xref: /openbmc/linux/mm/readahead.c (revision e868d61272caa648214046a096e5a6bfc068dc8c)
1 /*
2  * mm/readahead.c - address_space-level file readahead.
3  *
4  * Copyright (C) 2002, Linus Torvalds
5  *
6  * 09Apr2002	akpm@zip.com.au
7  *		Initial version.
8  */
9 
10 #include <linux/kernel.h>
11 #include <linux/fs.h>
12 #include <linux/mm.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 
19 void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
20 {
21 }
22 EXPORT_SYMBOL(default_unplug_io_fn);
23 
24 struct backing_dev_info default_backing_dev_info = {
25 	.ra_pages	= (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
26 	.state		= 0,
27 	.capabilities	= BDI_CAP_MAP_COPY,
28 	.unplug_io_fn	= default_unplug_io_fn,
29 };
30 EXPORT_SYMBOL_GPL(default_backing_dev_info);
31 
32 /*
33  * Initialise a struct file's readahead state.  Assumes that the caller has
34  * memset *ra to zero.
35  */
36 void
37 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
38 {
39 	ra->ra_pages = mapping->backing_dev_info->ra_pages;
40 	ra->prev_index = -1;
41 }
42 EXPORT_SYMBOL_GPL(file_ra_state_init);
43 
44 /*
45  * Return max readahead size for this inode in number-of-pages.
46  */
47 static inline unsigned long get_max_readahead(struct file_ra_state *ra)
48 {
49 	return ra->ra_pages;
50 }
51 
52 static inline unsigned long get_min_readahead(struct file_ra_state *ra)
53 {
54 	return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
55 }
56 
57 static inline void reset_ahead_window(struct file_ra_state *ra)
58 {
59 	/*
60 	 * ... but preserve ahead_start + ahead_size value,
61 	 * see 'recheck:' label in page_cache_readahead().
62 	 * Note: We never use ->ahead_size as rvalue without
63 	 * checking ->ahead_start != 0 first.
64 	 */
65 	ra->ahead_size += ra->ahead_start;
66 	ra->ahead_start = 0;
67 }
68 
69 static inline void ra_off(struct file_ra_state *ra)
70 {
71 	ra->start = 0;
72 	ra->flags = 0;
73 	ra->size = 0;
74 	reset_ahead_window(ra);
75 	return;
76 }
77 
78 /*
79  * Set the initial window size, round to next power of 2 and square
80  * for small size, x 4 for medium, and x 2 for large
81  * for 128k (32 page) max ra
82  * 1-8 page = 32k initial, > 8 page = 128k initial
83  */
84 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
85 {
86 	unsigned long newsize = roundup_pow_of_two(size);
87 
88 	if (newsize <= max / 32)
89 		newsize = newsize * 4;
90 	else if (newsize <= max / 4)
91 		newsize = newsize * 2;
92 	else
93 		newsize = max;
94 	return newsize;
95 }
96 
97 /*
98  * Set the new window size, this is called only when I/O is to be submitted,
99  * not for each call to readahead.  If a cache miss occured, reduce next I/O
100  * size, else increase depending on how close to max we are.
101  */
102 static inline unsigned long get_next_ra_size(struct file_ra_state *ra)
103 {
104 	unsigned long max = get_max_readahead(ra);
105 	unsigned long min = get_min_readahead(ra);
106 	unsigned long cur = ra->size;
107 	unsigned long newsize;
108 
109 	if (ra->flags & RA_FLAG_MISS) {
110 		ra->flags &= ~RA_FLAG_MISS;
111 		newsize = max((cur - 2), min);
112 	} else if (cur < max / 16) {
113 		newsize = 4 * cur;
114 	} else {
115 		newsize = 2 * cur;
116 	}
117 	return min(newsize, max);
118 }
119 
120 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
121 
122 /**
123  * read_cache_pages - populate an address space with some pages & start reads against them
124  * @mapping: the address_space
125  * @pages: The address of a list_head which contains the target pages.  These
126  *   pages have their ->index populated and are otherwise uninitialised.
127  * @filler: callback routine for filling a single page.
128  * @data: private data for the callback routine.
129  *
130  * Hides the details of the LRU cache etc from the filesystems.
