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/mm.h>
13 #include <linux/module.h>
14 #include <linux/blkdev.h>
15 #include <linux/backing-dev.h>
16 #include <linux/task_io_accounting_ops.h>
17 #include <linux/pagevec.h>
18 #include <linux/pagemap.h>
19
20 /*
21 * Initialise a struct file's readahead state. Assumes that the caller has
22 * memset *ra to zero.
23 */
24 void
25 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
26 {
27 ra->ra_pages = mapping->backing_dev_info->ra_pages;
28 ra->prev_pos = -1;
29 }
30 EXPORT_SYMBOL_GPL(file_ra_state_init);
31
32 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
33
34 /*
35 * see if a page needs releasing upon read_cache_pages() failure
36 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
37 * before calling, such as the NFS fs marking pages that are cached locally
38 * on disk, thus we need to give the fs a chance to clean up in the event of
39 * an error
40 */
41 static void read_cache_pages_invalidate_page(struct address_space *mapping,
42 struct page *page)
43 {
44 if (page_has_private(page)) {
45 if (!trylock_page(page))
46 BUG();
47 page->mapping = mapping;
48 do_invalidatepage(page, 0);
49 page->mapping = NULL;
50 unlock_page(page);
51 }
52 page_cache_release(page);
53 }
54
55 /*
56 * release a list of pages, invalidating them first if need be
57 */
58 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
59 struct list_head *pages)
60 {
61 struct page *victim;
62
63 while (!list_empty(pages)) {
64 victim = list_to_page(pages);
65 list_del(&victim->lru);
66 read_cache_pages_invalidate_page(mapping, victim);
67 }
68 }
69
70 /**
71 * read_cache_pages - populate an address space with some pages & start reads against them
72 * @mapping: the address_space
73 * @pages: The address of a list_head which contains the target pages. These
74 * pages have their ->index populated and are otherwise uninitialised.
75 * @filler: callback routine for filling a single page.
76 * @data: private data for the callback routine.
77 *
78 * Hides the details of the LRU cache etc from the filesystems.
79 */
80 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
81 int (*filler)(void *, struct page *), void *data)
82 {
83 struct page *page;
84 int ret = 0;
85
86 while (!list_empty(pages)) {
87 page = list_to_page(pages);
88 list_del(&page->lru);
89 if (add_to_page_cache_lru(page, mapping,
90 page->index, GFP_KERNEL)) {
91 read_cache_pages_invalidate_page(mapping, page);
92 continue;
93 }
94 page_cache_release(page);
95
96 ret = filler(data, page);
97 if (unlikely(ret)) {
98 read_cache_pages_invalidate_pages(mapping, pages);
99 break;
100 }
101 task_io_account_read(PAGE_CACHE_SIZE);
102 }
103 return ret;
104 }
105
106 EXPORT_SYMBOL(read_cache_pages);
107
108 static int read_pages(struct address_space *mapping, struct file *filp,
109 struct list_head *pages, unsigned nr_pages)
110 {
111 unsigned page_idx;
112 int ret;
113
114 if (mapping->a_ops->readpages) {
115 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
116 /* Clean up the remaining pages */
117 put_pages_list(pages);
118 goto out;
119 }
120
121 for (page_idx = 0; page_idx < nr_pages; page_idx++) {
122 struct page *page = list_to_page(pages);
123 list_del(&page->lru);
124 if (!add_to_page_cache_lru(page, mapping,
125 page->index, GFP_KERNEL)) {
126 mapping->a_ops->readpage(filp, page);
127 }
128 page_cache_release(page);
129 }
130 ret = 0;
131 out:
132 return ret;
133 }
134
135 /*
136 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all
137 * the pages first, then submits them all for I/O. This avoids the very bad
138 * behaviour which would occur if page allocations are causing VM writeback.
139 * We really don't want to intermingle reads and writes like that.
140 *
141 * Returns the number of pages requested, or the maximum amount of I/O allowed.
