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1da177e4 LT |
1 | /* |
2 | * mm/readahead.c - address_space-level file readahead. | |
3 | * | |
4 | * Copyright (C) 2002, Linus Torvalds | |
5 | * | |
6 | * 09Apr2002 akpm@zip.com.au | |
7 | * Initial version. | |
8 | */ | |
9 | ||
10 | #include <linux/kernel.h> | |
11 | #include <linux/fs.h> | |
12 | #include <linux/mm.h> | |
13 | #include <linux/module.h> | |
14 | #include <linux/blkdev.h> | |
15 | #include <linux/backing-dev.h> | |
16 | #include <linux/pagevec.h> | |
17 | ||
18 | void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page) | |
19 | { | |
20 | } | |
21 | EXPORT_SYMBOL(default_unplug_io_fn); | |
22 | ||
23 | struct backing_dev_info default_backing_dev_info = { | |
24 | .ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE, | |
25 | .state = 0, | |
26 | .capabilities = BDI_CAP_MAP_COPY, | |
27 | .unplug_io_fn = default_unplug_io_fn, | |
28 | }; | |
29 | EXPORT_SYMBOL_GPL(default_backing_dev_info); | |
30 | ||
31 | /* | |
32 | * Initialise a struct file's readahead state. Assumes that the caller has | |
33 | * memset *ra to zero. | |
34 | */ | |
35 | void | |
36 | file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping) | |
37 | { | |
38 | ra->ra_pages = mapping->backing_dev_info->ra_pages; | |
39 | ra->prev_page = -1; | |
40 | } | |
41 | ||
42 | /* | |
43 | * Return max readahead size for this inode in number-of-pages. | |
44 | */ | |
45 | static inline unsigned long get_max_readahead(struct file_ra_state *ra) | |
46 | { | |
47 | return ra->ra_pages; | |
48 | } | |
49 | ||
50 | static inline unsigned long get_min_readahead(struct file_ra_state *ra) | |
51 | { | |
52 | return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE; | |
53 | } | |
54 | ||
55 | static inline void ra_off(struct file_ra_state *ra) | |
56 | { | |
57 | ra->start = 0; | |
58 | ra->flags = 0; | |
59 | ra->size = 0; | |
60 | ra->ahead_start = 0; | |
61 | ra->ahead_size = 0; | |
62 | return; | |
63 | } | |
64 | ||
65 | /* | |
66 | * Set the initial window size, round to next power of 2 and square | |
67 | * for small size, x 4 for medium, and x 2 for large | |
68 | * for 128k (32 page) max ra | |
69 | * 1-8 page = 32k initial, > 8 page = 128k initial | |
70 | */ | |
71 | static unsigned long get_init_ra_size(unsigned long size, unsigned long max) | |
72 | { | |
73 | unsigned long newsize = roundup_pow_of_two(size); | |
74 | ||
75 | if (newsize <= max / 64) | |
76 | newsize = newsize * newsize; | |
77 | else if (newsize <= max / 4) | |
78 | newsize = max / 4; | |
79 | else | |
80 | newsize = max; | |
81 | return newsize; | |
82 | } | |
83 | ||
84 | /* | |
85 | * Set the new window size, this is called only when I/O is to be submitted, | |
86 | * not for each call to readahead. If a cache miss occured, reduce next I/O | |
87 | * size, else increase depending on how close to max we are. | |
88 | */ | |
89 | static inline unsigned long get_next_ra_size(struct file_ra_state *ra) | |
90 | { | |
91 | unsigned long max = get_max_readahead(ra); | |
92 | unsigned long min = get_min_readahead(ra); | |
93 | unsigned long cur = ra->size; | |
94 | unsigned long newsize; | |
95 | ||
96 | if (ra->flags & RA_FLAG_MISS) { | |
97 | ra->flags &= ~RA_FLAG_MISS; | |
98 | newsize = max((cur - 2), min); | |
99 | } else if (cur < max / 16) { | |
100 | newsize = 4 * cur; | |
101 | } else { | |
102 | newsize = 2 * cur; | |
103 | } | |
104 | return min(newsize, max); | |
105 | } | |
106 | ||
107 | #define list_to_page(head) (list_entry((head)->prev, struct page, lru)) | |
108 | ||
109 | /** | |
110 | * read_cache_pages - populate an address space with some pages, and | |
