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Linux/mm/readahead.c

  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/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>
 22 
 23 #include "internal.h"
 24 
 25 /*
 26  * Initialise a struct file's readahead state.  Assumes that the caller has
 27  * memset *ra to zero.
 28  */
 29 void
 30 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
 31 {
 32         ra->ra_pages = inode_to_bdi(mapping->host)->ra_pages;
 33         ra->prev_pos = -1;
 34 }
 35 EXPORT_SYMBOL_GPL(file_ra_state_init);
 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_SIZE);
 52                 page->mapping = NULL;
 53                 unlock_page(page);
 54         }
 55         put_page(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 = lru_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 = lru_to_page(pages);
 91                 list_del(&page->lru);
 92                 if (add_to_page_cache_lru(page, mapping, page->index,
 93                                 readahead_gfp_mask(mapping))) {
 94                         read_cache_pages_invalidate_page(mapping, page);
 95                         continue;
 96                 }
 97                 put_page(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_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 int nr_pages, gfp_t gfp)
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 = 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);
132                 put_page(page);
133         }
134         ret = 0;
135 
136 out:
137         blk_finish_plug(&plug);
138 
139         return ret;
140 }
141 
142 /*
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.
147  *
148  * Returns the number of pages requested, or the maximum amount of I/O allowed.
149  */
150 int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
151                         pgoff_t offset, unsigned long nr_to_read,
152                         unsigned long lookahead_size)
153 {
154         struct inode *inode = mapping->host;
155         struct page *page;
156         unsigned long end_index;        /* The last page we want to read */
157         LIST_HEAD(page_pool);
158         int page_idx;
159         int ret = 0;
160         loff_t isize = i_size_read(inode);
161         gfp_t gfp_mask = readahead_gfp_mask(mapping);
162 
163         if (isize == 0)
164                 goto out;
165 
166         end_index = ((isize - 1) >> PAGE_SHIFT);
167 
168         /*
169          * Preallocate as many pages as we will need.
170          */
171         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
172                 pgoff_t page_offset = offset + page_idx;
173 
174                 if (page_offset > end_index)
175                         break;
176 
177                 rcu_read_lock();
178                 page = radix_tree_lookup(&mapping->page_tree, page_offset);
179                 rcu_read_unlock();
180                 if (page && !radix_tree_exceptional_entry(page))
181                         continue;
182 
183                 page = __page_cache_alloc(gfp_mask);
184                 if (!page)
185                         break;
186                 page->index = page_offset;
187                 list_add(&page->lru, &page_pool);
188                 if (page_idx == nr_to_read - lookahead_size)
189                         SetPageReadahead(page);
190                 ret++;
191         }
192 
193         /*
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.
197          */
198         if (ret)
199                 read_pages(mapping, filp, &page_pool, ret, gfp_mask);
200         BUG_ON(!list_empty(&page_pool));
201 out:
202         return ret;
203 }
204 
205 /*
206  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
207  * memory at once.
208  */
209 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
210                                pgoff_t offset, unsigned long nr_to_read)
211 {
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;
215 
216         if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
217                 return -EINVAL;
218 
219         /*
220          * If the request exceeds the readahead window, allow the read to
221          * be up to the optimal hardware IO size
222          */
223         max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
224         nr_to_read = min(nr_to_read, max_pages);
225         while (nr_to_read) {
226                 int err;
227 
228                 unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
229 
230                 if (this_chunk > nr_to_read)
231                         this_chunk = nr_to_read;
232                 err = __do_page_cache_readahead(mapping, filp,
233                                                 offset, this_chunk, 0);
234                 if (err < 0)
235                         return err;
236 
237                 offset += this_chunk;
238                 nr_to_read -= this_chunk;
239         }
240         return 0;
241 }
242 
243 /*
244  * Set the initial window size, round to next power of 2 and square
245  * for small size, x 4 for medium, and x 2 for large
246  * for 128k (32 page) max ra
247  * 1-8 page = 32k initial, > 8 page = 128k initial
248  */
249 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
250 {
251         unsigned long newsize = roundup_pow_of_two(size);
252 
253         if (newsize <= max / 32)
254                 newsize = newsize * 4;
255         else if (newsize <= max / 4)
256                 newsize = newsize * 2;
257         else
258                 newsize = max;
259 
260         return newsize;
261 }
262 
263 /*
264  *  Get the previous window size, ramp it up, and
265  *  return it as the new window size.
266  */
267 static unsigned long get_next_ra_size(struct file_ra_state *ra,
268                                                 unsigned long max)
269 {
270         unsigned long cur = ra->size;
271         unsigned long newsize;
272 
273         if (cur < max / 16)
274                 newsize = 4 * cur;
275         else
276                 newsize = 2 * cur;
277 
278         return min(newsize, max);
279 }
280 
281 /*
282  * On-demand readahead design.
