Root/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/fs.h>
12#include <linux/gfp.h>
13#include <linux/mm.h>
14#include <linux/module.h>
15#include <linux/blkdev.h>
16#include <linux/backing-dev.h>
17#include <linux/task_io_accounting_ops.h>
18#include <linux/pagevec.h>
19#include <linux/pagemap.h>
20
21/*
22 * Initialise a struct file's readahead state. Assumes that the caller has
23 * memset *ra to zero.
24 */
25void
26file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
27{
28    ra->ra_pages = mapping->backing_dev_info->ra_pages;
29    ra->prev_pos = -1;
30}
31EXPORT_SYMBOL_GPL(file_ra_state_init);
32
33#define list_to_page(head) (list_entry((head)->prev, struct page, lru))
34
35/*
36 * see if a page needs releasing upon read_cache_pages() failure
37 * - the caller of read_cache_pages() may have set PG_private or PG_fscache
38 * before calling, such as the NFS fs marking pages that are cached locally
39 * on disk, thus we need to give the fs a chance to clean up in the event of
40 * an error
41 */
42static void read_cache_pages_invalidate_page(struct address_space *mapping,
43                         struct page *page)
44{
45    if (page_has_private(page)) {
46        if (!trylock_page(page))
47            BUG();
48        page->mapping = mapping;
49        do_invalidatepage(page, 0);
50        page->mapping = NULL;
51        unlock_page(page);
52    }
53    page_cache_release(page);
54}
55
56/*
57 * release a list of pages, invalidating them first if need be
58 */
59static void read_cache_pages_invalidate_pages(struct address_space *mapping,
60                          struct list_head *pages)
61{
62    struct page *victim;
63
64    while (!list_empty(pages)) {
65        victim = list_to_page(pages);
66        list_del(&victim->lru);
67        read_cache_pages_invalidate_page(mapping, victim);
68    }
69}
70
71/**
72 * read_cache_pages - populate an address space with some pages & start reads against them
73 * @mapping: the address_space
74 * @pages: The address of a list_head which contains the target pages. These
75 * pages have their ->index populated and are otherwise uninitialised.
76 * @filler: callback routine for filling a single page.
77 * @data: private data for the callback routine.
78 *
79 * Hides the details of the LRU cache etc from the filesystems.
80 */
81int read_cache_pages(struct address_space *mapping, struct list_head *pages,
82            int (*filler)(void *, struct page *), void *data)
83{
84    struct page *page;
85    int ret = 0;
86
87    while (!list_empty(pages)) {
88        page = list_to_page(pages);
89        list_del(&page->lru);
90        if (add_to_page_cache_lru(page, mapping,
91                    page->index, GFP_KERNEL)) {
92            read_cache_pages_invalidate_page(mapping, page);
93            continue;
94        }
95        page_cache_release(page);
96
97        ret = filler(data, page);
98        if (unlikely(ret)) {
99            read_cache_pages_invalidate_pages(mapping, pages);
100            break;
101        }
102        task_io_account_read(PAGE_CACHE_SIZE);
103    }
104    return ret;
105}
106
107EXPORT_SYMBOL(read_cache_pages);
108
109static int read_pages(struct address_space *mapping, struct file *filp,
110        struct list_head *pages, unsigned nr_pages)
111{
112    struct blk_plug plug;
113    unsigned page_idx;
114    int ret;
115
116    blk_start_plug(&plug);
117
118    if (mapping->a_ops->readpages) {
119        ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
120        /* Clean up the remaining pages */
121        put_pages_list(pages);
122        goto out;
123    }
124
125    for (page_idx = 0; page_idx < nr_pages; page_idx++) {
126        struct page *page = list_to_page(pages);
127        list_del(&page->lru);
128        if (!add_to_page_cache_lru(page, mapping,
129                    page->index, GFP_KERNEL)) {
130            mapping->a_ops->readpage(filp, page);
131        }
132        page_cache_release(page);
133    }
134    ret = 0;
135
136out:
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 */
150static int
151__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
152            pgoff_t offset, unsigned long nr_to_read,
153            unsigned long lookahead_size)
154{
155    struct inode *inode = mapping->host;
