Contents of /alx-src/tags/kernel26-2.6.12-alx-r9/mm/readahead.c
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
File MIME type: text/plain
File size: 16141 byte(s)
Tag kernel26-2.6.12-alx-r9
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; |
161 | int ret = 0; |
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)) { |
174 | mapping->a_ops->readpage(filp, page); |
175 | if (!pagevec_add(&lru_pvec, page)) |
176 | __pagevec_lru_add(&lru_pvec); |
177 | } else { |
178 | page_cache_release(page); |
179 | } |
180 | } |
181 | pagevec_lru_add(&lru_pvec); |
182 | out: |
183 | return ret; |
184 | } |
185 | |
186 | /* |
187 | * Readahead design. |
188 | * |
189 | * The fields in struct file_ra_state represent the most-recently-executed |
190 | * readahead attempt: |
191 | * |
192 | * start: Page index at which we started the readahead |
193 | * size: Number of pages in that read |
194 | * Together, these form the "current window". |
195 | * Together, start and size represent the `readahead window'. |
196 | * prev_page: The page which the readahead algorithm most-recently inspected. |
197 | * It is mainly used to detect sequential file reading. |
198 | * If page_cache_readahead sees that it is again being called for |
199 | * a page which it just looked at, it can return immediately without |
200 | * making any state changes. |
201 | * ahead_start, |
202 | * ahead_size: Together, these form the "ahead window". |
203 | * ra_pages: The externally controlled max readahead for this fd. |
204 | * |
205 | * When readahead is in the off state (size == 0), readahead is disabled. |
206 | * In this state, prev_page is used to detect the resumption of sequential I/O. |
207 | * |
208 | * The readahead code manages two windows - the "current" and the "ahead" |
209 | * windows. The intent is that while the application is walking the pages |
210 | * in the current window, I/O is underway on the ahead window. When the |
211 | * current window is fully traversed, it is replaced by the ahead window |
212 | * and the ahead window is invalidated. When this copying happens, the |
213 | * new current window's pages are probably still locked. So |
214 | * we submit a new batch of I/O immediately, creating a new ahead window. |
215 | * |
216 | * So: |
217 | * |
218 | * ----|----------------|----------------|----- |
219 | * ^start ^start+size |
220 | * ^ahead_start ^ahead_start+ahead_size |
221 | * |
222 | * ^ When this page is read, we submit I/O for the |
223 | * ahead window. |
224 | * |
225 | * A `readahead hit' occurs when a read request is made against a page which is |
226 | * the next sequential page. Ahead window calculations are done only when it |
227 | * is time to submit a new IO. The code ramps up the size agressively at first, |
228 | * but slow down as it approaches max_readhead. |
229 | * |
230 | * Any seek/ramdom IO will result in readahead being turned off. It will resume |
231 | * at the first sequential access. |
232 | * |
233 | * There is a special-case: if the first page which the application tries to |
234 | * read happens to be the first page of the file, it is assumed that a linear |
235 | * read is about to happen and the window is immediately set to the initial size |
236 | * based on I/O request size and the max_readahead. |
237 | * |
238 | * This function is to be called for every read request, rather than when |
239 | * it is time to perform readahead. It is called only once for the entire I/O |
240 | * regardless of size unless readahead is unable to start enough I/O to satisfy |
241 | * the request (I/O request > max_readahead). |
242 | */ |
243 | |
244 | /* |
245 | * do_page_cache_readahead actually reads a chunk of disk. It allocates all |
246 | * the pages first, then submits them all for I/O. This avoids the very bad |
247 | * behaviour which would occur if page allocations are causing VM writeback. |
248 | * We really don't want to intermingle reads and writes like that. |
249 | * |
250 | * Returns the number of pages requested, or the maximum amount of I/O allowed. |
251 | * |
252 | * do_page_cache_readahead() returns -1 if it encountered request queue |
253 | * congestion. |
254 | */ |
255 | static int |
256 | __do_page_cache_readahead(struct address_space *mapping, struct file *filp, |
257 | unsigned long offset, unsigned long nr_to_read) |
258 | { |
259 | struct inode *inode = mapping->host; |
260 | struct page *page; |
261 | unsigned long end_index; /* The last page we want to read */ |
262 | LIST_HEAD(page_pool); |
263 | int page_idx; |
264 | int ret = 0; |
265 | loff_t isize = i_size_read(inode); |
266 | |
267 | if (isize == 0) |
268 | goto out; |
269 | |
270 | end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); |
271 | |
272 | /* |
273 | * Preallocate as many pages as we will need. |
