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/pagevec.h>
18 void default_unplug_io_fn(struct backing_dev_info
*bdi
, struct page
*page
)
21 EXPORT_SYMBOL(default_unplug_io_fn
);
23 struct backing_dev_info default_backing_dev_info
= {
24 .ra_pages
= (VM_MAX_READAHEAD
* 1024) / PAGE_CACHE_SIZE
,
26 .capabilities
= BDI_CAP_MAP_COPY
,
27 .unplug_io_fn
= default_unplug_io_fn
,
29 EXPORT_SYMBOL_GPL(default_backing_dev_info
);
32 * Initialise a struct file's readahead state. Assumes that the caller has
36 file_ra_state_init(struct file_ra_state
*ra
, struct address_space
*mapping
)
38 ra
->ra_pages
= mapping
->backing_dev_info
->ra_pages
;
41 EXPORT_SYMBOL_GPL(file_ra_state_init
);
44 * Return max readahead size for this inode in number-of-pages.
46 static inline unsigned long get_max_readahead(struct file_ra_state
*ra
)
51 static inline unsigned long get_min_readahead(struct file_ra_state
*ra
)
53 return (VM_MIN_READAHEAD
* 1024) / PAGE_CACHE_SIZE
;
56 static inline void ra_off(struct file_ra_state
*ra
)
67 * Set the initial window size, round to next power of 2 and square
68 * for small size, x 4 for medium, and x 2 for large
69 * for 128k (32 page) max ra
70 * 1-8 page = 32k initial, > 8 page = 128k initial
72 static unsigned long get_init_ra_size(unsigned long size
, unsigned long max
)
74 unsigned long newsize
= roundup_pow_of_two(size
);
76 if (newsize
<= max
/ 64)
77 newsize
= newsize
* newsize
;
78 else if (newsize
<= max
/ 4)
86 * Set the new window size, this is called only when I/O is to be submitted,
87 * not for each call to readahead. If a cache miss occured, reduce next I/O
88 * size, else increase depending on how close to max we are.
90 static inline unsigned long get_next_ra_size(struct file_ra_state
*ra
)
92 unsigned long max
= get_max_readahead(ra
);
93 unsigned long min
= get_min_readahead(ra
);
94 unsigned long cur
= ra
->size
;
95 unsigned long newsize
;
97 if (ra
->flags
& RA_FLAG_MISS
) {
98 ra
->flags
&= ~RA_FLAG_MISS
;
99 newsize
= max((cur
- 2), min
);
100 } else if (cur
< max
/ 16) {
105 return min(newsize
, max
);
108 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
111 * read_cache_pages - populate an address space with some pages, and
112 * start reads against them.
113 * @mapping: the address_space
114 * @pages: The address of a list_head which contains the target pages. These
115 * pages have their ->index populated and are otherwise uninitialised.
116 * @filler: callback routine for filling a single page.
117 * @data: private data for the callback routine.
119 * Hides the details of the LRU cache etc from the filesystems.
