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Linux/mm/readahead.c

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  1 /*
  2  * mm/readahead.c - address_space-level file readahead.
  3  *
  4  * Copyright (C) 2002, Linus Torvalds
  5  *
  6  * 09Apr2002    Andrew Morton
  7  *              Initial version.
  8  */
  9 
 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>
 20 
 21 #include "internal.h"
 22 
 23 /*
 24  * Initialise a struct file's readahead state.  Assumes that the caller has
 25  * memset *ra to zero.
 26  */
 27 void
 28 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
 29 {
 30         ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
 31         ra->prev_pos = -1;
 32 }
 33 EXPORT_SYMBOL_GPL(file_ra_state_init);
 34 
 35 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
 36 
 37 /*
 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
 42  *   an error
 43  */
 44 static void read_cache_pages_invalidate_page(struct address_space *mapping,
 45                                              struct page *page)
 46 {
 47         if (page_has_private(page)) {
 48                 if (!trylock_page(page))
 49                         BUG();
 50                 page->mapping = mapping;
 51                 do_invalidatepage(page, 0, PAGE_CACHE_SIZE);
 52                 page->mapping = NULL;
 53                 unlock_page(page);
 54         }
 55         page_cache_release(page);
 56 }
 57 
 58 /*
 59  * release a list of pages, invalidating them first if need be
 60  */
 61 static void read_cache_pages_invalidate_pages(struct address_space *mapping,
 62                                               struct list_head *pages)
 63 {
 64         struct page *victim;
 65 
 66         while (!list_empty(pages)) {
 67                 victim = list_to_page(pages);
 68                 list_del(&victim->lru);
 69                 read_cache_pages_invalidate_page(mapping, victim);
 70         }
 71 }
 72 
 73 /**
 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.
 80  *
 81  * Hides the details of the LRU cache etc from the filesystems.
 82  */
 83 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
 84                         int (*filler)(void *, struct page *), void *data)
 85 {
 86         struct page *page;
 87         int ret = 0;
 88 
 89         while (!list_empty(pages)) {
 90                 page = list_to_page(pages);
 91                 list_del(&page->lru);
 92                 if (add_to_page_cache_lru(page, mapping,
 93                                         page->index, GFP_KERNEL)) {
 94                         read_cache_pages_invalidate_page(mapping, page);
 95                         continue;
 96                 }
 97                 page_cache_release(page);
 98 
 99                 ret = filler(data, page);
100                 if (unlikely(ret)) {
101                         read_cache_pages_invalidate_pages(mapping, pages);
102                         break;
103                 }
104                 task_io_account_read(PAGE_CACHE_SIZE);
105         }
106         return ret;
107 }
108 
109 EXPORT_SYMBOL(read_cache_pages);
110 
111 static int read_pages(struct address_space *mapping, struct file *filp,
112                 struct list_head *pages, unsigned nr_pages)
113 {
114         struct blk_plug plug;
115         unsigned page_idx;
116         int ret;
117 
118         blk_start_plug(&plug);
119 
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);
124                 goto out;
125         }
126 
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,
131                                         page->index, GFP_KERNEL)) {
132                         mapping->a_ops->readpage(filp, page);
133                 }
134                 page_cache_release(page);
135         }
136         ret = 0;
137 
138 out:
139         blk_finish_plug(&plug);
140 
141         return ret;
142 }
143 
144 /*
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.
149  *
150  * Returns the number of pages requested, or the maximum amount of I/O allowed.
151  */
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)
155 {
156         struct inode *inode = mapping->host;
157         struct page *page;
158         unsigned long end_index;        /* The last page we want to read */
159         LIST_HEAD(page_pool);
160         int page_idx;
161         int ret = 0;
162         loff_t isize = i_size_read(inode);
163 
164         if (isize == 0)
165                 goto out;
166 
167         end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
168 
169         /*
170          * Preallocate as many pages as we will need.
