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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/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 | */ |
25 | void |
26 | file_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 | } |
31 | EXPORT_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 | */ |
42 | static 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 | */ |
59 | static 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 | */ |
81 | int 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 | |
107 | EXPORT_SYMBOL(read_cache_pages); |
108 | |
109 | static int read_pages(struct address_space *mapping, struct file *filp, |
110 | struct list_head *pages, unsigned nr_pages) |
111 | { |
112 | unsigned page_idx; |
113 | int ret; |
114 | |
115 | if (mapping->a_ops->readpages) { |
116 | ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages); |
117 | /* Clean up the remaining pages */ |
118 | put_pages_list(pages); |
119 | goto out; |
120 | } |
121 | |
122 | for (page_idx = 0; page_idx < nr_pages; page_idx++) { |
123 | struct page *page = list_to_page(pages); |
124 | list_del(&page->lru); |
125 | if (!add_to_page_cache_lru(page, mapping, |
126 | page->index, GFP_KERNEL)) { |
127 | mapping->a_ops->readpage(filp, page); |
128 | } |
129 | page_cache_release(page); |
130 | } |
131 | ret = 0; |
132 | out: |
133 | return ret; |
134 | } |
135 | |
136 | /* |
137 | * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all |
138 | * the pages first, then submits them all for I/O. This avoids the very bad |
139 | * behaviour which would occur if page allocations are causing VM writeback. |
140 | * We really don't want to intermingle reads and writes like that. |
141 | * |
142 | * Returns the number of pages requested, or the maximum amount of I/O allowed. |
143 | */ |
144 | static int |
145 | __do_page_cache_readahead(struct address_space *mapping, struct file *filp, |
146 | pgoff_t offset, unsigned long nr_to_read, |
147 | unsigned long lookahead_size) |
148 | { |
149 | struct inode *inode = mapping->host; |
150 | struct page *page; |
151 | unsigned long end_index; /* The last page we want to read */ |
152 | LIST_HEAD(page_pool); |
153 | int page_idx; |
154 | int ret = 0; |
155 | loff_t isize = i_size_read(inode); |
156 | |
157 | if (isize == 0) |
158 | goto out; |
159 | |
160 | end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); |
161 | |
162 | /* |
163 | * Preallocate as many pages as we will need. |
164 | */ |
165 | for (page_idx = 0; page_idx < nr_to_read; page_idx++) { |
166 | pgoff_t page_offset = offset + page_idx; |
167 | |
168 | if (page_offset > end_index) |
169 | break; |
170 | |
171 | rcu_read_lock(); |
172 | page = radix_tree_lookup(&mapping->page_tree, page_offset); |
173 | rcu_read_unlock(); |
174 | if (page) |
175 | continue; |
176 | |
177 | page = page_cache_alloc_cold(mapping); |
178 | if (!page) |
179 | break; |
180 | page->index = page_offset; |
181 | list_add(&page->lru, &page_pool); |
182 | if (page_idx == nr_to_read - lookahead_size) |
183 | SetPageReadahead(page); |
184 | ret++; |
185 | } |
186 | |
187 | /* |
188 | * Now start the IO. We ignore I/O errors - if the page is not |
189 | * uptodate then the caller will launch readpage again, and |
190 | * will then handle the error. |
191 | */ |
192 | if (ret) |
193 | read_pages(mapping, filp, &page_pool, ret); |
194 | BUG_ON(!list_empty(&page_pool)); |
195 | out: |
196 | return ret; |
197 | } |
198 | |
199 | /* |
200 | * Chunk the readahead into 2 megabyte units, so that we don't pin too much |
201 | * memory at once. |
202 | */ |
203 | int force_page_cache_readahead(struct address_space *mapping, struct file *filp, |
204 | pgoff_t offset, unsigned long nr_to_read) |
205 | { |
206 | int ret = 0; |
207 | |
208 | if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) |
209 | return -EINVAL; |
210 | |
211 | nr_to_read = max_sane_readahead(nr_to_read); |
212 | while (nr_to_read) { |
213 | int err; |
214 | |
215 | unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE; |
216 | |
217 | if (this_chunk > nr_to_read) |
218 | this_chunk = nr_to_read; |