142 */
143 static int
144 __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
145 pgoff_t offset, unsigned long nr_to_read,
146 unsigned long lookahead_size)
147 {
148 struct inode *inode = mapping->host;
149 struct page *page;
150 unsigned long end_index; /* The last page we want to read */
151 LIST_HEAD(page_pool);
152 int page_idx;
153 int ret = 0;
154 loff_t isize = i_size_read(inode);
155
156 if (isize == 0)
157 goto out;
158
159 end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
160
161 /*
162 * Preallocate as many pages as we will need.
163 */
164 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
165 pgoff_t page_offset = offset + page_idx;
166
167 if (page_offset > end_index)
168 break;
169
170 rcu_read_lock();
171 page = radix_tree_lookup(&mapping->page_tree, page_offset);
172 rcu_read_unlock();
173 if (page)
174 continue;
175
176 page = page_cache_alloc_cold(mapping);
177 if (!page)
178 break;
179 page->index = page_offset;
180 list_add(&page->lru, &page_pool);
181 if (page_idx == nr_to_read - lookahead_size)
182 SetPageReadahead(page);
183 ret++;
184 }
185
186 /*
187 * Now start the IO. We ignore I/O errors - if the page is not
188 * uptodate then the caller will launch readpage again, and
189 * will then handle the error.
190 */
191 if (ret)
192 read_pages(mapping, filp, &page_pool, ret);
193 BUG_ON(!list_empty(&page_pool));
194 out:
195 return ret;
196 }
197
198 /*
199 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
200 * memory at once.
201 */
202 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
203 pgoff_t offset, unsigned long nr_to_read)
204 {
205 int ret = 0;
206
207 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
208 return -EINVAL;
209
210 nr_to_read = max_sane_readahead(nr_to_read);
211 while (nr_to_read) {
212 int err;
213
214 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
215
216 if (this_chunk > nr_to_read)
217 this_chunk = nr_to_read;
218 err = __do_page_cache_readahead(mapping, filp,
219 offset, this_chunk, 0);
220 if (err < 0) {
221 ret = err;
222 break;
223 }
224 ret += err;
225 offset += this_chunk;
226 nr_to_read -= this_chunk;
227 }
228 return ret;
229 }
230
231 /*
232 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
233 * sensible upper limit.
234 */
235 unsigned long max_sane_readahead(unsigned long nr)
236 {
237 return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
238 + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
239 }
240
241 /*
242 * Submit IO for the read-ahead request in file_ra_state.
243 */
244 unsigned long ra_submit(struct file_ra_state *ra,
245 struct address_space *mapping, struct file *filp)
246 {
247 int actual;
248
249 actual = __do_page_cache_readahead(mapping, filp,
250 ra->start, ra->size, ra->async_size);
251
252 return actual;
253 }
254
255 /*
256 * Set the initial window size, round to next power of 2 and square
257 * for small size, x 4 for medium, and x 2 for large
258 * for 128k (32 page) max ra
259 * 1-8 page = 32k initial, > 8 page = 128k initial
260 */
261 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
262 {
263 unsigned long newsize = roundup_pow_of_two(size);
264
265 if (newsize <= max / 32)
266 newsize = newsize * 4;
267 else if (newsize <= max / 4)
268 newsize = newsize * 2;
269 else
270 newsize = max;
271
272 return newsize;
273 }
274
275 /*
276 * Get the previous window size, ramp it up, and
277 * return it as the new window size.
278 */
279 static unsigned long get_next_ra_size(struct file_ra_state *ra,
280 unsigned long max)
281 {
282 unsigned long cur = ra->size;
283 unsigned long newsize;
284
285 if (cur < max / 16)
286 newsize = 4 * cur;
287 else
288 newsize = 2 * cur;
289
290 return min(newsize, max);
291 }
292
293 /*
294 * On-demand readahead design.