156    struct page *page;
157    unsigned long end_index; /* The last page we want to read */
158    LIST_HEAD(page_pool);
159    int page_idx;
160    int ret = 0;
161    loff_t isize = i_size_read(inode);
162
163    if (isize == 0)
164        goto out;
165
166    end_index = ((isize - 1) >> PAGE_CACHE_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)
181            continue;
182
183        page = page_cache_alloc_readahead(mapping);
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);
200    BUG_ON(!list_empty(&page_pool));
201out:
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 */
209int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
210        pgoff_t offset, unsigned long nr_to_read)
211{
212    int ret = 0;
213
214    if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
215        return -EINVAL;
216
217    nr_to_read = max_sane_readahead(nr_to_read);
218    while (nr_to_read) {
219        int err;
220
221        unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
222
223        if (this_chunk > nr_to_read)
224            this_chunk = nr_to_read;
225        err = __do_page_cache_readahead(mapping, filp,
226                        offset, this_chunk, 0);
227        if (err < 0) {
228            ret = err;
229            break;
230        }
231        ret += err;
232        offset += this_chunk;
233        nr_to_read -= this_chunk;
234    }
235    return ret;
236}
237
238/*
239 * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
240 * sensible upper limit.
241 */
242unsigned long max_sane_readahead(unsigned long nr)
243{
244    return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)
245        + node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
246}
247
248/*
249 * Submit IO for the read-ahead request in file_ra_state.
250 */
251unsigned long ra_submit(struct file_ra_state *ra,
252               struct address_space *mapping, struct file *filp)
253{
254    int actual;
255
256    actual = __do_page_cache_readahead(mapping, filp,
257                    ra->start, ra->size, ra->async_size);
258
259    return actual;
260}
261
262/*
263 * Set the initial window size, round to next power of 2 and square
264 * for small size, x 4 for medium, and x 2 for large
265 * for 128k (32 page) max ra
266 * 1-8 page = 32k initial, > 8 page = 128k initial
267 */
268static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
269{
270    unsigned long newsize = roundup_pow_of_two(size);
271
272    if (newsize <= max / 32)
273        newsize = newsize * 4;
274    else if (newsize <= max / 4)
275        newsize = newsize * 2;
276    else
277        newsize = max;
278
279    return newsize;
280}
281
282/*
283 * Get the previous window size, ramp it up, and
284 * return it as the new window size.
285 */
286static unsigned long get_next_ra_size(struct file_ra_state *ra,
287                        unsigned long max)
288{
289    unsigned long cur = ra->size;
290    unsigned long newsize;
291
292    if (cur < max / 16)
293        newsize = 4 * cur;
294    else
295        newsize = 2 * cur;
296
297    return min(newsize, max);
298}
299
300/*
301 * On-demand readahead design.
302 *
303 * The fields in struct file_ra_state represent the most-recently-executed
304 * readahead attempt:
305 *
306 * |<----- async_size ---------|
307 * |------------------- size -------------------->|
308 * |==================#===========================|
309 * ^start ^page marked with PG_readahead
310 *
311 * To overlap application thinking time and disk I/O time, we do
312 * `readahead pipelining': Do not wait until the application consumed all
313 * readahead pages and stalled on the missing page at readahead_index;
314 * Instead, submit an asynchronous readahead I/O as soon as there are
315 * only async_size pages left in the readahead window. Normally async_size
316 * will be equal to size, for maximum pipelining.
317 *
318 * In interleaved sequential reads, concurrent streams on the same fd can
319 * be invalidating each other's readahead state. So we flag the new readahead
320 * page at (start+size-async_size) with PG_readahead, and use it as readahead
321 * indicator. The flag won't be set on already cached pages, to avoid the
322 * readahead-for-nothing fuss, saving pointless page cache lookups.
