2 * mm/readahead.c - address_space-level file readahead.
4 * Copyright (C) 2002, Linus Torvalds
6 * 09Apr2002 akpm@zip.com.au
10 #include <linux/kernel.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>
20 void default_unplug_io_fn(struct backing_dev_info
*bdi
, struct page
*page
)
23 EXPORT_SYMBOL(default_unplug_io_fn
);
25 struct backing_dev_info default_backing_dev_info
= {
26 .ra_pages
= VM_MAX_READAHEAD
* 1024 / PAGE_CACHE_SIZE
,
28 .capabilities
= BDI_CAP_MAP_COPY
,
29 .unplug_io_fn
= default_unplug_io_fn
,
31 EXPORT_SYMBOL_GPL(default_backing_dev_info
);
34 * Initialise a struct file's readahead state. Assumes that the caller has
38 file_ra_state_init(struct file_ra_state
*ra
, struct address_space
*mapping
)
40 ra
->ra_pages
= mapping
->backing_dev_info
->ra_pages
;
43 EXPORT_SYMBOL_GPL(file_ra_state_init
);
45 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
48 * read_cache_pages - populate an address space with some pages & start reads against them
49 * @mapping: the address_space
50 * @pages: The address of a list_head which contains the target pages. These
51 * pages have their ->index populated and are otherwise uninitialised.
52 * @filler: callback routine for filling a single page.
53 * @data: private data for the callback routine.
55 * Hides the details of the LRU cache etc from the filesystems.
57 int read_cache_pages(struct address_space
*mapping
, struct list_head
*pages
,
58 int (*filler
)(void *, struct page
*), void *data
)
61 struct pagevec lru_pvec
;
64 pagevec_init(&lru_pvec
, 0);
66 while (!list_empty(pages
)) {
67 page
= list_to_page(pages
);
69 if (add_to_page_cache(page
, mapping
, page
->index
, GFP_KERNEL
)) {
70 page_cache_release(page
);
73 ret
= filler(data
, page
);
74 if (!pagevec_add(&lru_pvec
, page
))
75 __pagevec_lru_add(&lru_pvec
);
77 put_pages_list(pages
);
80 task_io_account_read(PAGE_CACHE_SIZE
);
82 pagevec_lru_add(&lru_pvec
);
86 EXPORT_SYMBOL(read_cache_pages
);
88 static int read_pages(struct address_space
*mapping
, struct file
*filp
,
89 struct list_head
*pages
, unsigned nr_pages
)
92 struct pagevec lru_pvec
;
95 if (mapping
->a_ops
->readpages
) {
96 ret
= mapping
->a_ops
->readpages(filp
, mapping
, pages
, nr_pages
);
97 /* Clean up the remaining pages */
98 put_pages_list(pages
);
102 pagevec_init(&lru_pvec
, 0);
103 for (page_idx
= 0; page_idx
< nr_pages
; page_idx
++) {
104 struct page
*page
= list_to_page(pages
);
105 list_del(&page
->lru
);
106 if (!add_to_page_cache(page
, mapping
,
107 page
->index
, GFP_KERNEL
)) {
108 mapping
->a_ops
->readpage(filp
, page
);
109 if (!pagevec_add(&lru_pvec
, page
))
110 __pagevec_lru_add(&lru_pvec
);
112 page_cache_release(page
);
114 pagevec_lru_add(&lru_pvec
);
121 * do_page_cache_readahead actually reads a chunk of disk. It allocates all
122 * the pages first, then submits them all for I/O. This avoids the very bad
123 * behaviour which would occur if page allocations are causing VM writeback.
124 * We really don't want to intermingle reads and writes like that.
126 * Returns the number of pages requested, or the maximum amount of I/O allowed.
128 * do_page_cache_readahead() returns -1 if it encountered request queue
132 __do_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
133 pgoff_t offset
, unsigned long nr_to_read
,
134 unsigned long lookahead_size
)
136 struct inode
*inode
= mapping
->host
;
138 unsigned long end_index
; /* The last page we want to read */
139 LIST_HEAD(page_pool
);
142 loff_t isize
= i_size_read(inode
);
147 end_index
= ((isize
- 1) >> PAGE_CACHE_SHIFT
);
150 * Preallocate as many pages as we will need.