295 *
296 * The fields in struct file_ra_state represent the most-recently-executed
297 * readahead attempt:
298 *
299 * |<----- async_size ---------|
300 * |------------------- size -------------------->|
301 * |==================#===========================|
302 * ^start ^page marked with PG_readahead
303 *
304 * To overlap application thinking time and disk I/O time, we do
305 * `readahead pipelining': Do not wait until the application consumed all
306 * readahead pages and stalled on the missing page at readahead_index;
307 * Instead, submit an asynchronous readahead I/O as soon as there are
308 * only async_size pages left in the readahead window. Normally async_size
309 * will be equal to size, for maximum pipelining.
310 *
311 * In interleaved sequential reads, concurrent streams on the same fd can
312 * be invalidating each other's readahead state. So we flag the new readahead
313 * page at (start+size-async_size) with PG_readahead, and use it as readahead
314 * indicator. The flag won't be set on already cached pages, to avoid the
315 * readahead-for-nothing fuss, saving pointless page cache lookups.
316 *
317 * prev_pos tracks the last visited byte in the _previous_ read request.
318 * It should be maintained by the caller, and will be used for detecting
319 * small random reads. Note that the readahead algorithm checks loosely
320 * for sequential patterns. Hence interleaved reads might be served as
321 * sequential ones.
322 *
323 * There is a special-case: if the first page which the application tries to
324 * read happens to be the first page of the file, it is assumed that a linear
325 * read is about to happen and the window is immediately set to the initial size
326 * based on I/O request size and the max_readahead.
327 *
328 * The code ramps up the readahead size aggressively at first, but slow down as
329 * it approaches max_readhead.
330 */
331
332 /*
333 * Count contiguously cached pages from @offset-1 to @offset-@max,
334 * this count is a conservative estimation of
335 * - length of the sequential read sequence, or
336 * - thrashing threshold in memory tight systems
337 */
338 static pgoff_t count_history_pages(struct address_space *mapping,
339 struct file_ra_state *ra,
340 pgoff_t offset, unsigned long max)
341 {
342 pgoff_t head;
343
344 rcu_read_lock();
345 head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
346 rcu_read_unlock();
347
348 return offset - 1 - head;
349 }
350
351 /*
352 * page cache context based read-ahead
353 */
354 static int try_context_readahead(struct address_space *mapping,
355 struct file_ra_state *ra,
356 pgoff_t offset,
357 unsigned long req_size,
358 unsigned long max)
359 {
360 pgoff_t size;
361
362 size = count_history_pages(mapping, ra, offset, max);
363
364 /*
365 * no history pages:
366 * it could be a random read
367 */
368 if (!size)
369 return 0;
370
371 /*
372 * starts from beginning of file:
373 * it is a strong indication of long-run stream (or whole-file-read)
374 */
375 if (size >= offset)
376 size *= 2;
377
378 ra->start = offset;
379 ra->size = get_init_ra_size(size + req_size, max);
380 ra->async_size = ra->size;
381
382 return 1;
383 }
384
385 /*
386 * A minimal readahead algorithm for trivial sequential/random reads.
387 */
388 static unsigned long
389 ondemand_readahead(struct address_space *mapping,
390 struct file_ra_state *ra, struct file *filp,
391 bool hit_readahead_marker, pgoff_t offset,
392 unsigned long req_size)
393 {
394 unsigned long max = max_sane_readahead(ra->ra_pages);
395
396 /*
397 * start of file
398 */
399 if (!offset)
400 goto initial_readahead;
401
402 /*
403 * It's the expected callback offset, assume sequential access.
404 * Ramp up sizes, and push forward the readahead window.
405 */
406 if ((offset == (ra->start + ra->size - ra->async_size) ||
407 offset == (ra->start + ra->size))) {
408 ra->start += ra->size;
409 ra->size = get_next_ra_size(ra, max);
410 ra->async_size = ra->size;
411 goto readit;
412 }
413
414 /*
415 * Hit a marked page without valid readahead state.
416 * E.g. interleaved reads.
417 * Query the pagecache for async_size, which normally equals to
418 * readahead size. Ramp it up and use it as the new readahead size.
