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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 | */ | |
28 | void | |
29 | file_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 | } | |
34 | EXPORT_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 | */ | |
43 | static 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_CACHE_SIZE); | |
51 | page->mapping = NULL; | |
52 | unlock_page(page); | |
53 | } | |
54 | page_cache_release(page); | |
55 | } | |
56 | ||
57 | /* | |
58 | * release a list of pages, invalidating them first if need be | |
59 | */ | |
60 | static 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 | */ | |
82 | int 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 | page_cache_release(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_CACHE_SIZE); | |
104 | } | |
105 | return ret; | |
106 | } | |
107 | ||
108 | EXPORT_SYMBOL(read_cache_pages); | |
109 | ||
110 | static 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 | page_cache_release(page); | |
134 | } | |
135 | ret = 0; | |
136 | ||
137 | out: | |
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 | */ | |
151 | int __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 && !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)); | |
201 | out: | |
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 | */ | |
209 | int 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_CACHE_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 | */ | |
240 | static 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 | */ | |
258 | static 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 | */ | |
317 | static 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 | */ | |
332 | static 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 | */ | |
366 | static unsigned long | |
367 | ondemand_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_CACHE_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 | ||
445 | initial_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 | ||
450 | readit: | |
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 | */ | |
478 | void 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 | } | |
495 | EXPORT_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 | */ | |
512 | void | |
513 | page_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 | } | |
539 | EXPORT_SYMBOL_GPL(page_cache_async_readahead); | |
540 | ||
541 | static ssize_t | |
542 | do_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 | ||
551 | SYSCALL_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_CACHE_SHIFT; | |
562 | pgoff_t end = (offset + count - 1) >> PAGE_CACHE_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 | } |