121 int read_cache_pages(struct address_space
*mapping
, struct list_head
*pages
,
122 int (*filler
)(void *, struct page
*), void *data
)
125 struct pagevec lru_pvec
;
128 pagevec_init(&lru_pvec
, 0);
130 while (!list_empty(pages
)) {
131 page
= list_to_page(pages
);
132 list_del(&page
->lru
);
133 if (add_to_page_cache(page
, mapping
, page
->index
, GFP_KERNEL
)) {
134 page_cache_release(page
);
137 ret
= filler(data
, page
);
138 if (!pagevec_add(&lru_pvec
, page
))
139 __pagevec_lru_add(&lru_pvec
);
141 while (!list_empty(pages
)) {
144 victim
= list_to_page(pages
);
145 list_del(&victim
->lru
);
146 page_cache_release(victim
);
151 pagevec_lru_add(&lru_pvec
);
155 EXPORT_SYMBOL(read_cache_pages
);
157 static int read_pages(struct address_space
*mapping
, struct file
*filp
,
158 struct list_head
*pages
, unsigned nr_pages
)
161 struct pagevec lru_pvec
;
164 if (mapping
->a_ops
->readpages
) {
165 ret
= mapping
->a_ops
->readpages(filp
, mapping
, pages
, nr_pages
);
169 pagevec_init(&lru_pvec
, 0);
170 for (page_idx
= 0; page_idx
< nr_pages
; page_idx
++) {
171 struct page
*page
= list_to_page(pages
);
172 list_del(&page
->lru
);
173 if (!add_to_page_cache(page
, mapping
,
174 page
->index
, GFP_KERNEL
)) {
175 ret
= mapping
->a_ops
->readpage(filp
, page
);
176 if (ret
!= AOP_TRUNCATED_PAGE
) {
177 if (!pagevec_add(&lru_pvec
, page
))
178 __pagevec_lru_add(&lru_pvec
);
180 } /* else fall through to release */
182 page_cache_release(page
);
184 pagevec_lru_add(&lru_pvec
);
193 * The fields in struct file_ra_state represent the most-recently-executed
196 * start: Page index at which we started the readahead
197 * size: Number of pages in that read
198 * Together, these form the "current window".
199 * Together, start and size represent the `readahead window'.
200 * prev_page: The page which the readahead algorithm most-recently inspected.
201 * It is mainly used to detect sequential file reading.
202 * If page_cache_readahead sees that it is again being called for
203 * a page which it just looked at, it can return immediately without
204 * making any state changes.
206 * ahead_size: Together, these form the "ahead window".
207 * ra_pages: The externally controlled max readahead for this fd.
209 * When readahead is in the off state (size == 0), readahead is disabled.
210 * In this state, prev_page is used to detect the resumption of sequential I/O.
212 * The readahead code manages two windows - the "current" and the "ahead"
213 * windows. The intent is that while the application is walking the pages
214 * in the current window, I/O is underway on the ahead window. When the
215 * current window is fully traversed, it is replaced by the ahead window
216 * and the ahead window is invalidated. When this copying happens, the
217 * new current window's pages are probably still locked. So
218 * we submit a new batch of I/O immediately, creating a new ahead window.
222 * ----|----------------|----------------|-----
224 * ^ahead_start ^ahead_start+ahead_size
226 * ^ When this page is read, we submit I/O for the
229 * A `readahead hit' occurs when a read request is made against a page which is
230 * the next sequential page. Ahead window calculations are done only when it
231 * is time to submit a new IO. The code ramps up the size agressively at first,
232 * but slow down as it approaches max_readhead.
234 * Any seek/ramdom IO will result in readahead being turned off. It will resume
235 * at the first sequential access.
237 * There is a special-case: if the first page which the application tries to
238 * read happens to be the first page of the file, it is assumed that a linear
239 * read is about to happen and the window is immediately set to the initial size
240 * based on I/O request size and the max_readahead.
242 * This function is to be called for every read request, rather than when
243 * it is time to perform readahead. It is called only once for the entire I/O
244 * regardless of size unless readahead is unable to start enough I/O to satisfy
245 * the request (I/O request > max_readahead).
249 * do_page_cache_readahead actually reads a chunk of disk. It allocates all
250 * the pages first, then submits them all for I/O. This avoids the very bad
251 * behaviour which would occur if page allocations are causing VM writeback.
252 * We really don't want to intermingle reads and writes like that.
254 * Returns the number of pages requested, or the maximum amount of I/O allowed.
256 * do_page_cache_readahead() returns -1 if it encountered request queue
260 __do_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
261 pgoff_t offset
, unsigned long nr_to_read
)
263 struct inode
*inode
= mapping
->host
;
265 unsigned long end_index
; /* The last page we want to read */
266 LIST_HEAD(page_pool
);
269 loff_t isize
= i_size_read(inode
);
274 end_index
= ((isize
- 1) >> PAGE_CACHE_SHIFT
);
277 * Preallocate as many pages as we will need.