283  *
284  * The fields in struct file_ra_state represent the most-recently-executed
285  * readahead attempt:
286  *
287  *                        |<----- async_size ---------|
288  *     |------------------- size -------------------->|
289  *     |==================#===========================|
290  *     ^start             ^page marked with PG_readahead
291  *
292  * To overlap application thinking time and disk I/O time, we do
293  * `readahead pipelining': Do not wait until the application consumed all
294  * readahead pages and stalled on the missing page at readahead_index;
295  * Instead, submit an asynchronous readahead I/O as soon as there are
296  * only async_size pages left in the readahead window. Normally async_size
297  * will be equal to size, for maximum pipelining.
298  *
299  * In interleaved sequential reads, concurrent streams on the same fd can
300  * be invalidating each other's readahead state. So we flag the new readahead
301  * page at (start+size-async_size) with PG_readahead, and use it as readahead
302  * indicator. The flag won't be set on already cached pages, to avoid the
303  * readahead-for-nothing fuss, saving pointless page cache lookups.
304  *
305  * prev_pos tracks the last visited byte in the _previous_ read request.
306  * It should be maintained by the caller, and will be used for detecting
307  * small random reads. Note that the readahead algorithm checks loosely
308  * for sequential patterns. Hence interleaved reads might be served as
309  * sequential ones.
310  *
311  * There is a special-case: if the first page which the application tries to
312  * read happens to be the first page of the file, it is assumed that a linear
313  * read is about to happen and the window is immediately set to the initial size
314  * based on I/O request size and the max_readahead.
315  *
316  * The code ramps up the readahead size aggressively at first, but slow down as
317  * it approaches max_readhead.
318  */
319 
320 /*
321  * Count contiguously cached pages from @offset-1 to @offset-@max,
322  * this count is a conservative estimation of
323  *      - length of the sequential read sequence, or
324  *      - thrashing threshold in memory tight systems
325  */
326 static pgoff_t count_history_pages(struct address_space *mapping,
327                                    pgoff_t offset, unsigned long max)
328 {
329         pgoff_t head;
330 
331         rcu_read_lock();
332         head = page_cache_prev_hole(mapping, offset - 1, max);
333         rcu_read_unlock();
334 
335         return offset - 1 - head;
336 }
337 
338 /*
339  * page cache context based read-ahead
340  */
341 static int try_context_readahead(struct address_space *mapping,
342                                  struct file_ra_state *ra,
343                                  pgoff_t offset,
344                                  unsigned long req_size,
345                                  unsigned long max)
346 {
347         pgoff_t size;
348 
349         size = count_history_pages(mapping, offset, max);
350 
351         /*
352          * not enough history pages:
353          * it could be a random read
354          */
355         if (size <= req_size)
356                 return 0;
357 
358         /*
359          * starts from beginning of file:
360          * it is a strong indication of long-run stream (or whole-file-read)
361          */
362         if (size >= offset)
363                 size *= 2;
364 
365         ra->start = offset;
366         ra->size = min(size + req_size, max);
367         ra->async_size = 1;
368 
369         return 1;
370 }
371 
372 /*
373  * A minimal readahead algorithm for trivial sequential/random reads.
374  */
375 static unsigned long
376 ondemand_readahead(struct address_space *mapping,
377                    struct file_ra_state *ra, struct file *filp,
378                    bool hit_readahead_marker, pgoff_t offset,
379                    unsigned long req_size)
380 {
381         struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
382         unsigned long max_pages = ra->ra_pages;
383         pgoff_t prev_offset;
384 
385         /*
386          * If the request exceeds the readahead window, allow the read to
387          * be up to the optimal hardware IO size
388          */
389         if (req_size > max_pages && bdi->io_pages > max_pages)
390                 max_pages = min(req_size, bdi->io_pages);
391 
392         /*
393          * start of file
394          */
395         if (!offset)
396                 goto initial_readahead;
397 
398         /*
399          * It's the expected callback offset, assume sequential access.
400          * Ramp up sizes, and push forward the readahead window.
401          */
402         if ((offset == (ra->start + ra->size - ra->async_size) ||
403              offset == (ra->start + ra->size))) {
404                 ra->start += ra->size;
405                 ra->size = get_next_ra_size(ra, max_pages);
406                 ra->async_size = ra->size;
407                 goto readit;
408         }
409 
410         /*
411          * Hit a marked page without valid readahead state.
412          * E.g. interleaved reads.
413          * Query the pagecache for async_size, which normally equals to
414          * readahead size. Ramp it up and use it as the new readahead size.
415          */
416         if (hit_readahead_marker) {
417                 pgoff_t start;
418 
419                 rcu_read_lock();
420                 start = page_cache_next_hole(mapping, offset + 1, max_pages);
421                 rcu_read_unlock();
422 
423                 if (!start || start - offset > max_pages)
424                         return 0;
425 
426                 ra->start = start;
427                 ra->size = start - offset;      /* old async_size */
428                 ra->size += req_size;
429                 ra->size = get_next_ra_size(ra, max_pages);
430                 ra->async_size = ra->size;
431                 goto readit;
432         }
433 
434         /*
435          * oversize read
436          */
437         if (req_size > max_pages)
438                 goto initial_readahead;
439 
440         /*
441          * sequential cache miss
442          * trivial case: (offset - prev_offset) == 1
443          * unaligned reads: (offset - prev_offset) == 0
444          */
445         prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
446         if (offset - prev_offset <= 1UL)
447                 goto initial_readahead;
448 
449         /*
450          * Query the page cache and look for the traces(cached history pages)
451          * that a sequential stream would leave behind.