274 | */ |
275 | read_lock_irq(&mapping->tree_lock); |
276 | for (page_idx = 0; page_idx < nr_to_read; page_idx++) { |
277 | unsigned long page_offset = offset + page_idx; |
278 | |
279 | if (page_offset > end_index) |
280 | break; |
281 | |
282 | page = radix_tree_lookup(&mapping->page_tree, page_offset); |
283 | if (page) |
284 | continue; |
285 | |
286 | read_unlock_irq(&mapping->tree_lock); |
287 | page = page_cache_alloc_cold(mapping); |
288 | read_lock_irq(&mapping->tree_lock); |
289 | if (!page) |
290 | break; |
291 | page->index = page_offset; |
292 | list_add(&page->lru, &page_pool); |
293 | ret++; |
294 | } |
295 | read_unlock_irq(&mapping->tree_lock); |
296 | |
297 | /* |
298 | * Now start the IO. We ignore I/O errors - if the page is not |
299 | * uptodate then the caller will launch readpage again, and |
300 | * will then handle the error. |
301 | */ |
302 | if (ret) |
303 | read_pages(mapping, filp, &page_pool, ret); |
304 | BUG_ON(!list_empty(&page_pool)); |
305 | out: |
306 | return ret; |
307 | } |
308 | |
309 | /* |
310 | * Chunk the readahead into 2 megabyte units, so that we don't pin too much |
311 | * memory at once. |
312 | */ |
313 | int force_page_cache_readahead(struct address_space *mapping, struct file *filp, |
314 | unsigned long offset, unsigned long nr_to_read) |
315 | { |
316 | int ret = 0; |
317 | |
318 | if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages)) |
319 | return -EINVAL; |
320 | |
321 | while (nr_to_read) { |
322 | int err; |
323 | |
324 | unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE; |
325 | |
326 | if (this_chunk > nr_to_read) |
327 | this_chunk = nr_to_read; |
328 | err = __do_page_cache_readahead(mapping, filp, |
329 | offset, this_chunk); |
330 | if (err < 0) { |
331 | ret = err; |
332 | break; |
333 | } |
334 | ret += err; |
335 | offset += this_chunk; |
336 | nr_to_read -= this_chunk; |
337 | } |
338 | return ret; |
339 | } |
340 | |
341 | /* |
342 | * Check how effective readahead is being. If the amount of started IO is |
343 | * less than expected then the file is partly or fully in pagecache and |
344 | * readahead isn't helping. |
345 | * |
346 | */ |
347 | static inline int check_ra_success(struct file_ra_state *ra, |
348 | unsigned long nr_to_read, unsigned long actual) |
349 | { |
350 | if (actual == 0) { |
351 | ra->cache_hit += nr_to_read; |
352 | if (ra->cache_hit >= VM_MAX_CACHE_HIT) { |
353 | ra_off(ra); |
354 | ra->flags |= RA_FLAG_INCACHE; |
355 | return 0; |
356 | } |
357 | } else { |
358 | ra->cache_hit=0; |
359 | } |
360 | return 1; |
361 | } |
362 | |
363 | /* |
364 | * This version skips the IO if the queue is read-congested, and will tell the |
365 | * block layer to abandon the readahead if request allocation would block. |
366 | * |
367 | * force_page_cache_readahead() will ignore queue congestion and will block on |
368 | * request queues. |
369 | */ |
370 | int do_page_cache_readahead(struct address_space *mapping, struct file *filp, |
371 | unsigned long offset, unsigned long nr_to_read) |
372 | { |
373 | if (bdi_read_congested(mapping->backing_dev_info)) |
374 | return -1; |
375 | |
376 | return __do_page_cache_readahead(mapping, filp, offset, nr_to_read); |
377 | } |
378 | |
379 | /* |
380 | * Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block' |
381 | * is set wait till the read completes. Otherwise attempt to read without |
382 | * blocking. |
383 | * Returns 1 meaning 'success' if read is succesfull without switching off |
384 | * readhaead mode. Otherwise return failure. |
385 | */ |
386 | static int |
387 | blockable_page_cache_readahead(struct address_space *mapping, struct file *filp, |
388 | unsigned long offset, unsigned long nr_to_read, |
389 | struct file_ra_state *ra, int block) |
390 | { |
391 | int actual; |
392 | |
393 | if (!block && bdi_read_congested(mapping->backing_dev_info)) |
394 | return 0; |
395 | |
396 | actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read); |
397 | |
398 | return check_ra_success(ra, nr_to_read, actual); |
399 | } |
400 | |
401 | static int make_ahead_window(struct address_space *mapping, struct file *filp, |
402 | struct file_ra_state *ra, int force) |
403 | { |
404 | int block, ret; |
405 | |
406 | ra->ahead_size = get_next_ra_size(ra); |
407 | ra->ahead_start = ra->start + ra->size; |
408 | |
409 | block = force || (ra->prev_page >= ra->ahead_start); |
410 | ret = blockable_page_cache_readahead(mapping, filp, |
411 | ra->ahead_start, ra->ahead_size, ra, block); |
412 | |
413 | if (!ret && !force) { |
414 | /* A read failure in blocking mode, implies pages are |
415 | * all cached. So we can safely assume we have taken |
416 | * care of all the pages requested in this call. |
417 | * A read failure in non-blocking mode, implies we are |