111 | * start reads against them. | |
112 | * @mapping: the address_space | |
113 | * @pages: The address of a list_head which contains the target pages. These | |
114 | * pages have their ->index populated and are otherwise uninitialised. | |
115 | * @filler: callback routine for filling a single page. | |
116 | * @data: private data for the callback routine. | |
117 | * | |
118 | * Hides the details of the LRU cache etc from the filesystems. | |
119 | */ | |
120 | int read_cache_pages(struct address_space *mapping, struct list_head *pages, | |
121 | int (*filler)(void *, struct page *), void *data) | |
122 | { | |
123 | struct page *page; | |
124 | struct pagevec lru_pvec; | |
125 | int ret = 0; | |
126 | ||
127 | pagevec_init(&lru_pvec, 0); | |
128 | ||
129 | while (!list_empty(pages)) { | |
130 | page = list_to_page(pages); | |
131 | list_del(&page->lru); | |
132 | if (add_to_page_cache(page, mapping, page->index, GFP_KERNEL)) { | |
133 | page_cache_release(page); | |
134 | continue; | |
135 | } | |
136 | ret = filler(data, page); | |
137 | if (!pagevec_add(&lru_pvec, page)) | |
138 | __pagevec_lru_add(&lru_pvec); | |
139 | if (ret) { | |
140 | while (!list_empty(pages)) { | |
141 | struct page *victim; | |
142 | ||
143 | victim = list_to_page(pages); | |
144 | list_del(&victim->lru); | |
145 | page_cache_release(victim); | |
146 | } | |
147 | break; | |
148 | } | |
149 | } | |
150 | pagevec_lru_add(&lru_pvec); | |
151 | return ret; | |
152 | } | |
153 | ||
154 | EXPORT_SYMBOL(read_cache_pages); | |
155 | ||
156 | static int read_pages(struct address_space *mapping, struct file *filp, | |
157 | struct list_head *pages, unsigned nr_pages) | |
158 | { | |
159 | unsigned page_idx; | |
160 | struct pagevec lru_pvec; | |
994fc28c | 161 | int ret; |
1da177e4 LT |
162 | |
163 | if (mapping->a_ops->readpages) { | |
164 | ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages); | |
165 | goto out; | |
166 | } | |
167 | ||
168 | pagevec_init(&lru_pvec, 0); | |
169 | for (page_idx = 0; page_idx < nr_pages; page_idx++) { | |
170 | struct page *page = list_to_page(pages); | |
171 | list_del(&page->lru); | |
172 | if (!add_to_page_cache(page, mapping, | |
173 | page->index, GFP_KERNEL)) { | |
994fc28c ZB |
174 | ret = mapping->a_ops->readpage(filp, page); |
175 | if (ret != AOP_TRUNCATED_PAGE) { | |
176 | if (!pagevec_add(&lru_pvec, page)) | |
177 | __pagevec_lru_add(&lru_pvec); | |
178 | continue; | |
179 | } /* else fall through to release */ | |
1da177e4 | 180 | } |
994fc28c | 181 | page_cache_release(page); |
1da177e4 LT |
182 | } |
183 | pagevec_lru_add(&lru_pvec); | |
994fc28c | 184 | ret = 0; |
1da177e4 LT |
185 | out: |
186 | return ret; | |
187 | } | |
188 | ||
189 | /* | |
190 | * Readahead design. | |
191 | * | |
192 | * The fields in struct file_ra_state represent the most-recently-executed | |
193 | * readahead attempt: | |
194 | * | |
195 | * start: Page index at which we started the readahead | |
196 | * size: Number of pages in that read | |
197 | * Together, these form the "current window". | |
198 | * Together, start and size represent the `readahead window'. | |
199 | * prev_page: The page which the readahead algorithm most-recently inspected. | |
200 | * It is mainly used to detect sequential file reading. | |
201 | * If page_cache_readahead sees that it is again being called for | |
202 | * a page which it just looked at, it can return immediately without | |
203 | * making any state changes. | |
204 | * ahead_start, | |
205 | * ahead_size: Together, these form the "ahead window". | |
206 | * ra_pages: The externally controlled max readahead for this fd. | |
