize * 2;
276 else
277 newsize = max;
278
279 return newsize;
280}
281
282/*
283 * Get the previous window size, ramp it up, and
284 * return it as the new window size.
285 */
286static unsigned long get_next_ra_size(struct file_ra_state *ra,
287 unsigned long max)
288{
289 unsigned long cur = ra->size;
290 unsigned long newsize;
291
292 if (cur < max / 16)
293 newsize = 4 * cur;
294 else
295 newsize = 2 * cur;
296
297 return min(newsize, max);
298}
299
300/*
301 * On-demand readahead design.
302 *
303 * The fields in struct file_ra_state represent the most-recently-executed
304 * readahead attempt:
305 *
306 * |<----- async_size ---------|
307 * |------------------- size -------------------->|
308 * |==================#===========================|
309 * ^start ^page marked with PG_readahead
310 *
311 * To overlap application thinking time and disk I/O time, we do
312 * `readahead pipelining': Do not wait until the application consumed all
313 * readahead pages and stalled on the missing page at readahead_index;
314 * Instead, submit an asynchronous readahead I/O as soon as there are
315 * only async_size pages left in the readahead window. Normally async_size
316 * will be equal to size, for maximum pipelining.
317 *
318 * In interleaved sequential reads, concurrent streams on the same fd can
319 * be invalidating each other's readahead state. So we flag the new readahead
320 * page at (start+size-async_size) with PG_readahead, and use it as readahead
321 * indicator. The flag won't be set on already cached pages, to avoid the
322 * readahead-for-nothing fuss, saving pointless page cache lookups.
323 *
324 * prev_pos tracks the last visited byte in the _previous_ read request.
325 * It should be maintained by the caller, and will be used for detecting
326 * small random reads. Note that the readahead algorithm checks loosely
327 * for sequential patterns. Hence interleaved reads might be served as
328 * sequential ones.
329 *
330 * There is a special-case: if the first page which the application tries to
331 * read happens to be the first page of the file, it is assumed that a linear
332 * read is about to happen and the window is immediately set to the initial size
333 * based on I/O request size and the max_readahead.
334 *
335 * The code ramps up the readahead size aggressively at first, but slow down as
336 * it approaches max_readhead.
337 */
338
339/*
340 * Count contiguously cached pages from @offset-1 to @offset-@max,
341 * this count is a conservative estimation of
342 * - length of the sequential read sequence, or
343 * - thrashing threshold in memory tight systems
344 */
345static pgoff_t count_history_pages(struct address_space *mapping,
346 struct file_ra_state *ra,
347 pgoff_t offset, unsigned long max)
348{
349 pgoff_t head;
350
351 rcu_read_lock();
352 head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
353 rcu_read_unlock();
354
355 return offset - 1 - head;
356}
357
358/*
359 * page cache context based read-ahead
360 */
361static int try_context_readahead(struct address_space *mapping,
362 struct file_ra_state *ra,
363 pgoff_t offset,
364 unsigned long req_size,
365 unsigned long max)
366{
367 pgoff_t size;
368
369 size = count_history_pages(mapping, ra, offset, max);
370
371 /*
372 * no history pages:
373 * it could be a random read
374 */
375 if (!size)
376 return 0;
377
378 /*
379 * starts from beginning of file:
380 * it is a strong indication of long-run stream (or whole-file-read)
381 */
382 if (size >= offset)
383 size *= 2;
384
385 ra->start = offset;
386 ra->size = get_init_ra_size(size + req_size, max);
387 ra->async_size = ra->size;
388
389 return 1;
390}
391
392/*
393 * A minimal readahead algorithm for trivial sequential/random reads.
394 */
395static unsigned long
396ondemand_readahead(struct address_space *mapping,
397 struct file_ra_state *ra, struct file *filp,
398 bool hit_readahead_marker, pgoff_t offset,
399 unsigned long req_size)
400{
401 unsigned long max = max_sane_readahead(ra->ra_pages);
402
403 /*
404 * start of file
405 */
406 if (!offset)
407 goto initial_readahead;
408
409 /*
410 * It's the expected callback offset, assume sequential access.
411 * Ramp up sizes, and push forward the readahead window.
412 */
413 if ((offset == (ra->start + ra->size - ra->async_size) ||
414 offset == (ra->start + ra->size))) {
415 ra->start += ra->size;
416 ra->size = get_next_ra_size(ra, max);
417 ra->async_size = ra->size;
418 goto readit;
419 }
420
421 /*
422 * Hit a marked page without valid readahead state.