418 | * reading more pages than requested in this call. So |
419 | * we safely assume we have taken care of all the pages |
420 | * requested in this call. |
421 | * |
422 | * Just reset the ahead window in case we failed due to |
423 | * congestion. The ahead window will any way be closed |
424 | * in case we failed due to excessive page cache hits. |
425 | */ |
426 | ra->ahead_start = 0; |
427 | ra->ahead_size = 0; |
428 | } |
429 | |
430 | return ret; |
431 | } |
432 | |
433 | /* |
434 | * page_cache_readahead is the main function. If performs the adaptive |
435 | * readahead window size management and submits the readahead I/O. |
436 | */ |
437 | unsigned long |
438 | page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra, |
439 | struct file *filp, unsigned long offset, |
440 | unsigned long req_size) |
441 | { |
442 | unsigned long max, newsize; |
443 | int sequential; |
444 | |
445 | /* |
446 | * We avoid doing extra work and bogusly perturbing the readahead |
447 | * window expansion logic. |
448 | */ |
449 | if (offset == ra->prev_page && --req_size) |
450 | ++offset; |
451 | |
452 | /* Note that prev_page == -1 if it is a first read */ |
453 | sequential = (offset == ra->prev_page + 1); |
454 | ra->prev_page = offset; |
455 | |
456 | max = get_max_readahead(ra); |
457 | newsize = min(req_size, max); |
458 | |
459 | /* No readahead or sub-page sized read or file already in cache */ |
460 | if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE)) |
461 | goto out; |
462 | |
463 | ra->prev_page += newsize - 1; |
464 | |
465 | /* |
466 | * Special case - first read at start of file. We'll assume it's |
467 | * a whole-file read and grow the window fast. Or detect first |
468 | * sequential access |
469 | */ |
470 | if (sequential && ra->size == 0) { |
471 | ra->size = get_init_ra_size(newsize, max); |
472 | ra->start = offset; |
473 | if (!blockable_page_cache_readahead(mapping, filp, offset, |
474 | ra->size, ra, 1)) |
475 | goto out; |
476 | |
477 | /* |
478 | * If the request size is larger than our max readahead, we |
479 | * at least want to be sure that we get 2 IOs in flight and |
480 | * we know that we will definitly need the new I/O. |
481 | * once we do this, subsequent calls should be able to overlap |
482 | * IOs,* thus preventing stalls. so issue the ahead window |
483 | * immediately. |
484 | */ |
485 | if (req_size >= max) |
486 | make_ahead_window(mapping, filp, ra, 1); |
487 | |
488 | goto out; |
489 | } |
490 | |
491 | /* |
492 | * Now handle the random case: |
493 | * partial page reads and first access were handled above, |
494 | * so this must be the next page otherwise it is random |
495 | */ |
496 | if (!sequential) { |
497 | ra_off(ra); |
498 | blockable_page_cache_readahead(mapping, filp, offset, |
499 | newsize, ra, 1); |
500 | goto out; |
501 | } |
502 | |
503 | /* |
504 | * If we get here we are doing sequential IO and this was not the first |
505 | * occurence (ie we have an existing window) |
506 | */ |
507 | |
508 | if (ra->ahead_start == 0) { /* no ahead window yet */ |
509 | if (!make_ahead_window(mapping, filp, ra, 0)) |
510 | goto out; |
511 | } |
512 | /* |
513 | * Already have an ahead window, check if we crossed into it. |
514 | * If so, shift windows and issue a new ahead window. |
515 | * Only return the #pages that are in the current window, so that |
516 | * we get called back on the first page of the ahead window which |
517 | * will allow us to submit more IO. |
518 | */ |
519 | if (ra->prev_page >= ra->ahead_start) { |
520 | ra->start = ra->ahead_start; |
521 | ra->size = ra->ahead_size; |
522 | make_ahead_window(mapping, filp, ra, 0); |
523 | } |
524 | |
525 | out: |
526 | return ra->prev_page + 1; |
527 | } |
528 | |
529 | /* |
530 | * handle_ra_miss() is called when it is known that a page which should have |
531 | * been present in the pagecache (we just did some readahead there) was in fact |
532 | * not found. This will happen if it was evicted by the VM (readahead |
533 | * thrashing) |
534 | * |
535 | * Turn on the cache miss flag in the RA struct, this will cause the RA code |
536 | * to reduce the RA size on the next read. |
537 | */ |
538 | void handle_ra_miss(struct address_space *mapping, |
539 | struct file_ra_state *ra, pgoff_t offset) |
540 | { |
541 | ra->flags |= RA_FLAG_MISS; |
542 | ra->flags &= ~RA_FLAG_INCACHE; |
543 | } |
544 | |
545 | /* |
546 | * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a |
547 | * sensible upper limit. |
548 | */ |
549 | unsigned long max_sane_readahead(unsigned long nr) |
550 | { |
551 | unsigned long active; |
552 | unsigned long inactive; |
553 | unsigned long free; |
554 | |
555 | __get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id())); |
556 | return min(nr, (inactive + free) / 2); |
557 | } |