279 read_lock_irq(&mapping
->tree_lock
);
280 for (page_idx
= 0; page_idx
< nr_to_read
; page_idx
++) {
281 pgoff_t page_offset
= offset
+ page_idx
;
283 if (page_offset
> end_index
)
286 page
= radix_tree_lookup(&mapping
->page_tree
, page_offset
);
290 read_unlock_irq(&mapping
->tree_lock
);
291 page
= page_cache_alloc_cold(mapping
);
292 read_lock_irq(&mapping
->tree_lock
);
295 page
->index
= page_offset
;
296 list_add(&page
->lru
, &page_pool
);
299 read_unlock_irq(&mapping
->tree_lock
);
302 * Now start the IO. We ignore I/O errors - if the page is not
303 * uptodate then the caller will launch readpage again, and
304 * will then handle the error.
307 read_pages(mapping
, filp
, &page_pool
, ret
);
308 BUG_ON(!list_empty(&page_pool
));
314 * Chunk the readahead into 2 megabyte units, so that we don't pin too much
317 int force_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
318 pgoff_t offset
, unsigned long nr_to_read
)
322 if (unlikely(!mapping
->a_ops
->readpage
&& !mapping
->a_ops
->readpages
))
328 unsigned long this_chunk
= (2 * 1024 * 1024) / PAGE_CACHE_SIZE
;
330 if (this_chunk
> nr_to_read
)
331 this_chunk
= nr_to_read
;
332 err
= __do_page_cache_readahead(mapping
, filp
,
339 offset
+= this_chunk
;
340 nr_to_read
-= this_chunk
;
346 * Check how effective readahead is being. If the amount of started IO is
347 * less than expected then the file is partly or fully in pagecache and
348 * readahead isn't helping.
351 static inline int check_ra_success(struct file_ra_state
*ra
,
352 unsigned long nr_to_read
, unsigned long actual
)
355 ra
->cache_hit
+= nr_to_read
;
356 if (ra
->cache_hit
>= VM_MAX_CACHE_HIT
) {
358 ra
->flags
|= RA_FLAG_INCACHE
;
368 * This version skips the IO if the queue is read-congested, and will tell the
369 * block layer to abandon the readahead if request allocation would block.
371 * force_page_cache_readahead() will ignore queue congestion and will block on
374 int do_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
375 pgoff_t offset
, unsigned long nr_to_read
)
377 if (bdi_read_congested(mapping
->backing_dev_info
))
380 return __do_page_cache_readahead(mapping
, filp
, offset
, nr_to_read
);
384 * Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block'
385 * is set wait till the read completes. Otherwise attempt to read without
387 * Returns 1 meaning 'success' if read is succesfull without switching off
388 * readhaead mode. Otherwise return failure.
391 blockable_page_cache_readahead(struct address_space
*mapping
, struct file
*filp
,
392 pgoff_t offset
, unsigned long nr_to_read
,
393 struct file_ra_state
*ra
, int block
)
397 if (!block
&& bdi_read_congested(mapping
->backing_dev_info
))
400 actual
= __do_page_cache_readahead(mapping
, filp
, offset
, nr_to_read
);
402 return check_ra_success(ra
, nr_to_read
, actual
);
405 static int make_ahead_window(struct address_space
*mapping
, struct file
*filp
,
406 struct file_ra_state
*ra
, int force
)
410 ra
->ahead_size
= get_next_ra_size(ra
);
411 ra
->ahead_start
= ra
->start
+ ra
->size
;
413 block
= force
|| (ra
->prev_page
>= ra
->ahead_start
);
414 ret
= blockable_page_cache_readahead(mapping
, filp
,
415 ra
->ahead_start
, ra
->ahead_size
, ra
, block
);
417 if (!ret
&& !force
) {
418 /* A read failure in blocking mode, implies pages are
419 * all cached. So we can safely assume we have taken
420 * care of all the pages requested in this call.
421 * A read failure in non-blocking mode, implies we are
422 * reading more pages than requested in this call. So
423 * we safely assume we have taken care of all the pages
424 * requested in this call.
426 * Just reset the ahead window in case we failed due to
427 * congestion. The ahead window will any way be closed
428 * in case we failed due to excessive page cache hits.