219 | err = __do_page_cache_readahead(mapping, filp, |
220 | offset, this_chunk, 0); |
221 | if (err < 0) { |
222 | ret = err; |
223 | break; |
224 | } |
225 | ret += err; |
226 | offset += this_chunk; |
227 | nr_to_read -= this_chunk; |
228 | } |
229 | return ret; |
230 | } |
231 | |
232 | /* |
233 | * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a |
234 | * sensible upper limit. |
235 | */ |
236 | unsigned long max_sane_readahead(unsigned long nr) |
237 | { |
238 | return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE) |
239 | + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2); |
240 | } |
241 | |
242 | /* |
243 | * Submit IO for the read-ahead request in file_ra_state. |
244 | */ |
245 | unsigned long ra_submit(struct file_ra_state *ra, |
246 | struct address_space *mapping, struct file *filp) |
247 | { |
248 | int actual; |
249 | |
250 | actual = __do_page_cache_readahead(mapping, filp, |
251 | ra->start, ra->size, ra->async_size); |
252 | |
253 | return actual; |
254 | } |
255 | |
256 | /* |
257 | * Set the initial window size, round to next power of 2 and square |
258 | * for small size, x 4 for medium, and x 2 for large |
259 | * for 128k (32 page) max ra |
260 | * 1-8 page = 32k initial, > 8 page = 128k initial |
261 | */ |
262 | static unsigned long get_init_ra_size(unsigned long size, unsigned long max) |
263 | { |
264 | unsigned long newsize = roundup_pow_of_two(size); |
265 | |
266 | if (newsize <= max / 32) |
267 | newsize = newsize * 4; |
268 | else if (newsize <= max / 4) |
269 | newsize = newsize * 2; |
270 | else |
271 | newsize = max; |
272 | |
273 | return newsize; |
274 | } |
275 | |
276 | /* |
277 | * Get the previous window size, ramp it up, and |
278 | * return it as the new window size. |
279 | */ |
280 | static unsigned long get_next_ra_size(struct file_ra_state *ra, |
281 | unsigned long max) |
282 | { |
283 | unsigned long cur = ra->size; |
284 | unsigned long newsize; |
285 | |
286 | if (cur < max / 16) |
287 | newsize = 4 * cur; |
288 | else |
289 | newsize = 2 * cur; |
290 | |
291 | return min(newsize, max); |
292 | } |
293 | |
294 | /* |
295 | * On-demand readahead design. |
296 | * |
297 | * The fields in struct file_ra_state represent the most-recently-executed |
298 | * readahead attempt: |
299 | * |
300 | * |<----- async_size ---------| |
301 | * |------------------- size -------------------->| |
302 | * |==================#===========================| |
303 | * ^start ^page marked with PG_readahead |
304 | * |
305 | * To overlap application thinking time and disk I/O time, we do |
306 | * `readahead pipelining': Do not wait until the application consumed all |
307 | * readahead pages and stalled on the missing page at readahead_index; |
308 | * Instead, submit an asynchronous readahead I/O as soon as there are |
309 | * only async_size pages left in the readahead window. Normally async_size |
310 | * will be equal to size, for maximum pipelining. |
311 | * |
312 | * In interleaved sequential reads, concurrent streams on the same fd can |
313 | * be invalidating each other's readahead state. So we flag the new readahead |
314 | * page at (start+size-async_size) with PG_readahead, and use it as readahead |
315 | * indicator. The flag won't be set on already cached pages, to avoid the |
316 | * readahead-for-nothing fuss, saving pointless page cache lookups. |
317 | * |
318 | * prev_pos tracks the last visited byte in the _previous_ read request. |
319 | * It should be maintained by the caller, and will be used for detecting |
320 | * small random reads. Note that the readahead algorithm checks loosely |
321 | * for sequential patterns. Hence interleaved reads might be served as |
322 | * sequential ones. |
323 | * |
324 | * There is a special-case: if the first page which the application tries to |
325 | * read happens to be the first page of the file, it is assumed that a linear |
326 | * read is about to happen and the window is immediately set to the initial size |
327 | * based on I/O request size and the max_readahead. |
328 | * |
329 | * The code ramps up the readahead size aggressively at first, but slow down as |
