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/dax.h>
12 #include <linux/gfp.h>
13 #include <linux/export.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 #include <linux/syscalls.h>
20 #include <linux/file.h>
21 #include <linux/mm_inline.h>
26 * Initialise a struct file's readahead state. Assumes that the caller has
30 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
32 ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
35 EXPORT_SYMBOL_GPL(file_ra_state_init);
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_SIZE);
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 = lru_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 = lru_to_page(pages);
92 if (add_to_page_cache_lru(page, mapping, page->index,
93 readahead_gfp_mask(mapping))) {
94 read_cache_pages_invalidate_page(mapping, page);
99 ret = filler(data, page);
101 read_cache_pages_invalidate_pages(mapping, pages);
104 task_io_account_read(PAGE_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 int nr_pages, gfp_t gfp)
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 = lru_to_page(pages);
129 list_del(&page->lru);
130 if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
131 mapping->a_ops->readpage(filp, page);
137 blk_finish_plug(&plug);
143 * __do_page_cache_readahead() actually reads a chunk of disk. It allocates all
144 * the pages first, then submits them all for I/O. This avoids the very bad
145 * behaviour which would occur if page allocations are causing VM writeback.
146 * We really don't want to intermingle reads and writes like that.
148 * Returns the number of pages requested, or the maximum amount of I/O allowed.
150 unsigned int __do_page_cache_readahead(struct address_space *mapping,
151 struct file *filp, pgoff_t offset, unsigned long nr_to_read,
152 unsigned long lookahead_size)
154 struct inode *inode = mapping->host;
156 unsigned long end_index; /* The last page we want to read */
157 LIST_HEAD(page_pool);
159 unsigned int nr_pages = 0;
160 loff_t isize = i_size_read(inode);
161 gfp_t gfp_mask = readahead_gfp_mask(mapping);
166 end_index = ((isize - 1) >> PAGE_SHIFT);
169 * Preallocate as many pages as we will need.
171 for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
172 pgoff_t page_offset = offset + page_idx;
174 if (page_offset > end_index)
178 page = radix_tree_lookup(&mapping->i_pages, page_offset);
180 if (page && !radix_tree_exceptional_entry(page))
183 page = __page_cache_alloc(gfp_mask);
186 page->index = page_offset;
187 list_add(&page->lru, &page_pool);
188 if (page_idx == nr_to_read - lookahead_size)
189 SetPageReadahead(page);
194 * Now start the IO. We ignore I/O errors - if the page is not
195 * uptodate then the caller will launch readpage again, and
196 * will then handle the error.
199 read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
200 BUG_ON(!list_empty(&page_pool));
206 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
209 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
210 pgoff_t offset, unsigned long nr_to_read)
212 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
213 struct file_ra_state *ra = &filp->f_ra;
214 unsigned long max_pages;
216 if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
220 * If the request exceeds the readahead window, allow the read to
221 * be up to the optimal hardware IO size
223 max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
224 nr_to_read = min(nr_to_read, max_pages);
226 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
228 if (this_chunk > nr_to_read)
229 this_chunk = nr_to_read;
230 __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
232 offset += this_chunk;
233 nr_to_read -= this_chunk;
239 * Set the initial window size, round to next power of 2 and square
240 * for small size, x 4 for medium, and x 2 for large
241 * for 128k (32 page) max ra
242 * 1-8 page = 32k initial, > 8 page = 128k initial
244 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
246 unsigned long newsize = roundup_pow_of_two(size);
248 if (newsize <= max / 32)
249 newsize = newsize * 4;
250 else if (newsize <= max / 4)
251 newsize = newsize * 2;
259 * Get the previous window size, ramp it up, and
260 * return it as the new window size.
262 static unsigned long get_next_ra_size(struct file_ra_state *ra,
265 unsigned long cur = ra->size;
266 unsigned long newsize;
273 return min(newsize, max);
277 * On-demand readahead design.
279 * The fields in struct file_ra_state represent the most-recently-executed
282 * |<----- async_size ---------|
283 * |------------------- size -------------------->|
284 * |==================#===========================|
285 * ^start ^page marked with PG_readahead
287 * To overlap application thinking time and disk I/O time, we do
288 * `readahead pipelining': Do not wait until the application consumed all
289 * readahead pages and stalled on the missing page at readahead_index;
290 * Instead, submit an asynchronous readahead I/O as soon as there are
291 * only async_size pages left in the readahead window. Normally async_size
292 * will be equal to size, for maximum pipelining.
294 * In interleaved sequential reads, concurrent streams on the same fd can
295 * be invalidating each other's readahead state. So we flag the new readahead
296 * page at (start+size-async_size) with PG_readahead, and use it as readahead
297 * indicator. The flag won't be set on already cached pages, to avoid the
298 * readahead-for-nothing fuss, saving pointless page cache lookups.
300 * prev_pos tracks the last visited byte in the _previous_ read request.
301 * It should be maintained by the caller, and will be used for detecting
302 * small random reads. Note that the readahead algorithm checks loosely
303 * for sequential patterns. Hence interleaved reads might be served as
306 * There is a special-case: if the first page which the application tries to
307 * read happens to be the first page of the file, it is assumed that a linear
308 * read is about to happen and the window is immediately set to the initial size
309 * based on I/O request size and the max_readahead.
311 * The code ramps up the readahead size aggressively at first, but slow down as
312 * it approaches max_readhead.