207 | * | |
208 | * When readahead is in the off state (size == 0), readahead is disabled. | |
209 | * In this state, prev_page is used to detect the resumption of sequential I/O. | |
210 | * | |
211 | * The readahead code manages two windows - the "current" and the "ahead" | |
212 | * windows. The intent is that while the application is walking the pages | |
213 | * in the current window, I/O is underway on the ahead window. When the | |
214 | * current window is fully traversed, it is replaced by the ahead window | |
215 | * and the ahead window is invalidated. When this copying happens, the | |
216 | * new current window's pages are probably still locked. So | |
217 | * we submit a new batch of I/O immediately, creating a new ahead window. | |
218 | * | |
219 | * So: | |
220 | * | |
221 | * ----|----------------|----------------|----- | |
222 | * ^start ^start+size | |
223 | * ^ahead_start ^ahead_start+ahead_size | |
224 | * | |
225 | * ^ When this page is read, we submit I/O for the | |
226 | * ahead window. | |
227 | * | |
228 | * A `readahead hit' occurs when a read request is made against a page which is | |
229 | * the next sequential page. Ahead window calculations are done only when it | |
230 | * is time to submit a new IO. The code ramps up the size agressively at first, | |
231 | * but slow down as it approaches max_readhead. | |
232 | * | |
233 | * Any seek/ramdom IO will result in readahead being turned off. It will resume | |
234 | * at the first sequential access. | |
235 | * | |
236 | * There is a special-case: if the first page which the application tries to | |
237 | * read happens to be the first page of the file, it is assumed that a linear | |
238 | * read is about to happen and the window is immediately set to the initial size | |
239 | * based on I/O request size and the max_readahead. | |
240 | * | |
241 | * This function is to be called for every read request, rather than when | |
242 | * it is time to perform readahead. It is called only once for the entire I/O | |
243 | * regardless of size unless readahead is unable to start enough I/O to satisfy | |
244 | * the request (I/O request > max_readahead). | |
245 | */ | |
246 | ||
247 | /* | |
248 | * do_page_cache_readahead actually reads a chunk of disk. It allocates all | |
249 | * the pages first, then submits them all for I/O. This avoids the very bad | |
250 | * behaviour which would occur if page allocations are causing VM writeback. | |
251 | * We really don't want to intermingle reads and writes like that. | |
252 | * | |
253 | * Returns the number of pages requested, or the maximum amount of I/O allowed. | |
254 | * | |
255 | * do_page_cache_readahead() returns -1 if it encountered request queue | |
256 | * congestion. | |
257 | */ | |
258 | static int | |
259 | __do_page_cache_readahead(struct address_space *mapping, struct file *filp, | |
7361f4d8 | 260 | pgoff_t offset, unsigned long nr_to_read) |
1da177e4 LT |
261 | { |
262 | struct inode *inode = mapping->host; | |
263 | struct page *page; | |
264 | unsigned long end_index; /* The last page we want to read */ | |
265 | LIST_HEAD(page_pool); | |
266 | int page_idx; | |
267 | int ret = 0; | |
268 | loff_t isize = i_size_read(inode); | |
269 | ||
270 | if (isize == 0) | |
271 | goto out; | |
272 | ||
273 | end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); | |
274 | ||
275 | /* | |
276 | * Preallocate as many pages as we will need. | |
277 | */ | |
278 | read_lock_irq(&mapping->tree_lock); | |
279 | for (page_idx = 0; page_idx < nr_to_read; page_idx++) { | |
7361f4d8 | 280 | pgoff_t page_offset = offset + page_idx; |