452          */
453         if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
454                 goto readit;
455 
456         /*
457          * standalone, small random read
458          * Read as is, and do not pollute the readahead state.
459          */
460         return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
461 
462 initial_readahead:
463         ra->start = offset;
464         ra->size = get_init_ra_size(req_size, max_pages);
465         ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
466 
467 readit:
468         /*
469          * Will this read hit the readahead marker made by itself?
470          * If so, trigger the readahead marker hit now, and merge
471          * the resulted next readahead window into the current one.
472          */
473         if (offset == ra->start && ra->size == ra->async_size) {
474                 ra->async_size = get_next_ra_size(ra, max_pages);
475                 ra->size += ra->async_size;
476         }
477 
478         return ra_submit(ra, mapping, filp);
479 }
480 
481 /**
482  * page_cache_sync_readahead - generic file readahead
483  * @mapping: address_space which holds the pagecache and I/O vectors
484  * @ra: file_ra_state which holds the readahead state
485  * @filp: passed on to ->readpage() and ->readpages()
486  * @offset: start offset into @mapping, in pagecache page-sized units
487  * @req_size: hint: total size of the read which the caller is performing in
488  *            pagecache pages
489  *
490  * page_cache_sync_readahead() should be called when a cache miss happened:
491  * it will submit the read.  The readahead logic may decide to piggyback more
492  * pages onto the read request if access patterns suggest it will improve
493  * performance.
494  */
495 void page_cache_sync_readahead(struct address_space *mapping,
496                                struct file_ra_state *ra, struct file *filp,
497                                pgoff_t offset, unsigned long req_size)
498 {
499         /* no read-ahead */
500         if (!ra->ra_pages)
501                 return;
502 
503         /* be dumb */
504         if (filp && (filp->f_mode & FMODE_RANDOM)) {
505                 force_page_cache_readahead(mapping, filp, offset, req_size);
506                 return;
507         }
508 
509         /* do read-ahead */
510         ondemand_readahead(mapping, ra, filp, false, offset, req_size);
511 }
512 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
513 
514 /**
515  * page_cache_async_readahead - file readahead for marked pages
516  * @mapping: address_space which holds the pagecache and I/O vectors
517  * @ra: file_ra_state which holds the readahead state
518  * @filp: passed on to ->readpage() and ->readpages()
519  * @page: the page at @offset which has the PG_readahead flag set
520  * @offset: start offset into @mapping, in pagecache page-sized units
521  * @req_size: hint: total size of the read which the caller is performing in
522  *            pagecache pages
523  *
524  * page_cache_async_readahead() should be called when a page is used which
525  * has the PG_readahead flag; this is a marker to suggest that the application
526  * has used up enough of the readahead window that we should start pulling in
527  * more pages.
528  */
529 void
530 page_cache_async_readahead(struct address_space *mapping,
531                            struct file_ra_state *ra, struct file *filp,
532                            struct page *page, pgoff_t offset,
533                            unsigned long req_size)
534 {
535         /* no read-ahead */
536         if (!ra->ra_pages)
537                 return;
538 
539         /*
540          * Same bit is used for PG_readahead and PG_reclaim.
541          */
542         if (PageWriteback(page))
543                 return;
544 
545         ClearPageReadahead(page);
546 
547         /*
548          * Defer asynchronous read-ahead on IO congestion.
549          */
550         if (inode_read_congested(mapping->host))
551                 return;
552 
553         /* do read-ahead */
554         ondemand_readahead(mapping, ra, filp, true, offset, req_size);
555 }
556 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
557 
558 static ssize_t
559 do_readahead(struct address_space *mapping, struct file *filp,
560              pgoff_t index, unsigned long nr)
561 {
562         if (!mapping || !mapping->a_ops)
563                 return -EINVAL;
564 
565         /*
566          * Readahead doesn't make sense for DAX inodes, but we don't want it
567          * to report a failure either.  Instead, we just return success and
568          * don't do any work.
569          */
570         if (dax_mapping(mapping))
571                 return 0;
572 
573         return force_page_cache_readahead(mapping, filp, index, nr);
574 }
575 
576 SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
577 {
578         ssize_t ret;
579         struct fd f;
580 
581         ret = -EBADF;
582         f = fdget(fd);
583         if (f.file) {
584                 if (f.file->f_mode & FMODE_READ) {
585                         struct address_space *mapping = f.file->f_mapping;
586                         pgoff_t start = offset >> PAGE_SHIFT;
587                         pgoff_t end = (offset + count - 1) >> PAGE_SHIFT;
588                         unsigned long len = end - start + 1;
589                         ret = do_readahead(mapping, f.file, start, len);
590                 }
591                 fdput(f);
592         }
593         return ret;
594 }
595 

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