171          */
172         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
173                 pgoff_t page_offset = offset + page_idx;
174 
175                 if (page_offset > end_index)
176                         break;
177 
178                 rcu_read_lock();
179                 page = radix_tree_lookup(&mapping->page_tree, page_offset);
180                 rcu_read_unlock();
181                 if (page && !radix_tree_exceptional_entry(page))
182                         continue;
183 
184                 page = page_cache_alloc_readahead(mapping);
185                 if (!page)
186                         break;
187                 page->index = page_offset;
188                 list_add(&page->lru, &page_pool);
189                 if (page_idx == nr_to_read - lookahead_size)
190                         SetPageReadahead(page);
191                 ret++;
192         }
193 
194         /*
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.
198          */
199         if (ret)
200                 read_pages(mapping, filp, &page_pool, ret);
201         BUG_ON(!list_empty(&page_pool));
202 out:
203         return ret;
204 }
205 
206 /*
207  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
208  * memory at once.
209  */
210 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
211                 pgoff_t offset, unsigned long nr_to_read)
212 {
213         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
214                 return -EINVAL;
215 
216         nr_to_read = max_sane_readahead(nr_to_read);
217         while (nr_to_read) {
218                 int err;
219 
220                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
221 
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);
226                 if (err < 0)
227                         return err;
228 
229                 offset += this_chunk;
230                 nr_to_read -= this_chunk;
231         }
232         return 0;
233 }
234 
235 #define MAX_READAHEAD   ((512*4096)/PAGE_CACHE_SIZE)
236 /*
237  * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
238  * sensible upper limit.
239  */
240 unsigned long max_sane_readahead(unsigned long nr)
241 {
242         return min(nr, MAX_READAHEAD);
243 }
244 
245 /*
246  * Set the initial window size, round to next power of 2 and square
247  * for small size, x 4 for medium, and x 2 for large
248  * for 128k (32 page) max ra
249  * 1-8 page = 32k initial, > 8 page = 128k initial
250  */
251 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
252 {
253         unsigned long newsize = roundup_pow_of_two(size);
254 
255         if (newsize <= max / 32)
256                 newsize = newsize * 4;
257         else if (newsize <= max / 4)
258                 newsize = newsize * 2;
259         else
260                 newsize = max;
261 
262         return newsize;
263 }
264 
265 /*
266  *  Get the previous window size, ramp it up, and
267  *  return it as the new window size.
268  */
269 static unsigned long get_next_ra_size(struct file_ra_state *ra,
270                                                 unsigned long max)
271 {
272         unsigned long cur = ra->size;
273         unsigned long newsize;
274 
275         if (cur < max / 16)
276                 newsize = 4 * cur;
277         else
278                 newsize = 2 * cur;
279 
280         return min(newsize, max);
281 }
282 
283 /*
284  * On-demand readahead design.
285  *
286  * The fields in struct file_ra_state represent the most-recently-executed
287  * readahead attempt:
288  *
289  *                        |<----- async_size ---------|
290  *     |------------------- size -------------------->|
291  *     |==================#===========================|
292  *     ^start             ^page marked with PG_readahead
293  *
294  * To overlap application thinking time and disk I/O time, we do
295  * `readahead pipelining': Do not wait until the application consumed all
296  * readahead pages and stalled on the missing page at readahead_index;
297  * Instead, submit an asynchronous readahead I/O as soon as there are
298  * only async_size pages left in the readahead window. Normally async_size
299  * will be equal to size, for maximum pipelining.
300  *
301  * In interleaved sequential reads, concurrent streams on the same fd can
302  * be invalidating each other's readahead state. So we flag the new readahead
303  * page at (start+size-async_size) with PG_readahead, and use it as readahead
304  * indicator. The flag won't be set on already cached pages, to avoid the
305  * readahead-for-nothing fuss, saving pointless page cache lookups.
306  *
307  * prev_pos tracks the last visited byte in the _previous_ read request.
308  * It should be maintained by the caller, and will be used for detecting
309  * small random reads. Note that the readahead algorithm checks loosely
310  * for sequential patterns. Hence interleaved reads might be served as
311  * sequential ones.
312  *
313  * There is a special-case: if the first page which the application tries to
314  * read happens to be the first page of the file, it is assumed that a linear
315  * read is about to happen and the window is immediately set to the initial size
316  * based on I/O request size and the max_readahead.