1da177e4 LT |
281 | |
282 | if (page_offset > end_index) | |
283 | break; | |
284 | ||
285 | page = radix_tree_lookup(&mapping->page_tree, page_offset); | |
286 | if (page) | |
287 | continue; | |
288 | ||
289 | read_unlock_irq(&mapping->tree_lock); | |
290 | page = page_cache_alloc_cold(mapping); | |
291 | read_lock_irq(&mapping->tree_lock); | |
292 | if (!page) | |
293 | break; | |
294 | page->index = page_offset; | |
295 | list_add(&page->lru, &page_pool); | |
296 | ret++; | |
297 | } | |
298 | read_unlock_irq(&mapping->tree_lock); | |
299 | ||
300 | /* | |
301 | * Now start the IO. We ignore I/O errors - if the page is not | |
302 | * uptodate then the caller will launch readpage again, and | |
303 | * will then handle the error. | |
304 | */ | |
305 | if (ret) | |
306 | read_pages(mapping, filp, &page_pool, ret); | |
307 | BUG_ON(!list_empty(&page_pool)); | |
308 | out: | |
309 | return ret; | |
310 | } | |
311 | ||
312 | /* | |
313 | * Chunk the readahead into 2 megabyte units, so that we don't pin too much | |
314 | * memory at once. | |
315 | */ | |
316 | int force_page_cache_readahead(struct address_space *mapping, struct file *filp, | |
7361f4d8 | 317 | pgoff_t offset, unsigned long nr_to_read) |
1da177e4 LT |
318 | { |
319 | int ret = 0; | |
320 | ||
321 | if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) | |
322 | return -EINVAL; | |
323 | ||
324 | while (nr_to_read) { | |
325 | int err; | |
326 | ||
327 | unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE; | |
328 | ||
329 | if (this_chunk > nr_to_read) | |
330 | this_chunk = nr_to_read; | |
331 | err = __do_page_cache_readahead(mapping, filp, | |
332 | offset, this_chunk); | |
333 | if (err < 0) { | |
334 | ret = err; | |
335 | break; | |
336 | } | |
337 | ret += err; | |
338 | offset += this_chunk; | |
339 | nr_to_read -= this_chunk; | |
340 | } | |
341 | return ret; | |
342 | } | |
343 | ||
344 | /* | |
345 | * Check how effective readahead is being. If the amount of started IO is | |
346 | * less than expected then the file is partly or fully in pagecache and | |
347 | * readahead isn't helping. | |
348 | * | |
349 | */ | |
350 | static inline int check_ra_success(struct file_ra_state *ra, | |
351 | unsigned long nr_to_read, unsigned long actual) | |
352 | { | |
353 | if (actual == 0) { | |
354 | ra->cache_hit += nr_to_read; | |
355 | if (ra->cache_hit >= VM_MAX_CACHE_HIT) { | |
356 | ra_off(ra); | |
357 | ra->flags |= RA_FLAG_INCACHE; | |
358 | return 0; | |
359 | } | |
360 | } else { | |
361 | ra->cache_hit=0; | |
362 | } | |
363 | return 1; | |
364 | } | |
365 | ||
366 | /* | |
367 | * This version skips the IO if the queue is read-congested, and will tell the | |
368 | * block layer to abandon the readahead if request allocation would block. | |
369 | * | |
370 | * force_page_cache_readahead() will ignore queue congestion and will block on | |
371 | * request queues. | |
372 | */ | |
373 | int do_page_cache_readahead(struct address_space *mapping, struct file *filp, | |
7361f4d8 | 374 | pgoff_t offset, unsigned long nr_to_read) |
1da177e4 LT |
375 | { |
376 | if (bdi_read_congested(mapping->backing_dev_info)) | |
377 | return -1; | |
378 | ||
379 | return __do_page_cache_readahead(mapping, filp, offset, nr_to_read); | |
380 | } | |
381 | ||
382 | /* | |
383 | * Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block' | |
384 | * is set wait till the read completes. Otherwise attempt to read without | |
385 | * blocking. | |
386 | * Returns 1 meaning 'success' if read is succesfull without switching off | |
387 | * readhaead mode. Otherwise return failure. | |