438 * page_cache_readahead - generic adaptive readahead
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 * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
443 * @req_size: hint: total size of the read which the caller is performing in
444 * PAGE_CACHE_SIZE units
446 * page_cache_readahead() is the main function. If performs the adaptive
447 * readahead window size management and submits the readahead I/O.
449 * Note that @filp is purely used for passing on to the ->readpage[s]()
450 * handler: it may refer to a different file from @mapping (so we may not use
451 * @filp->f_mapping or @filp->f_dentry->d_inode here).
452 * Also, @ra may not be equal to &@filp->f_ra.
456 page_cache_readahead(struct address_space
*mapping
, struct file_ra_state
*ra
,
457 struct file
*filp
, pgoff_t offset
, unsigned long req_size
)
459 unsigned long max
, newsize
;
463 * We avoid doing extra work and bogusly perturbing the readahead
464 * window expansion logic.
466 if (offset
== ra
->prev_page
&& --req_size
)
469 /* Note that prev_page == -1 if it is a first read */
470 sequential
= (offset
== ra
->prev_page
+ 1);
471 ra
->prev_page
= offset
;
473 max
= get_max_readahead(ra
);
474 newsize
= min(req_size
, max
);
476 /* No readahead or sub-page sized read or file already in cache */
477 if (newsize
== 0 || (ra
->flags
& RA_FLAG_INCACHE
))
480 ra
->prev_page
+= newsize
- 1;
483 * Special case - first read at start of file. We'll assume it's
484 * a whole-file read and grow the window fast. Or detect first
487 if (sequential
&& ra
->size
== 0) {
488 ra
->size
= get_init_ra_size(newsize
, max
);
490 if (!blockable_page_cache_readahead(mapping
, filp
, offset
,
495 * If the request size is larger than our max readahead, we
496 * at least want to be sure that we get 2 IOs in flight and
497 * we know that we will definitly need the new I/O.
498 * once we do this, subsequent calls should be able to overlap
499 * IOs,* thus preventing stalls. so issue the ahead window
503 make_ahead_window(mapping
, filp
, ra
, 1);
509 * Now handle the random case:
510 * partial page reads and first access were handled above,
511 * so this must be the next page otherwise it is random
515 blockable_page_cache_readahead(mapping
, filp
, offset
,
521 * If we get here we are doing sequential IO and this was not the first
522 * occurence (ie we have an existing window)
525 if (ra
->ahead_start
== 0) { /* no ahead window yet */
526 if (!make_ahead_window(mapping
, filp
, ra
, 0))
530 * Already have an ahead window, check if we crossed into it.
531 * If so, shift windows and issue a new ahead window.
532 * Only return the #pages that are in the current window, so that
533 * we get called back on the first page of the ahead window which
534 * will allow us to submit more IO.
536 if (ra
->prev_page
>= ra
->ahead_start
) {
537 ra
->start
= ra
->ahead_start
;
538 ra
->size
= ra
->ahead_size
;
539 make_ahead_window(mapping
, filp
, ra
, 0);
543 return ra
->prev_page
+ 1;
547 * handle_ra_miss() is called when it is known that a page which should have
548 * been present in the pagecache (we just did some readahead there) was in fact
549 * not found. This will happen if it was evicted by the VM (readahead
552 * Turn on the cache miss flag in the RA struct, this will cause the RA code
553 * to reduce the RA size on the next read.
555 void handle_ra_miss(struct address_space
*mapping
,
556 struct file_ra_state
*ra
, pgoff_t offset
)
558 ra
->flags
|= RA_FLAG_MISS
;
559 ra
->flags
&= ~RA_FLAG_INCACHE
;
564 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
565 * sensible upper limit.
567 unsigned long max_sane_readahead(unsigned long nr
)
569 unsigned long active
;
570 unsigned long inactive
;
573 __get_zone_counts(&active
, &inactive
, &free
, NODE_DATA(numa_node_id()));
574 return min(nr
, (inactive
+ free
) / 2);