2 * mm/readahead.c - address_space-level file readahead.
4 * Copyright (C) 2002, Linus Torvalds
6 * 09Apr2002 Andrew Morton
10 #include <linux/kernel.h>
12 #include <linux/gfp.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>
22 * Initialise a struct file's readahead state. Assumes that the caller has
26 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
28 ra->ra_pages = mapping->backing_dev_info->ra_pages;
31 EXPORT_SYMBOL_GPL(file_ra_state_init);
33 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
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
42 static void read_cache_pages_invalidate_page(struct address_space *mapping,
45 if (page_has_private(page)) {
46 if (!trylock_page(page))
48 page->mapping = mapping;
49 do_invalidatepage(page, 0);
53 page_cache_release(page);
57 * release a list of pages, invalidating them first if need be
59 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
60 struct list_head *pages)
64 while (!list_empty(pages)) {
65 victim = list_to_page(pages);
66 list_del(&victim->lru);
67 read_cache_pages_invalidate_page(mapping, victim);
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.
79 * Hides the details of the LRU cache etc from the filesystems.
81 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
82 int (*filler)(void *, struct page *), void *data)
87 while (!list_empty(pages)) {
88 page = list_to_page(pages);
90 if (add_to_page_cache_lru(page, mapping,
91 page->index, GFP_KERNEL)) {
92 read_cache_pages_invalidate_page(mapping, page);
95 page_cache_release(page);
97 ret = filler(data, page);
99 read_cache_pages_invalidate_pages(mapping, pages);
102 task_io_account_read(PAGE_CACHE_SIZE);
107 EXPORT_SYMBOL(read_cache_pages);
109 static int read_pages(struct address_space *mapping, struct file *filp,
110 struct list_head *pages, unsigned nr_pages)
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);
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);
129 page_cache_release(page);
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.
142 * Returns the number of pages requested, or the maximum amount of I/O allowed.
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)
149 struct inode *inode = mapping->host;
151 unsigned long end_index; /* The last page we want to read */
152 LIST_HEAD(page_pool);
155 loff_t isize = i_size_read(inode);
160 end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
163 * Preallocate as many pages as we will need.
165 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
166 pgoff_t page_offset = offset + page_idx;
168 if (page_offset > end_index)
172 page = radix_tree_lookup(&mapping->page_tree, page_offset);
177 page = page_cache_alloc_cold(mapping);
180 page->index = page_offset;
181 list_add(&page->lru, &page_pool);
182 if (page_idx == nr_to_read - lookahead_size)
183 SetPageReadahead(page);
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.
193 read_pages(mapping, filp, &page_pool, ret);
194 BUG_ON(!list_empty(&page_pool));
200 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
203 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
204 pgoff_t offset, unsigned long nr_to_read)
208 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
211 nr_to_read = max_sane_readahead(nr_to_read);
215 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
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);
226 offset += this_chunk;
227 nr_to_read -= this_chunk;
233 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
234 * sensible upper limit.
236 unsigned long max_sane_readahead(unsigned long nr)
238 return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
239 + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
243 * Submit IO for the read-ahead request in file_ra_state.
245 unsigned long ra_submit(struct file_ra_state *ra,
246 struct address_space *mapping, struct file *filp)
250 actual = __do_page_cache_readahead(mapping, filp,
251 ra->start, ra->size, ra->async_size);
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
262 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
264 unsigned long newsize = roundup_pow_of_two(size);
266 if (newsize <= max / 32)
267 newsize = newsize * 4;
268 else if (newsize <= max / 4)
269 newsize = newsize * 2;
277 * Get the previous window size, ramp it up, and
278 * return it as the new window size.
280 static unsigned long get_next_ra_size(struct file_ra_state *ra,
283 unsigned long cur = ra->size;
284 unsigned long newsize;
291 return min(newsize, max);
295 * On-demand readahead design.
297 * The fields in struct file_ra_state represent the most-recently-executed
300 * |<----- async_size ---------|
301 * |------------------- size -------------------->|
302 * |==================#===========================|
303 * ^start ^page marked with PG_readahead
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.
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.
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
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.
329 * The code ramps up the readahead size aggressively at first, but slow down as
330 * it approaches max_readhead.
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
339 static pgoff_t count_history_pages(struct address_space *mapping,
340 struct file_ra_state *ra,
341 pgoff_t offset, unsigned long max)
346 head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
349 return offset - 1 - head;
353 * page cache context based read-ahead
355 static int try_context_readahead(struct address_space *mapping,
356 struct file_ra_state *ra,
358 unsigned long req_size,
363 size = count_history_pages(mapping, ra, offset, max);
367 * it could be a random read
373 * starts from beginning of file:
374 * it is a strong indication of long-run stream (or whole-file-read)
380 ra->size = get_init_ra_size(size + req_size, max);
381 ra->async_size = ra->size;
387 * A minimal readahead algorithm for trivial sequential/random reads.
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)
395 unsigned long max = max_sane_readahead(ra->ra_pages);
401 goto initial_readahead;
404 * It's the expected callback offset, assume sequential access.
405 * Ramp up sizes, and push forward the readahead window.
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;
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.
421 if (hit_readahead_marker) {
425 start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
428 if (!start || start - offset > max)
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;
443 goto initial_readahead;
446 * sequential cache miss
448 if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
449 goto initial_readahead;
452 * Query the page cache and look for the traces(cached history pages)
453 * that a sequential stream would leave behind.
455 if (try_context_readahead(mapping, ra, offset, req_size, max))
459 * standalone, small random read
460 * Read as is, and do not pollute the readahead state.
462 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
466 ra->size = get_init_ra_size(req_size, max);
467 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
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.
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;
480 return ra_submit(ra, mapping, filp);
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
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
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)
506 if (filp && (filp->f_mode & FMODE_RANDOM)) {
507 force_page_cache_readahead(mapping, filp, offset, req_size);
512 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
514 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
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
526 * page_cache_async_ondemand() 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
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)
542 * Same bit is used for PG_readahead and PG_reclaim.
544 if (PageWriteback(page))
547 ClearPageReadahead(page);
550 * Defer asynchronous read-ahead on IO congestion.
552 if (bdi_read_congested(mapping->backing_dev_info))
556 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
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.
566 if (PageUptodate(page))
567 blk_run_backing_dev(mapping->backing_dev_info, NULL);
570 EXPORT_SYMBOL_GPL(page_cache_async_readahead);