388 | */ | |
389 | static int | |
390 | blockable_page_cache_readahead(struct address_space *mapping, struct file *filp, | |
7361f4d8 | 391 | pgoff_t offset, unsigned long nr_to_read, |
1da177e4 LT |
392 | struct file_ra_state *ra, int block) |
393 | { | |
394 | int actual; | |
395 | ||
396 | if (!block && bdi_read_congested(mapping->backing_dev_info)) | |
397 | return 0; | |
398 | ||
399 | actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read); | |
400 | ||
401 | return check_ra_success(ra, nr_to_read, actual); | |
402 | } | |
403 | ||
404 | static int make_ahead_window(struct address_space *mapping, struct file *filp, | |
405 | struct file_ra_state *ra, int force) | |
406 | { | |
407 | int block, ret; | |
408 | ||
409 | ra->ahead_size = get_next_ra_size(ra); | |
410 | ra->ahead_start = ra->start + ra->size; | |
411 | ||
412 | block = force || (ra->prev_page >= ra->ahead_start); | |
413 | ret = blockable_page_cache_readahead(mapping, filp, | |
414 | ra->ahead_start, ra->ahead_size, ra, block); | |
415 | ||
416 | if (!ret && !force) { | |
417 | /* A read failure in blocking mode, implies pages are | |
418 | * all cached. So we can safely assume we have taken | |
419 | * care of all the pages requested in this call. | |
420 | * A read failure in non-blocking mode, implies we are | |
421 | * reading more pages than requested in this call. So | |
422 | * we safely assume we have taken care of all the pages | |
423 | * requested in this call. | |
424 | * | |
425 | * Just reset the ahead window in case we failed due to | |
426 | * congestion. The ahead window will any way be closed | |
427 | * in case we failed due to excessive page cache hits. | |
428 | */ | |
429 | ra->ahead_start = 0; | |
430 | ra->ahead_size = 0; | |
431 | } | |
432 | ||
433 | return ret; | |
434 | } | |
435 | ||
7361f4d8 AM |
436 | /** |
437 | * page_cache_readahead - generic adaptive readahead | |
438 | * @mapping: address_space which holds the pagecache and I/O vectors | |
439 | * @ra: file_ra_state which holds the readahead state | |
440 | * @filp: passed on to ->readpage() and ->readpages() | |
441 | * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units | |
442 | * @req_size: hint: total size of the read which the caller is performing in | |
443 | * PAGE_CACHE_SIZE units | |
444 | * | |
445 | * page_cache_readahead() is the main function. If performs the adaptive | |
1da177e4 | 446 | * readahead window size management and submits the readahead I/O. |
7361f4d8 AM |
447 | * |
448 | * Note that @filp is purely used for passing on to the ->readpage[s]() | |
449 | * handler: it may refer to a different file from @mapping (so we may not use | |
450 | * @filp->f_mapping or @filp->f_dentry->d_inode here). | |
451 | * Also, @ra may not be equal to &@filp->f_ra. | |
452 | * | |
1da177e4 LT |
453 | */ |
454 | unsigned long | |
455 | page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra, | |
7361f4d8 | 456 | struct file *filp, pgoff_t offset, unsigned long req_size) |
1da177e4 LT |
457 | { |
458 | unsigned long max, newsize; | |
459 | int sequential; | |
460 | ||
461 | /* | |
462 | * We avoid doing extra work and bogusly perturbing the readahead | |
463 | * window expansion logic. | |
464 | */ | |
465 | if (offset == ra->prev_page && --req_size) | |
466 | ++offset; | |
467 | ||
468 | /* Note that prev_page == -1 if it is a first read */ | |
469 | sequential = (offset == ra->prev_page + 1); | |
470 | ra->prev_page = offset; | |
471 | ||
472 | max = get_max_readahead(ra); | |
473 | newsize = min(req_size, max); | |
474 | ||
475 | /* No readahead or sub-page sized read or file already in cache */ | |