317  *
318  * The code ramps up the readahead size aggressively at first, but slow down as
319  * it approaches max_readhead.
320  */
321 
322 /*
323  * Count contiguously cached pages from @offset-1 to @offset-@max,
324  * this count is a conservative estimation of
325  *      - length of the sequential read sequence, or
326  *      - thrashing threshold in memory tight systems
327  */
328 static pgoff_t count_history_pages(struct address_space *mapping,
329                                    pgoff_t offset, unsigned long max)
330 {
331         pgoff_t head;
332 
333         rcu_read_lock();
334         head = page_cache_prev_hole(mapping, offset - 1, max);
335         rcu_read_unlock();
336 
337         return offset - 1 - head;
338 }
339 
340 /*
341  * page cache context based read-ahead
342  */
343 static int try_context_readahead(struct address_space *mapping,
344                                  struct file_ra_state *ra,
345                                  pgoff_t offset,
346                                  unsigned long req_size,
347                                  unsigned long max)
348 {
349         pgoff_t size;
350 
351         size = count_history_pages(mapping, offset, max);
352 
353         /*
354          * not enough history pages:
355          * it could be a random read
356          */
357         if (size <= req_size)
358                 return 0;
359 
360         /*
361          * starts from beginning of file:
362          * it is a strong indication of long-run stream (or whole-file-read)
363          */
364         if (size >= offset)
365                 size *= 2;
366 
367         ra->start = offset;
368         ra->size = min(size + req_size, max);
369         ra->async_size = 1;
370 
371         return 1;
372 }
373 
374 /*
375  * A minimal readahead algorithm for trivial sequential/random reads.
376  */
377 static unsigned long
378 ondemand_readahead(struct address_space *mapping,
379                    struct file_ra_state *ra, struct file *filp,
380                    bool hit_readahead_marker, pgoff_t offset,
381                    unsigned long req_size)
382 {
383         unsigned long max = max_sane_readahead(ra->ra_pages);
384         pgoff_t prev_offset;
385 
386         /*
387          * start of file
388          */
389         if (!offset)
390                 goto initial_readahead;
391 
392         /*
393          * It's the expected callback offset, assume sequential access.
394          * Ramp up sizes, and push forward the readahead window.
395          */
396         if ((offset == (ra->start + ra->size - ra->async_size) ||
397              offset == (ra->start + ra->size))) {
398                 ra->start += ra->size;
399                 ra->size = get_next_ra_size(ra, max);
400                 ra->async_size = ra->size;
401                 goto readit;
402         }
403 
404         /*
405          * Hit a marked page without valid readahead state.
406          * E.g. interleaved reads.
407          * Query the pagecache for async_size, which normally equals to
408          * readahead size. Ramp it up and use it as the new readahead size.
409          */
410         if (hit_readahead_marker) {
411                 pgoff_t start;
412 
413                 rcu_read_lock();
414                 start = page_cache_next_hole(mapping, offset + 1, max);
415                 rcu_read_unlock();
416 
417                 if (!start || start - offset > max)
418                         return 0;
419 
420                 ra->start = start;
421                 ra->size = start - offset;      /* old async_size */
422                 ra->size += req_size;
423                 ra->size = get_next_ra_size(ra, max);
424                 ra->async_size = ra->size;
425                 goto readit;
426         }
427 
428         /*
429          * oversize read
430          */
431         if (req_size > max)
432                 goto initial_readahead;
433 
434         /*
435          * sequential cache miss
436          * trivial case: (offset - prev_offset) == 1
437          * unaligned reads: (offset - prev_offset) == 0
438          */
439         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_CACHE_SHIFT;
440         if (offset - prev_offset <= 1UL)
441                 goto initial_readahead;
442 
443         /*
444          * Query the page cache and look for the traces(cached history pages)
445          * that a sequential stream would leave behind.
446          */
447         if (try_context_readahead(mapping, ra, offset, req_size, max))
448                 goto readit;
449 
450         /*
451          * standalone, small random read
452          * Read as is, and do not pollute the readahead state.