152 for (page_idx
= 0; page_idx
< nr_to_read
; page_idx
++) {
153 pgoff_t page_offset
= offset
+ page_idx
;
155 if (page_offset
> end_index
)
159 page
= radix_tree_lookup(&mapping
->page_tree
, page_offset
);
164 page
= page_cache_alloc_cold(mapping
);
167 page
->index
= page_offset
;
168 list_add(&page
->lru
, &page_pool
);
169 if (page_idx
== nr_to_read
- lookahead_size
)
170 SetPageReadahead(page
);
175 * Now start the IO. We ignore I/O errors - if the page is not
176 * uptodate then the caller will launch readpage again, and
177 * will then handle the error.
180 read_pages(mapping
, filp
, &page_pool
, ret
);
181 BUG_ON(!list_empty(&page_pool
));
187 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
190 int force_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
191 pgoff_t offset
, unsigned long nr_to_read
)
195 if (unlikely(!mapping
->a_ops
->readpage
&& !mapping
->a_ops
->readpages
))
201 unsigned long this_chunk
= (2 * 1024 * 1024) / PAGE_CACHE_SIZE
;
203 if (this_chunk
> nr_to_read
)
204 this_chunk
= nr_to_read
;
205 err
= __do_page_cache_readahead(mapping
, filp
,
206 offset
, this_chunk
, 0);
212 offset
+= this_chunk
;
213 nr_to_read
-= this_chunk
;
219 * This version skips the IO if the queue is read-congested, and will tell the
220 * block layer to abandon the readahead if request allocation would block.
222 * force_page_cache_readahead() will ignore queue congestion and will block on
225 int do_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
226 pgoff_t offset
, unsigned long nr_to_read
)
228 if (bdi_read_congested(mapping
->backing_dev_info
))
231 return __do_page_cache_readahead(mapping
, filp
, offset
, nr_to_read
, 0);
235 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
236 * sensible upper limit.
238 unsigned long max_sane_readahead(unsigned long nr
)
240 return min(nr
, (node_page_state(numa_node_id(), NR_INACTIVE
)
241 + node_page_state(numa_node_id(), NR_FREE_PAGES
)) / 2);
245 * Submit IO for the read-ahead request in file_ra_state.
247 static unsigned long ra_submit(struct file_ra_state
*ra
,
248 struct address_space
*mapping
, struct file
*filp
)
252 actual
= __do_page_cache_readahead(mapping
, filp
,
253 ra
->start
, ra
->size
, ra
->async_size
);
259 * Set the initial window size, round to next power of 2 and square
260 * for small size, x 4 for medium, and x 2 for large
261 * for 128k (32 page) max ra
262 * 1-8 page = 32k initial, > 8 page = 128k initial
264 static unsigned long get_init_ra_size(unsigned long size
, unsigned long max
)
266 unsigned long newsize
= roundup_pow_of_two(size
);
268 if (newsize
<= max
/ 32)
269 newsize
= newsize
* 4;
270 else if (newsize
<= max
/ 4)
271 newsize
= newsize
* 2;
279 * Get the previous window size, ramp it up, and
280 * return it as the new window size.
282 static unsigned long get_next_ra_size(struct file_ra_state
*ra
,
285 unsigned long cur
= ra
->size
;
286 unsigned long newsize
;
293 return min(newsize
, max
);
297 * On-demand readahead design.
299 * The fields in struct file_ra_state represent the most-recently-executed
302 * |<----- async_size ---------|
303 * |------------------- size -------------------->|
304 * |==================#===========================|
305 * ^start ^page marked with PG_readahead
307 * To overlap application thinking time and disk I/O time, we do
308 * `readahead pipelining': Do not wait until the application consumed all
309 * readahead pages and stalled on the missing page at readahead_index;
310 * Instead, submit an asynchronous readahead I/O as soon as there are
311 * only async_size pages left in the readahead window. Normally async_size
312 * will be equal to size, for maximum pipelining.
314 * In interleaved sequential reads, concurrent streams on the same fd can
315 * be invalidating each other's readahead state. So we flag the new readahead
316 * page at (start+size-async_size) with PG_readahead, and use it as readahead
317 * indicator. The flag won't be set on already cached pages, to avoid the
318 * readahead-for-nothing fuss, saving pointless page cache lookups.
320 * prev_pos tracks the last visited byte in the _previous_ read request.
321 * It should be maintained by the caller, and will be used for detecting
322 * small random reads. Note that the readahead algorithm checks loosely
323 * for sequential patterns. Hence interleaved reads might be served as
326 * There is a special-case: if the first page which the application tries to
327 * read happens to be the first page of the file, it is assumed that a linear
328 * read is about to happen and the window is immediately set to the initial size
329 * based on I/O request size and the max_readahead.
331 * The code ramps up the readahead size aggressively at first, but slow down as
332 * it approaches max_readhead.