316 * Count contiguously cached pages from @offset-1 to @offset-@max,
317 * this count is a conservative estimation of
318 * - length of the sequential read sequence, or
319 * - thrashing threshold in memory tight systems
321 static pgoff_t count_history_pages(struct address_space *mapping,
322 pgoff_t offset, unsigned long max)
327 head = page_cache_prev_hole(mapping, offset - 1, max);
330 return offset - 1 - head;
334 * page cache context based read-ahead
336 static int try_context_readahead(struct address_space *mapping,
337 struct file_ra_state *ra,
339 unsigned long req_size,
344 size = count_history_pages(mapping, offset, max);
347 * not enough history pages:
348 * it could be a random read
350 if (size <= req_size)
354 * starts from beginning of file:
355 * it is a strong indication of long-run stream (or whole-file-read)
361 ra->size = min(size + req_size, max);
368 * A minimal readahead algorithm for trivial sequential/random reads.
371 ondemand_readahead(struct address_space *mapping,
372 struct file_ra_state *ra, struct file *filp,
373 bool hit_readahead_marker, pgoff_t offset,
374 unsigned long req_size)
376 struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
377 unsigned long max_pages = ra->ra_pages;
381 * If the request exceeds the readahead window, allow the read to
382 * be up to the optimal hardware IO size
384 if (req_size > max_pages && bdi->io_pages > max_pages)
385 max_pages = min(req_size, bdi->io_pages);
391 goto initial_readahead;
394 * It's the expected callback offset, assume sequential access.
395 * Ramp up sizes, and push forward the readahead window.
397 if ((offset == (ra->start + ra->size - ra->async_size) ||
398 offset == (ra->start + ra->size))) {
399 ra->start += ra->size;
400 ra->size = get_next_ra_size(ra, max_pages);
401 ra->async_size = ra->size;
406 * Hit a marked page without valid readahead state.
407 * E.g. interleaved reads.
408 * Query the pagecache for async_size, which normally equals to
409 * readahead size. Ramp it up and use it as the new readahead size.
411 if (hit_readahead_marker) {
415 start = page_cache_next_hole(mapping, offset + 1, max_pages);
418 if (!start || start - offset > max_pages)
422 ra->size = start - offset; /* old async_size */
423 ra->size += req_size;
424 ra->size = get_next_ra_size(ra, max_pages);
425 ra->async_size = ra->size;
432 if (req_size > max_pages)
433 goto initial_readahead;
436 * sequential cache miss
437 * trivial case: (offset - prev_offset) == 1
438 * unaligned reads: (offset - prev_offset) == 0
440 prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
441 if (offset - prev_offset <= 1UL)
442 goto initial_readahead;
445 * Query the page cache and look for the traces(cached history pages)
446 * that a sequential stream would leave behind.
448 if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
452 * standalone, small random read
453 * Read as is, and do not pollute the readahead state.
455 return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
459 ra->size = get_init_ra_size(req_size, max_pages);
460 ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
464 * Will this read hit the readahead marker made by itself?
465 * If so, trigger the readahead marker hit now, and merge
466 * the resulted next readahead window into the current one.
468 if (offset == ra->start && ra->size == ra->async_size) {
469 ra->async_size = get_next_ra_size(ra, max_pages);
470 ra->size += ra->async_size;
473 return ra_submit(ra, mapping, filp);
477 * page_cache_sync_readahead - generic file readahead
478 * @mapping: address_space which holds the pagecache and I/O vectors
479 * @ra: file_ra_state which holds the readahead state
480 * @filp: passed on to ->readpage() and ->readpages()
481 * @offset: start offset into @mapping, in pagecache page-sized units
482 * @req_size: hint: total size of the read which the caller is performing in
485 * page_cache_sync_readahead() should be called when a cache miss happened:
486 * it will submit the read. The readahead logic may decide to piggyback more
487 * pages onto the read request if access patterns suggest it will improve
490 void page_cache_sync_readahead(struct address_space *mapping,
491 struct file_ra_state *ra, struct file *filp,
492 pgoff_t offset, unsigned long req_size)
499 if (filp && (filp->f_mode & FMODE_RANDOM)) {
500 force_page_cache_readahead(mapping, filp, offset, req_size);
505 ondemand_readahead(mapping, ra, filp, false, offset, req_size);
507 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
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
519 * page_cache_async_readahead() 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
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)
535 * Same bit is used for PG_readahead and PG_reclaim.
537 if (PageWriteback(page))
540 ClearPageReadahead(page);
543 * Defer asynchronous read-ahead on IO congestion.
545 if (inode_read_congested(mapping->host))
549 ondemand_readahead(mapping, ra, filp, true, offset, req_size);
551 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
554 do_readahead(struct address_space *mapping, struct file *filp,
555 pgoff_t index, unsigned long nr)
557 if (!mapping || !mapping->a_ops)
561 * Readahead doesn't make sense for DAX inodes, but we don't want it
562 * to report a failure either. Instead, we just return success and
565 if (dax_mapping(mapping))
568 return force_page_cache_readahead(mapping, filp, index, nr);
571 ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
579 if (f.file->f_mode & FMODE_READ) {
580 struct address_space *mapping = f.file->f_mapping;
581 pgoff_t start = offset >> PAGE_SHIFT;
582 pgoff_t end = (offset + count - 1) >> PAGE_SHIFT;
583 unsigned long len = end - start + 1;
584 ret = do_readahead(mapping, f.file, start, len);
591 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
593 return ksys_readahead(fd, offset, count);