330 | * it approaches max_readhead. |
331 | */ |
332 | |
333 | /* |
334 | * Count contiguously cached pages from @offset-1 to @offset-@max, |
335 | * this count is a conservative estimation of |
336 | * - length of the sequential read sequence, or |
337 | * - thrashing threshold in memory tight systems |
338 | */ |
339 | static pgoff_t count_history_pages(struct address_space *mapping, |
340 | struct file_ra_state *ra, |
341 | pgoff_t offset, unsigned long max) |
342 | { |
343 | pgoff_t head; |
344 | |
345 | rcu_read_lock(); |
346 | head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max); |
347 | rcu_read_unlock(); |
348 | |
349 | return offset - 1 - head; |
350 | } |
351 | |
352 | /* |
353 | * page cache context based read-ahead |
354 | */ |
355 | static int try_context_readahead(struct address_space *mapping, |
356 | struct file_ra_state *ra, |
357 | pgoff_t offset, |
358 | unsigned long req_size, |
359 | unsigned long max) |
360 | { |
361 | pgoff_t size; |
362 | |
363 | size = count_history_pages(mapping, ra, offset, max); |
364 | |
365 | /* |
366 | * no history pages: |
367 | * it could be a random read |
368 | */ |
369 | if (!size) |
370 | return 0; |
371 | |
372 | /* |
373 | * starts from beginning of file: |
374 | * it is a strong indication of long-run stream (or whole-file-read) |
375 | */ |
376 | if (size >= offset) |
377 | size *= 2; |
378 | |
379 | ra->start = offset; |
380 | ra->size = get_init_ra_size(size + req_size, max); |
381 | ra->async_size = ra->size; |
382 | |
383 | return 1; |
384 | } |
385 | |
386 | /* |
387 | * A minimal readahead algorithm for trivial sequential/random reads. |
388 | */ |
389 | static unsigned long |
390 | ondemand_readahead(struct address_space *mapping, |
391 | struct file_ra_state *ra, struct file *filp, |
392 | bool hit_readahead_marker, pgoff_t offset, |
393 | unsigned long req_size) |
394 | { |
395 | unsigned long max = max_sane_readahead(ra->ra_pages); |
396 | |
397 | /* |
398 | * start of file |
399 | */ |
400 | if (!offset) |
401 | goto initial_readahead; |
402 | |
403 | /* |
404 | * It's the expected callback offset, assume sequential access. |
405 | * Ramp up sizes, and push forward the readahead window. |
406 | */ |
407 | if ((offset == (ra->start + ra->size - ra->async_size) || |
408 | offset == (ra->start + ra->size))) { |
409 | ra->start += ra->size; |
410 | ra->size = get_next_ra_size(ra, max); |
411 | ra->async_size = ra->size; |
412 | goto readit; |
413 | } |
414 | |
415 | /* |
416 | * Hit a marked page without valid readahead state. |
417 | * E.g. interleaved reads. |
418 | * Query the pagecache for async_size, which normally equals to |
419 | * readahead size. Ramp it up and use it as the new readahead size. |
420 | */ |
421 | if (hit_readahead_marker) { |
422 | pgoff_t start; |
423 | |
424 | rcu_read_lock(); |
425 | start = radix_tree_next_hole(&mapping->page_tree, offset+1,max); |
426 | rcu_read_unlock(); |
427 | |
428 | if (!start || start - offset > max) |
429 | return 0; |
430 | |
431 | ra->start = start; |
432 | ra->size = start - offset; /* old async_size */ |
433 | ra->size += req_size; |
434 | ra->size = get_next_ra_size(ra, max); |
435 | ra->async_size = ra->size; |
436 | goto readit; |
437 | } |
438 | |
439 | /* |
440 | * oversize read |
441 | */ |
442 | if (req_size > max) |
443 | goto initial_readahead; |
444 | |
445 | /* |
446 | * sequential cache miss |
447 | */ |
448 | if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL) |
449 | goto initial_readahead; |
450 | |
451 | /* |
452 | * Query the page cache and look for the traces(cached history pages) |
453 | * that a sequential stream would leave behind. |
454 | */ |
455 | if (try_context_readahead(mapping, ra, offset, req_size, max)) |
456 | goto readit; |
457 | |
458 | /* |
459 | * standalone, small random read |
460 | * Read as is, and do not pollute the readahead state. |
461 | */ |
462 | return __do_page_cache_readahead(mapping, filp, offset, req_size, 0); |
463 | |
464 | initial_readahead: |
465 | ra->start = offset; |
466 | ra->size = get_init_ra_size(req_size, max); |
