2 * mm/readahead.c - address_space-level file readahead.
4 * Copyright (C) 2002, Linus Torvalds
6 * 09Apr2002 Andrew Morton
10 #include <linux/kernel.h>
11 #include <linux/gfp.h>
12 #include <linux/export.h>
13 #include <linux/blkdev.h>
14 #include <linux/backing-dev.h>
15 #include <linux/task_io_accounting_ops.h>
16 #include <linux/pagevec.h>
17 #include <linux/pagemap.h>
18 #include <linux/syscalls.h>
19 #include <linux/file.h>
24 * Initialise a struct file's readahead state. Assumes that the caller has
28 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
30 ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
33 EXPORT_SYMBOL_GPL(file_ra_state_init);
35 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
38 * see if a page needs releasing upon read_cache_pages() failure
39 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
40 * before calling, such as the NFS fs marking pages that are cached locally
41 * on disk, thus we need to give the fs a chance to clean up in the event of
44 static void read_cache_pages_invalidate_page(struct address_space *mapping,
47 if (page_has_private(page)) {
48 if (!trylock_page(page))
50 page->mapping = mapping;
51 do_invalidatepage(page, 0, PAGE_CACHE_SIZE);
55 page_cache_release(page);
59 * release a list of pages, invalidating them first if need be
61 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
62 struct list_head *pages)
66 while (!list_empty(pages)) {
67 victim = list_to_page(pages);
68 list_del(&victim->lru);
69 read_cache_pages_invalidate_page(mapping, victim);
74 * read_cache_pages - populate an address space with some pages & start reads against them
75 * @mapping: the address_space
76 * @pages: The address of a list_head which contains the target pages. These
77 * pages have their ->index populated and are otherwise uninitialised.
78 * @filler: callback routine for filling a single page.
79 * @data: private data for the callback routine.
81 * Hides the details of the LRU cache etc from the filesystems.
83 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
84 int (*filler)(void *, struct page *), void *data)
89 while (!list_empty(pages)) {
90 page = list_to_page(pages);
92 if (add_to_page_cache_lru(page, mapping, page->index,
93 mapping_gfp_constraint(mapping, GFP_KERNEL))) {
94 read_cache_pages_invalidate_page(mapping, page);
97 page_cache_release(page);
99 ret = filler(data, page);
101 read_cache_pages_invalidate_pages(mapping, pages);
104 task_io_account_read(PAGE_CACHE_SIZE);
109 EXPORT_SYMBOL(read_cache_pages);
111 static int read_pages(struct address_space *mapping, struct file *filp,
112 struct list_head *pages, unsigned nr_pages)
114 struct blk_plug plug;
118 blk_start_plug(&plug);
120 if (mapping->a_ops->readpages) {
121 ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
122 /* Clean up the remaining pages */
123 put_pages_list(pages);
127 for (page_idx = 0; page_idx < nr_pages; page_idx++) {
128 struct page *page = list_to_page(pages);
129 list_del(&page->lru);
130 if (!add_to_page_cache_lru(page, mapping, page->index,
131 mapping_gfp_constraint(mapping, GFP_KERNEL))) {
132 mapping->a_ops->readpage(filp, page);
134 page_cache_release(page);
139 blk_finish_plug(&plug);
145 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all
146 * the pages first, then submits them all for I/O. This avoids the very bad
147 * behaviour which would occur if page allocations are causing VM writeback.
148 * We really don't want to intermingle reads and writes like that.
150 * Returns the number of pages requested, or the maximum amount of I/O allowed.
152 int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
153 pgoff_t offset, unsigned long nr_to_read,
154 unsigned long lookahead_size)
156 struct inode *inode = mapping->host;
158 unsigned long end_index; /* The last page we want to read */
159 LIST_HEAD(page_pool);
162 loff_t isize = i_size_read(inode);
167 end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
170 * Preallocate as many pages as we will need.
172 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
173 pgoff_t page_offset = offset + page_idx;
175 if (page_offset > end_index)
179 page = radix_tree_lookup(&mapping->page_tree, page_offset);
181 if (page && !radix_tree_exceptional_entry(page))
184 page = page_cache_alloc_readahead(mapping);
187 page->index = page_offset;
188 list_add(&page->lru, &page_pool);
189 if (page_idx == nr_to_read - lookahead_size)
190 SetPageReadahead(page);
195 * Now start the IO. We ignore I/O errors - if the page is not
196 * uptodate then the caller will launch readpage again, and
197 * will then handle the error.