453          */
454         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
455 
456 initial_readahead:
457         ra->start = offset;
458         ra->size = get_init_ra_size(req_size, max);
459         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
460 
461 readit:
462         /*
463          * Will this read hit the readahead marker made by itself?
464          * If so, trigger the readahead marker hit now, and merge
465          * the resulted next readahead window into the current one.
466          */
467         if (offset == ra->start && ra->size == ra->async_size) {
468                 ra->async_size = get_next_ra_size(ra, max);
469                 ra->size += ra->async_size;
470         }
471 
472         return ra_submit(ra, mapping, filp);
473 }
474 
475 /**
476  * page_cache_sync_readahead - generic file readahead
477  * @mapping: address_space which holds the pagecache and I/O vectors
478  * @ra: file_ra_state which holds the readahead state
479  * @filp: passed on to ->readpage() and ->readpages()
480  * @offset: start offset into @mapping, in pagecache page-sized units
481  * @req_size: hint: total size of the read which the caller is performing in
482  *            pagecache pages
483  *
484  * page_cache_sync_readahead() should be called when a cache miss happened:
485  * it will submit the read.  The readahead logic may decide to piggyback more
486  * pages onto the read request if access patterns suggest it will improve
487  * performance.
488  */
489 void page_cache_sync_readahead(struct address_space *mapping,
490                                struct file_ra_state *ra, struct file *filp,
491                                pgoff_t offset, unsigned long req_size)
492 {
493         /* no read-ahead */
494         if (!ra->ra_pages)
495                 return;
496 
497         /* be dumb */
498         if (filp && (filp->f_mode & FMODE_RANDOM)) {
499                 force_page_cache_readahead(mapping, filp, offset, req_size);
500                 return;
501         }
502 
503         /* do read-ahead */
504         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
505 }
506 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
507 
508 /**
509  * page_cache_async_readahead - file readahead for marked pages
510  * @mapping: address_space which holds the pagecache and I/O vectors
511  * @ra: file_ra_state which holds the readahead state
512  * @filp: passed on to ->readpage() and ->readpages()
513  * @page: the page at @offset which has the PG_readahead flag set
514  * @offset: start offset into @mapping, in pagecache page-sized units
515  * @req_size: hint: total size of the read which the caller is performing in
516  *            pagecache pages
517  *
518  * page_cache_async_readahead() should be called when a page is used which
519  * has the PG_readahead flag; this is a marker to suggest that the application
520  * has used up enough of the readahead window that we should start pulling in
521  * more pages.
522  */
523 void
524 page_cache_async_readahead(struct address_space *mapping,
525                            struct file_ra_state *ra, struct file *filp,
526                            struct page *page, pgoff_t offset,
527                            unsigned long req_size)
528 {
529         /* no read-ahead */
530         if (!ra->ra_pages)
531                 return;
532 
533         /*
534          * Same bit is used for PG_readahead and PG_reclaim.
535          */
536         if (PageWriteback(page))
537                 return;
538 
539         ClearPageReadahead(page);
540 
541         /*
542          * Defer asynchronous read-ahead on IO congestion.
543          */
544         if (bdi_read_congested(inode_to_bdi(mapping->host)))
545                 return;
546 
547         /* do read-ahead */
548         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
549 }
550 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
551 
552 static ssize_t
553 do_readahead(struct address_space *mapping, struct file *filp,
554              pgoff_t index, unsigned long nr)
555 {
556         if (!mapping || !mapping->a_ops)
557                 return -EINVAL;
558 
559         return force_page_cache_readahead(mapping, filp, index, nr);
560 }
561 
562 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
563 {
564         ssize_t ret;
565         struct fd f;
566 
567         ret = -EBADF;
568         f = fdget(fd);
569         if (f.file) {
570                 if (f.file->f_mode & FMODE_READ) {
571                         struct address_space *mapping = f.file->f_mapping;
572                         pgoff_t start = offset >> PAGE_CACHE_SHIFT;
573                         pgoff_t end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
574                         unsigned long len = end - start + 1;
575                         ret = do_readahead(mapping, f.file, start, len);
576                 }
577                 fdput(f);
578         }
579         return ret;
580 }
581 

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