467 | ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size; |
468 | |
469 | readit: |
470 | /* |
471 | * Will this read hit the readahead marker made by itself? |
472 | * If so, trigger the readahead marker hit now, and merge |
473 | * the resulted next readahead window into the current one. |
474 | */ |
475 | if (offset == ra->start && ra->size == ra->async_size) { |
476 | ra->async_size = get_next_ra_size(ra, max); |
477 | ra->size += ra->async_size; |
478 | } |
479 | |
480 | return ra_submit(ra, mapping, filp); |
481 | } |
482 | |
483 | /** |
484 | * page_cache_sync_readahead - generic file readahead |
485 | * @mapping: address_space which holds the pagecache and I/O vectors |
486 | * @ra: file_ra_state which holds the readahead state |
487 | * @filp: passed on to ->readpage() and ->readpages() |
488 | * @offset: start offset into @mapping, in pagecache page-sized units |
489 | * @req_size: hint: total size of the read which the caller is performing in |
490 | * pagecache pages |
491 | * |
492 | * page_cache_sync_readahead() should be called when a cache miss happened: |
493 | * it will submit the read. The readahead logic may decide to piggyback more |
494 | * pages onto the read request if access patterns suggest it will improve |
495 | * performance. |
496 | */ |
497 | void page_cache_sync_readahead(struct address_space *mapping, |
498 | struct file_ra_state *ra, struct file *filp, |
499 | pgoff_t offset, unsigned long req_size) |
500 | { |
501 | /* no read-ahead */ |
502 | if (!ra->ra_pages) |
503 | return; |
504 | |
505 | /* be dumb */ |
506 | if (filp && (filp->f_mode & FMODE_RANDOM)) { |
507 | force_page_cache_readahead(mapping, filp, offset, req_size); |
508 | return; |
509 | } |
510 | |
511 | /* do read-ahead */ |
512 | ondemand_readahead(mapping, ra, filp, false, offset, req_size); |
513 | } |
514 | EXPORT_SYMBOL_GPL(page_cache_sync_readahead); |
515 | |
516 | /** |
517 | * page_cache_async_readahead - file readahead for marked pages |
518 | * @mapping: address_space which holds the pagecache and I/O vectors |
519 | * @ra: file_ra_state which holds the readahead state |
520 | * @filp: passed on to ->readpage() and ->readpages() |
521 | * @page: the page at @offset which has the PG_readahead flag set |
522 | * @offset: start offset into @mapping, in pagecache page-sized units |
523 | * @req_size: hint: total size of the read which the caller is performing in |
524 | * pagecache pages |
525 | * |
526 | * page_cache_async_readahead() should be called when a page is used which |
527 | * has the PG_readahead flag; this is a marker to suggest that the application |
528 | * has used up enough of the readahead window that we should start pulling in |
529 | * more pages. |
530 | */ |
531 | void |
532 | page_cache_async_readahead(struct address_space *mapping, |
533 | struct file_ra_state *ra, struct file *filp, |
534 | struct page *page, pgoff_t offset, |
535 | unsigned long req_size) |
536 | { |
537 | /* no read-ahead */ |
538 | if (!ra->ra_pages) |
539 | return; |
540 | |
541 | /* |
542 | * Same bit is used for PG_readahead and PG_reclaim. |
543 | */ |
544 | if (PageWriteback(page)) |
545 | return; |
546 | |
547 | ClearPageReadahead(page); |
548 | |
549 | /* |
550 | * Defer asynchronous read-ahead on IO congestion. |
551 | */ |
552 | if (bdi_read_congested(mapping->backing_dev_info)) |
553 | return; |
554 | |
555 | /* do read-ahead */ |
556 | ondemand_readahead(mapping, ra, filp, true, offset, req_size); |
557 | |
558 | #ifdef CONFIG_BLOCK |
559 | /* |
560 | * Normally the current page is !uptodate and lock_page() will be |
561 | * immediately called to implicitly unplug the device. However this |
562 | * is not always true for RAID conifgurations, where data arrives |
563 | * not strictly in their submission order. In this case we need to |
564 | * explicitly kick off the IO. |
565 | */ |
566 | if (PageUptodate(page)) |
567 | blk_run_backing_dev(mapping->backing_dev_info, NULL); |
568 | #endif |
569 | } |
570 | EXPORT_SYMBOL_GPL(page_cache_async_readahead); |
571 |
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