200 read_pages(mapping, filp, &page_pool, ret);
201 BUG_ON(!list_empty(&page_pool));
207 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
210 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
211 pgoff_t offset, unsigned long nr_to_read)
213 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
216 nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
220 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
222 if (this_chunk > nr_to_read)
223 this_chunk = nr_to_read;
224 err = __do_page_cache_readahead(mapping, filp,
225 offset, this_chunk, 0);
229 offset += this_chunk;
230 nr_to_read -= this_chunk;
236 * Set the initial window size, round to next power of 2 and square
237 * for small size, x 4 for medium, and x 2 for large
238 * for 128k (32 page) max ra
239 * 1-8 page = 32k initial, > 8 page = 128k initial
241 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
243 unsigned long newsize = roundup_pow_of_two(size);
245 if (newsize <= max / 32)
246 newsize = newsize * 4;
247 else if (newsize <= max / 4)
248 newsize = newsize * 2;
256 * Get the previous window size, ramp it up, and
257 * return it as the new window size.
259 static unsigned long get_next_ra_size(struct file_ra_state *ra,
262 unsigned long cur = ra->size;
263 unsigned long newsize;
270 return min(newsize, max);
274 * On-demand readahead design.
276 * The fields in struct file_ra_state represent the most-recently-executed
279 * |<----- async_size ---------|
280 * |------------------- size -------------------->|
281 * |==================#===========================|
282 * ^start ^page marked with PG_readahead
284 * To overlap application thinking time and disk I/O time, we do
285 * `readahead pipelining': Do not wait until the application consumed all
286 * readahead pages and stalled on the missing page at readahead_index;
287 * Instead, submit an asynchronous readahead I/O as soon as there are
288 * only async_size pages left in the readahead window. Normally async_size
289 * will be equal to size, for maximum pipelining.
291 * In interleaved sequential reads, concurrent streams on the same fd can
292 * be invalidating each other's readahead state. So we flag the new readahead
293 * page at (start+size-async_size) with PG_readahead, and use it as readahead
294 * indicator. The flag won't be set on already cached pages, to avoid the
295 * readahead-for-nothing fuss, saving pointless page cache lookups.
297 * prev_pos tracks the last visited byte in the _previous_ read request.
298 * It should be maintained by the caller, and will be used for detecting
299 * small random reads. Note that the readahead algorithm checks loosely
300 * for sequential patterns. Hence interleaved reads might be served as
303 * There is a special-case: if the first page which the application tries to
304 * read happens to be the first page of the file, it is assumed that a linear
305 * read is about to happen and the window is immediately set to the initial size
306 * based on I/O request size and the max_readahead.
308 * The code ramps up the readahead size aggressively at first, but slow down as
309 * it approaches max_readhead.
313 * Count contiguously cached pages from @offset-1 to @offset-@max,
314 * this count is a conservative estimation of
315 * - length of the sequential read sequence, or
316 * - thrashing threshold in memory tight systems
318 static pgoff_t count_history_pages(struct address_space *mapping,
319 pgoff_t offset, unsigned long max)
324 head = page_cache_prev_hole(mapping, offset - 1, max);
327 return offset - 1 - head;
331 * page cache context based read-ahead
333 static int try_context_readahead(struct address_space *mapping,
334 struct file_ra_state *ra,
336 unsigned long req_size,
341 size = count_history_pages(mapping, offset, max);
344 * not enough history pages:
345 * it could be a random read
347 if (size <= req_size)
351 * starts from beginning of file:
352 * it is a strong indication of long-run stream (or whole-file-read)
358 ra->size = min(size + req_size, max);
365 * A minimal readahead algorithm for trivial sequential/random reads.
368 ondemand_readahead(struct address_space *mapping,
369 struct file_ra_state *ra, struct file *filp,
370 bool hit_readahead_marker, pgoff_t offset,
371 unsigned long req_size)
373 unsigned long max = ra->ra_pages;
380 goto initial_readahead;
383 * It's the expected callback offset, assume sequential access.
384 * Ramp up sizes, and push forward the readahead window.