323 *
324 * prev_pos tracks the last visited byte in the _previous_ read request.
325 * It should be maintained by the caller, and will be used for detecting
326 * small random reads. Note that the readahead algorithm checks loosely
327 * for sequential patterns. Hence interleaved reads might be served as
328 * sequential ones.
329 *
330 * There is a special-case: if the first page which the application tries to
331 * read happens to be the first page of the file, it is assumed that a linear
332 * read is about to happen and the window is immediately set to the initial size
333 * based on I/O request size and the max_readahead.
334 *
335 * The code ramps up the readahead size aggressively at first, but slow down as
336 * it approaches max_readhead.
337 */
338
339/*
340 * Count contiguously cached pages from @offset-1 to @offset-@max,
341 * this count is a conservative estimation of
342 * - length of the sequential read sequence, or
343 * - thrashing threshold in memory tight systems
344 */
345static pgoff_t count_history_pages(struct address_space *mapping,
346                   struct file_ra_state *ra,
347                   pgoff_t offset, unsigned long max)
348{
349    pgoff_t head;
350
351    rcu_read_lock();
352    head = radix_tree_prev_hole(&mapping->page_tree, offset - 1, max);
353    rcu_read_unlock();
354
355    return offset - 1 - head;
356}
357
358/*
359 * page cache context based read-ahead
360 */
361static int try_context_readahead(struct address_space *mapping,
362                 struct file_ra_state *ra,
363                 pgoff_t offset,
364                 unsigned long req_size,
365                 unsigned long max)
366{
367    pgoff_t size;
368
369    size = count_history_pages(mapping, ra, offset, max);
370
371    /*
372     * no history pages:
373     * it could be a random read
374     */
375    if (!size)
376        return 0;
377
378    /*
379     * starts from beginning of file:
380     * it is a strong indication of long-run stream (or whole-file-read)
381     */
382    if (size >= offset)
383        size *= 2;
384
385    ra->start = offset;
386    ra->size = get_init_ra_size(size + req_size, max);
387    ra->async_size = ra->size;
388
389    return 1;
390}
391
392/*
393 * A minimal readahead algorithm for trivial sequential/random reads.
394 */
395static unsigned long
396ondemand_readahead(struct address_space *mapping,
397           struct file_ra_state *ra, struct file *filp,
398           bool hit_readahead_marker, pgoff_t offset,
399           unsigned long req_size)
400{
401    unsigned long max = max_sane_readahead(ra->ra_pages);
402
403    /*
404     * start of file
405     */
406    if (!offset)
407        goto initial_readahead;
408
409    /*
410     * It's the expected callback offset, assume sequential access.
411     * Ramp up sizes, and push forward the readahead window.
412     */
413    if ((offset == (ra->start + ra->size - ra->async_size) ||
414         offset == (ra->start + ra->size))) {
415        ra->start += ra->size;
416        ra->size = get_next_ra_size(ra, max);
417        ra->async_size = ra->size;
418        goto readit;
419    }
420
421    /*
422     * Hit a marked page without valid readahead state.
423     * E.g. interleaved reads.
424     * Query the pagecache for async_size, which normally equals to
425     * readahead size. Ramp it up and use it as the new readahead size.
426     */
427    if (hit_readahead_marker) {
428        pgoff_t start;
429
430        rcu_read_lock();
431        start = radix_tree_next_hole(&mapping->page_tree, offset+1,max);
432        rcu_read_unlock();
433
434        if (!start || start - offset > max)
435            return 0;
436
437        ra->start = start;
438        ra->size = start - offset; /* old async_size */
439        ra->size += req_size;
440        ra->size = get_next_ra_size(ra, max);
441        ra->async_size = ra->size;
442        goto readit;
443    }
444
445    /*
446     * oversize read
447     */
448    if (req_size > max)
449        goto initial_readahead;
450
451    /*
452     * sequential cache miss
453     */
454    if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL)
455        goto initial_readahead;
456
457    /*
458     * Query the page cache and look for the traces(cached history pages)
459     * that a sequential stream would leave behind.
