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