386 if ((offset == (ra->start + ra->size - ra->async_size) ||
387 offset == (ra->start + ra->size))) {
388 ra->start += ra->size;
389 ra->size = get_next_ra_size(ra, max);
390 ra->async_size = ra->size;
395 * Hit a marked page without valid readahead state.
396 * E.g. interleaved reads.
397 * Query the pagecache for async_size, which normally equals to
398 * readahead size. Ramp it up and use it as the new readahead size.
400 if (hit_readahead_marker) {
404 start = page_cache_next_hole(mapping, offset + 1, max);
407 if (!start || start - offset > max)
411 ra->size = start - offset; /* old async_size */
412 ra->size += req_size;
413 ra->size = get_next_ra_size(ra, max);
414 ra->async_size = ra->size;
422 goto initial_readahead;
425 * sequential cache miss
426 * trivial case: (offset - prev_offset) == 1
427 * unaligned reads: (offset - prev_offset) == 0
429 prev_offset = (unsigned long long)ra->prev_pos >> PAGE_CACHE_SHIFT;
430 if (offset - prev_offset <= 1UL)
431 goto initial_readahead;
434 * Query the page cache and look for the traces(cached history pages)
435 * that a sequential stream would leave behind.
437 if (try_context_readahead(mapping, ra, offset, req_size, max))
441 * standalone, small random read
442 * Read as is, and do not pollute the readahead state.
444 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
448 ra->size = get_init_ra_size(req_size, max);
449 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
453 * Will this read hit the readahead marker made by itself?
454 * If so, trigger the readahead marker hit now, and merge
455 * the resulted next readahead window into the current one.
457 if (offset == ra->start && ra->size == ra->async_size) {
458 ra->async_size = get_next_ra_size(ra, max);
459 ra->size += ra->async_size;
462 return ra_submit(ra, mapping, filp);
466 * page_cache_sync_readahead - generic file readahead
467 * @mapping: address_space which holds the pagecache and I/O vectors
468 * @ra: file_ra_state which holds the readahead state
469 * @filp: passed on to ->readpage() and ->readpages()
470 * @offset: start offset into @mapping, in pagecache page-sized units
471 * @req_size: hint: total size of the read which the caller is performing in
474 * page_cache_sync_readahead() should be called when a cache miss happened:
475 * it will submit the read. The readahead logic may decide to piggyback more
476 * pages onto the read request if access patterns suggest it will improve
479 void page_cache_sync_readahead(struct address_space *mapping,
480 struct file_ra_state *ra, struct file *filp,
481 pgoff_t offset, unsigned long req_size)
488 if (filp && (filp->f_mode & FMODE_RANDOM)) {
489 force_page_cache_readahead(mapping, filp, offset, req_size);
494 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
496 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
499 * page_cache_async_readahead - file readahead for marked pages
500 * @mapping: address_space which holds the pagecache and I/O vectors
501 * @ra: file_ra_state which holds the readahead state
502 * @filp: passed on to ->readpage() and ->readpages()
503 * @page: the page at @offset which has the PG_readahead flag set
504 * @offset: start offset into @mapping, in pagecache page-sized units
505 * @req_size: hint: total size of the read which the caller is performing in
508 * page_cache_async_readahead() should be called when a page is used which
509 * has the PG_readahead flag; this is a marker to suggest that the application
510 * has used up enough of the readahead window that we should start pulling in
514 page_cache_async_readahead(struct address_space *mapping,
515 struct file_ra_state *ra, struct file *filp,
516 struct page *page, pgoff_t offset,
517 unsigned long req_size)
524 * Same bit is used for PG_readahead and PG_reclaim.
526 if (PageWriteback(page))
529 ClearPageReadahead(page);
532 * Defer asynchronous read-ahead on IO congestion.
534 if (inode_read_congested(mapping->host))
538 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
540 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
543 do_readahead(struct address_space *mapping, struct file *filp,
544 pgoff_t index, unsigned long nr)
546 if (!mapping || !mapping->a_ops)
549 return force_page_cache_readahead(mapping, filp, index, nr);
552 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
560 if (f.file->f_mode & FMODE_READ) {
561 struct address_space *mapping = f.file->f_mapping;
562 pgoff_t start = offset >> PAGE_CACHE_SHIFT;
563 pgoff_t end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
564 unsigned long len = end - start + 1;
565 ret = do_readahead(mapping, f.file, start, len);