460     */
461    if (try_context_readahead(mapping, ra, offset, req_size, max))
462        goto readit;
463
464    /*
465     * standalone, small random read
466     * Read as is, and do not pollute the readahead state.
467     */
468    return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
469
470initial_readahead:
471    ra->start = offset;
472    ra->size = get_init_ra_size(req_size, max);
473    ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
474
475readit:
476    /*
477     * Will this read hit the readahead marker made by itself?
478     * If so, trigger the readahead marker hit now, and merge
479     * the resulted next readahead window into the current one.
480     */
481    if (offset == ra->start && ra->size == ra->async_size) {
482        ra->async_size = get_next_ra_size(ra, max);
483        ra->size += ra->async_size;
484    }
485
486    return ra_submit(ra, mapping, filp);
487}
488
489/**
490 * page_cache_sync_readahead - generic file readahead
491 * @mapping: address_space which holds the pagecache and I/O vectors
492 * @ra: file_ra_state which holds the readahead state
493 * @filp: passed on to ->readpage() and ->readpages()
494 * @offset: start offset into @mapping, in pagecache page-sized units
495 * @req_size: hint: total size of the read which the caller is performing in
496 * pagecache pages
497 *
498 * page_cache_sync_readahead() should be called when a cache miss happened:
499 * it will submit the read. The readahead logic may decide to piggyback more
500 * pages onto the read request if access patterns suggest it will improve
501 * performance.
502 */
503void page_cache_sync_readahead(struct address_space *mapping,
504                   struct file_ra_state *ra, struct file *filp,
505                   pgoff_t offset, unsigned long req_size)
506{
507    /* no read-ahead */
508    if (!ra->ra_pages)
509        return;
510
511    /* be dumb */
512    if (filp && (filp->f_mode & FMODE_RANDOM)) {
513        force_page_cache_readahead(mapping, filp, offset, req_size);
514        return;
515    }
516
517    /* do read-ahead */
518    ondemand_readahead(mapping, ra, filp, false, offset, req_size);
519}
520EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
521
522/**
523 * page_cache_async_readahead - file readahead for marked pages
524 * @mapping: address_space which holds the pagecache and I/O vectors
525 * @ra: file_ra_state which holds the readahead state
526 * @filp: passed on to ->readpage() and ->readpages()
527 * @page: the page at @offset which has the PG_readahead flag set
528 * @offset: start offset into @mapping, in pagecache page-sized units
529 * @req_size: hint: total size of the read which the caller is performing in
530 * pagecache pages
531 *
532 * page_cache_async_readahead() should be called when a page is used which
533 * has the PG_readahead flag; this is a marker to suggest that the application
534 * has used up enough of the readahead window that we should start pulling in
535 * more pages.
536 */
537void
538page_cache_async_readahead(struct address_space *mapping,
539               struct file_ra_state *ra, struct file *filp,
540               struct page *page, pgoff_t offset,
541               unsigned long req_size)
542{
543    /* no read-ahead */
544    if (!ra->ra_pages)
545        return;
546
547    /*
548     * Same bit is used for PG_readahead and PG_reclaim.
549     */
550    if (PageWriteback(page))
551        return;
552
553    ClearPageReadahead(page);
554
555    /*
556     * Defer asynchronous read-ahead on IO congestion.
557     */
558    if (bdi_read_congested(mapping->backing_dev_info))
559        return;
560
561    /* do read-ahead */
562    ondemand_readahead(mapping, ra, filp, true, offset, req_size);
563}
564EXPORT_SYMBOL_GPL(page_cache_async_readahead);
565

Archive Download this file



interactive