476 | if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE)) | |
477 | goto out; | |
478 | ||
479 | ra->prev_page += newsize - 1; | |
480 | ||
481 | /* | |
482 | * Special case - first read at start of file. We'll assume it's | |
483 | * a whole-file read and grow the window fast. Or detect first | |
484 | * sequential access | |
485 | */ | |
486 | if (sequential && ra->size == 0) { | |
487 | ra->size = get_init_ra_size(newsize, max); | |
488 | ra->start = offset; | |
489 | if (!blockable_page_cache_readahead(mapping, filp, offset, | |
490 | ra->size, ra, 1)) | |
491 | goto out; | |
492 | ||
493 | /* | |
494 | * If the request size is larger than our max readahead, we | |
495 | * at least want to be sure that we get 2 IOs in flight and | |
496 | * we know that we will definitly need the new I/O. | |
497 | * once we do this, subsequent calls should be able to overlap | |
498 | * IOs,* thus preventing stalls. so issue the ahead window | |
499 | * immediately. | |
500 | */ | |
501 | if (req_size >= max) | |
502 | make_ahead_window(mapping, filp, ra, 1); | |
503 | ||
504 | goto out; | |
505 | } | |
506 | ||
507 | /* | |
508 | * Now handle the random case: | |
509 | * partial page reads and first access were handled above, | |
510 | * so this must be the next page otherwise it is random | |
511 | */ | |
512 | if (!sequential) { | |
513 | ra_off(ra); | |
514 | blockable_page_cache_readahead(mapping, filp, offset, | |
515 | newsize, ra, 1); | |
516 | goto out; | |
517 | } | |
518 | ||
519 | /* | |
520 | * If we get here we are doing sequential IO and this was not the first | |
521 | * occurence (ie we have an existing window) | |
522 | */ | |
523 | ||
524 | if (ra->ahead_start == 0) { /* no ahead window yet */ | |
525 | if (!make_ahead_window(mapping, filp, ra, 0)) | |
526 | goto out; | |
527 | } | |
528 | /* | |
529 | * Already have an ahead window, check if we crossed into it. | |
530 | * If so, shift windows and issue a new ahead window. | |
531 | * Only return the #pages that are in the current window, so that | |
532 | * we get called back on the first page of the ahead window which | |
533 | * will allow us to submit more IO. | |
534 | */ | |
535 | if (ra->prev_page >= ra->ahead_start) { | |
536 | ra->start = ra->ahead_start; | |
537 | ra->size = ra->ahead_size; | |
538 | make_ahead_window(mapping, filp, ra, 0); | |
539 | } | |
540 | ||
541 | out: | |
542 | return ra->prev_page + 1; | |
543 | } | |
544 | ||
545 | /* | |
546 | * handle_ra_miss() is called when it is known that a page which should have | |
547 | * been present in the pagecache (we just did some readahead there) was in fact | |
548 | * not found. This will happen if it was evicted by the VM (readahead | |
549 | * thrashing) | |
550 | * | |
551 | * Turn on the cache miss flag in the RA struct, this will cause the RA code | |
552 | * to reduce the RA size on the next read. | |
553 | */ | |
554 | void handle_ra_miss(struct address_space *mapping, | |
555 | struct file_ra_state *ra, pgoff_t offset) | |
556 | { | |
557 | ra->flags |= RA_FLAG_MISS; | |
558 | ra->flags &= ~RA_FLAG_INCACHE; | |
3b30bbd9 | 559 | ra->cache_hit = 0; |
1da177e4 LT |
560 | } |
561 | ||
562 | /* | |
563 | * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a | |
564 | * sensible upper limit. | |
565 | */ | |
566 | unsigned long max_sane_readahead(unsigned long nr) | |
567 | { | |
568 | unsigned long active; | |
569 | unsigned long inactive; | |
570 | unsigned long free; | |
571 | ||
572 | __get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id())); | |
573 | return min(nr, (inactive + free) / 2); | |
574 | } |