336 * A minimal readahead algorithm for trivial sequential/random reads.
339 ondemand_readahead(struct address_space
*mapping
,
340 struct file_ra_state
*ra
, struct file
*filp
,
341 bool hit_readahead_marker
, pgoff_t offset
,
342 unsigned long req_size
)
344 int max
= ra
->ra_pages
; /* max readahead pages */
349 * It's the expected callback offset, assume sequential access.
350 * Ramp up sizes, and push forward the readahead window.
352 if (offset
&& (offset
== (ra
->start
+ ra
->size
- ra
->async_size
) ||
353 offset
== (ra
->start
+ ra
->size
))) {
354 ra
->start
+= ra
->size
;
355 ra
->size
= get_next_ra_size(ra
, max
);
356 ra
->async_size
= ra
->size
;
360 prev_offset
= ra
->prev_pos
>> PAGE_CACHE_SHIFT
;
361 sequential
= offset
- prev_offset
<= 1UL || req_size
> max
;
364 * Standalone, small read.
365 * Read as is, and do not pollute the readahead state.
367 if (!hit_readahead_marker
&& !sequential
) {
368 return __do_page_cache_readahead(mapping
, filp
,
369 offset
, req_size
, 0);
373 * Hit a marked page without valid readahead state.
374 * E.g. interleaved reads.
375 * Query the pagecache for async_size, which normally equals to
376 * readahead size. Ramp it up and use it as the new readahead size.
378 if (hit_readahead_marker
) {
381 read_lock_irq(&mapping
->tree_lock
);
382 start
= radix_tree_next_hole(&mapping
->page_tree
, offset
, max
+1);
383 read_unlock_irq(&mapping
->tree_lock
);
385 if (!start
|| start
- offset
> max
)
389 ra
->size
= start
- offset
; /* old async_size */
390 ra
->size
= get_next_ra_size(ra
, max
);
391 ra
->async_size
= ra
->size
;
397 * - first read on start of file
398 * - sequential cache miss
399 * - oversize random read
400 * Start readahead for it.
403 ra
->size
= get_init_ra_size(req_size
, max
);
404 ra
->async_size
= ra
->size
> req_size
? ra
->size
- req_size
: ra
->size
;
407 return ra_submit(ra
, mapping
, filp
);
411 * page_cache_sync_readahead - generic file readahead
412 * @mapping: address_space which holds the pagecache and I/O vectors
413 * @ra: file_ra_state which holds the readahead state
414 * @filp: passed on to ->readpage() and ->readpages()
415 * @offset: start offset into @mapping, in pagecache page-sized units
416 * @req_size: hint: total size of the read which the caller is performing in
419 * page_cache_sync_readahead() should be called when a cache miss happened:
420 * it will submit the read. The readahead logic may decide to piggyback more
421 * pages onto the read request if access patterns suggest it will improve
424 void page_cache_sync_readahead(struct address_space
*mapping
,
425 struct file_ra_state
*ra
, struct file
*filp
,
426 pgoff_t offset
, unsigned long req_size
)
433 ondemand_readahead(mapping
, ra
, filp
, false, offset
, req_size
);
435 EXPORT_SYMBOL_GPL(page_cache_sync_readahead
);
438 * page_cache_async_readahead - file readahead for marked pages
439 * @mapping: address_space which holds the pagecache and I/O vectors
440 * @ra: file_ra_state which holds the readahead state
441 * @filp: passed on to ->readpage() and ->readpages()
442 * @page: the page at @offset which has the PG_readahead flag set
443 * @offset: start offset into @mapping, in pagecache page-sized units
444 * @req_size: hint: total size of the read which the caller is performing in
447 * page_cache_async_ondemand() should be called when a page is used which
448 * has the PG_readahead flag: this is a marker to suggest that the application
449 * has used up enough of the readahead window that we should start pulling in
452 page_cache_async_readahead(struct address_space
*mapping
,
453 struct file_ra_state
*ra
, struct file
*filp
,
454 struct page
*page
, pgoff_t offset
,
455 unsigned long req_size
)
462 * Same bit is used for PG_readahead and PG_reclaim.
464 if (PageWriteback(page
))
467 ClearPageReadahead(page
);
470 * Defer asynchronous read-ahead on IO congestion.
472 if (bdi_read_congested(mapping
->backing_dev_info
))
476 ondemand_readahead(mapping
, ra
, filp
, true, offset
, req_size
);
478 EXPORT_SYMBOL_GPL(page_cache_async_readahead
);