419 */
420 if (hit_readahead_marker) {
421 pgoff_t start;
422
423 rcu_read_lock();
424 start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
425 rcu_read_unlock();
426
427 if (!start || start - offset > max)
428 return 0;
429
430 ra->start = start;
431 ra->size = start - offset; /* old async_size */
432 ra->size += req_size;
433 ra->size = get_next_ra_size(ra, max);
434 ra->async_size = ra->size;
435 goto readit;
436 }
437
438 /*
439 * oversize read
440 */
441 if (req_size > max)
442 goto initial_readahead;
443
444 /*
445 * sequential cache miss
446 */
447 if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
448 goto initial_readahead;
449
450 /*
451 * Query the page cache and look for the traces(cached history pages)
452 * that a sequential stream would leave behind.
453 */
454 if (try_context_readahead(mapping, ra, offset, req_size, max))
455 goto readit;
456
457 /*
458 * standalone, small random read
459 * Read as is, and do not pollute the readahead state.
460 */
461 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
462
463 initial_readahead:
464 ra->start = offset;
465 ra->size = get_init_ra_size(req_size, max);
466 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
467
468 readit:
469 /*
470 * Will this read hit the readahead marker made by itself?
471 * If so, trigger the readahead marker hit now, and merge
472 * the resulted next readahead window into the current one.
473 */
474 if (offset == ra->start && ra->size == ra->async_size) {
475 ra->async_size = get_next_ra_size(ra, max);
476 ra->size += ra->async_size;
477 }
478
479 return ra_submit(ra, mapping, filp);
480 }
481
482 /**
483 * page_cache_sync_readahead - generic file readahead
484 * @mapping: address_space which holds the pagecache and I/O vectors
485 * @ra: file_ra_state which holds the readahead state
486 * @filp: passed on to ->readpage() and ->readpages()
487 * @offset: start offset into @mapping, in pagecache page-sized units
488 * @req_size: hint: total size of the read which the caller is performing in
489 * pagecache pages
490 *
491 * page_cache_sync_readahead() should be called when a cache miss happened:
492 * it will submit the read. The readahead logic may decide to piggyback more
493 * pages onto the read request if access patterns suggest it will improve
494 * performance.
495 */
496 void page_cache_sync_readahead(struct address_space *mapping,
497 struct file_ra_state *ra, struct file *filp,
498 pgoff_t offset, unsigned long req_size)
499 {
500 /* no read-ahead */
501 if (!ra->ra_pages)
502 return;
503
504 /* do read-ahead */
505 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
506 }
507 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
508
509 /**
510 * page_cache_async_readahead - file readahead for marked pages
511 * @mapping: address_space which holds the pagecache and I/O vectors
512 * @ra: file_ra_state which holds the readahead state
513 * @filp: passed on to ->readpage() and ->readpages()
514 * @page: the page at @offset which has the PG_readahead flag set
515 * @offset: start offset into @mapping, in pagecache page-sized units
516 * @req_size: hint: total size of the read which the caller is performing in
517 * pagecache pages
518 *
519 * page_cache_async_ondemand() should be called when a page is used which
520 * has the PG_readahead flag; this is a marker to suggest that the application
521 * has used up enough of the readahead window that we should start pulling in
522 * more pages.
523 */
524 void
525 page_cache_async_readahead(struct address_space *mapping,
526 struct file_ra_state *ra, struct file *filp,
527 struct page *page, pgoff_t offset,
528 unsigned long req_size)
529 {
530 /* no read-ahead */
531 if (!ra->ra_pages)
532 return;
533
534 /*
535 * Same bit is used for PG_readahead and PG_reclaim.
536 */
537 if (PageWriteback(page))
538 return;
539
540 ClearPageReadahead(page);
541
542 /*
543 * Defer asynchronous read-ahead on IO congestion.
544 */
545 if (bdi_read_congested(mapping->backing_dev_info))
546 return;
547
548 /* do read-ahead */
549 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
550 }
551 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
552
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