423 * E.g. interleaved reads.
424 * Query the pagecache for async_size, which normally equals to
425 * readahead size. Ramp it up and use it as the new readahead size.
426 */
427 if (hit_readahead_marker) {
428 pgoff_t start;
429
430 rcu_read_lock();
431 start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
432 rcu_read_unlock();
433
434 if (!start || start - offset > max)
435 return 0;
436
437 ra->start = start;
438 ra->size = start - offset; /* old async_size */
439 ra->size += req_size;
440 ra->size = get_next_ra_size(ra, max);
441 ra->async_size = ra->size;
442 goto readit;
443 }
444
445 /*
446 * oversize read
447 */
448 if (req_size > max)
449 goto initial_readahead;
450
451 /*
452 * sequential cache miss
453 */
454 if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
455 goto initial_readahead;
456
457 /*
458 * Query the page cache and look for the traces(cached history pages)
459 * that a sequential stream would leave behind.
460 */
461 if (try_context_readahead(mapping, ra, offset, req_size, max))
462 goto readit;
463
464 /*
465 * standalone, small random read
466 * Read as is, and do not pollute the readahead state.
467 */
468 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
469
470initial_readahead:
471 ra->start = offset;
472 ra->size = get_init_ra_size(req_size, max);
473 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
474
475readit:
476 /*
477 * Will this read hit the readahead marker made by itself?
478 * If so, trigger the readahead marker hit now, and merge
479 * the resulted next readahead window into the current one.
480 */
481 if (offset == ra->start && ra->size == ra->async_size) {
482 ra->async_size = get_next_ra_size(ra, max);
483 ra->size += ra->async_size;
484 }
485
486 return ra_submit(ra, mapping, filp);
487}
488
489/**
490 * page_cache_sync_readahead - generic file readahead
491 * @mapping: address_space which holds the pagecache and I/O vectors
492 * @ra: file_ra_state which holds the readahead state
493 * @filp: passed on to ->readpage() and ->readpages()
494 * @offset: start offset into @mapping, in pagecache page-sized units
495 * @req_size: hint: total size of the read which the caller is performing in
496 * pagecache pages
497 *
498 * page_cache_sync_readahead() should be called when a cache miss happened:
499 * it will submit the read. The readahead logic may decide to piggyback more
500 * pages onto the read request if access patterns suggest it will improve
501 * performance.
502 */
503void page_cache_sync_readahead(struct address_space *mapping,
504 struct file_ra_state *ra, struct file *filp,
505 pgoff_t offset, unsigned long req_size)
506{
507 /* no read-ahead */
508 if (!ra->ra_pages)
509 return;
510
511 /* be dumb */
512 if (filp && (filp->f_mode & FMODE_RANDOM)) {
513 force_page_cache_readahead(mapping, filp, offset, req_size);
514 return;
515 }
516
517 /* do read-ahead */
518 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
519}
520EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
521
522/**
523 * page_cache_async_readahead - file readahead for marked pages
524 * @mapping: address_space which holds the pagecache and I/O vectors
525 * @ra: file_ra_state which holds the readahead state
526 * @filp: passed on to ->readpage() and ->readpages()
527 * @page: the page at @offset which has the PG_readahead flag set
528 * @offset: start offset into @mapping, in pagecache page-sized units
529 * @req_size: hint: total size of the read which the caller is performing in
530 * pagecache pages
531 *
532 * page_cache_async_readahead() should be called when a page is used which
533 * has the PG_readahead flag; this is a marker to suggest that the application
534 * has used up enough of the readahead window that we should start pulling in
535 * more pages.
536 */
537void
538page_cache_async_readahead(struct address_space *mapping,
539 struct file_ra_state *ra, struct file *filp,
540 struct page *page, pgoff_t offset,
541 unsigned long req_size)
542{
543 /* no read-ahead */
544 if (!ra->ra_pages)
545 return;
546
547 /*
548 * Same bit is used for PG_readahead and PG_reclaim.
549 */
550 if (PageWriteback(page))
551 return;
552
553 ClearPageReadahead(page);
554
555 /*
556 * Defer asynchronous read-ahead on IO congestion.
557 */
558 if (bdi_read_congested(mapping->backing_dev_info))
559 return;
560
561 /* do read-ahead */
562 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
563}
564EXPORT_SYMBOL_GPL(page_cache_async_readahead);