131  */
132 int read_cache_pages(struct address_space *mapping, struct list_head *pages,
133 			int (*filler)(void *, struct page *), void *data)
134 {
135 	struct page *page;
136 	struct pagevec lru_pvec;
137 	int ret = 0;
138 
139 	pagevec_init(&lru_pvec, 0);
140 
141 	while (!list_empty(pages)) {
142 		page = list_to_page(pages);
143 		list_del(&page->lru);
144 		if (add_to_page_cache(page, mapping, page->index, GFP_KERNEL)) {
145 			page_cache_release(page);
146 			continue;
147 		}
148 		ret = filler(data, page);
149 		if (!pagevec_add(&lru_pvec, page))
150 			__pagevec_lru_add(&lru_pvec);
151 		if (ret) {
152 			put_pages_list(pages);
153 			break;
154 		}
155 		task_io_account_read(PAGE_CACHE_SIZE);
156 	}
157 	pagevec_lru_add(&lru_pvec);
158 	return ret;
159 }
160 
161 EXPORT_SYMBOL(read_cache_pages);
162 
163 static int read_pages(struct address_space *mapping, struct file *filp,
164 		struct list_head *pages, unsigned nr_pages)
165 {
166 	unsigned page_idx;
167 	struct pagevec lru_pvec;
168 	int ret;
169 
170 	if (mapping->a_ops->readpages) {
171 		ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);
172 		/* Clean up the remaining pages */
173 		put_pages_list(pages);
174 		goto out;
175 	}
176 
177 	pagevec_init(&lru_pvec, 0);
178 	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
179 		struct page *page = list_to_page(pages);
180 		list_del(&page->lru);
181 		if (!add_to_page_cache(page, mapping,
182 					page->index, GFP_KERNEL)) {
183 			mapping->a_ops->readpage(filp, page);
184 			if (!pagevec_add(&lru_pvec, page))
185 				__pagevec_lru_add(&lru_pvec);
186 		} else
187 			page_cache_release(page);
188 	}
189 	pagevec_lru_add(&lru_pvec);
190 	ret = 0;
191 out:
192 	return ret;
193 }
194 
195 /*
196  * Readahead design.
197  *
198  * The fields in struct file_ra_state represent the most-recently-executed
199  * readahead attempt:
200  *
201  * start:	Page index at which we started the readahead
202  * size:	Number of pages in that read
203  *              Together, these form the "current window".
204  *              Together, start and size represent the `readahead window'.
205  * prev_index:  The page which the readahead algorithm most-recently inspected.
206  *              It is mainly used to detect sequential file reading.
207  *              If page_cache_readahead sees that it is again being called for
208  *              a page which it just looked at, it can return immediately without
209  *              making any state changes.
210  * offset:      Offset in the prev_index where the last read ended - used for
211  *              detection of sequential file reading.
212  * ahead_start,
213  * ahead_size:  Together, these form the "ahead window".
214  * ra_pages:	The externally controlled max readahead for this fd.
215  *
216  * When readahead is in the off state (size == 0), readahead is disabled.
217  * In this state, prev_index is used to detect the resumption of sequential I/O.
218  *
219  * The readahead code manages two windows - the "current" and the "ahead"
220  * windows.  The intent is that while the application is walking the pages
221  * in the current window, I/O is underway on the ahead window.  When the
222  * current window is fully traversed, it is replaced by the ahead window
223  * and the ahead window is invalidated.  When this copying happens, the
224  * new current window's pages are probably still locked.  So
225  * we submit a new batch of I/O immediately, creating a new ahead window.
226  *
227  * So:
228  *
229  *   ----|----------------|----------------|-----
230  *       ^start           ^start+size
231  *                        ^ahead_start     ^ahead_start+ahead_size
232  *
233  *         ^ When this page is read, we submit I/O for the
234  *           ahead window.
235  *
236  * A `readahead hit' occurs when a read request is made against a page which is
237  * the next sequential page. Ahead window calculations are done only when it
238  * is time to submit a new IO.  The code ramps up the size agressively at first,
239  * but slow down as it approaches max_readhead.
240  *
241  * Any seek/ramdom IO will result in readahead being turned off.  It will resume
242  * at the first sequential access.
243  *
244  * There is a special-case: if the first page which the application tries to
245  * read happens to be the first page of the file, it is assumed that a linear
246  * read is about to happen and the window is immediately set to the initial size
247  * based on I/O request size and the max_readahead.
248  *
249  * This function is to be called for every read request, rather than when
250  * it is time to perform readahead.  It is called only once for the entire I/O
251  * regardless of size unless readahead is unable to start enough I/O to satisfy
252  * the request (I/O request > max_readahead).
253  */
254 
255 /*
256  * do_page_cache_readahead actually reads a chunk of disk.  It allocates all
257  * the pages first, then submits them all for I/O. This avoids the very bad
258  * behaviour which would occur if page allocations are causing VM writeback.
259  * We really don't want to intermingle reads and writes like that.
260  *
261  * Returns the number of pages requested, or the maximum amount of I/O allowed.
262  *
263  * do_page_cache_readahead() returns -1 if it encountered request queue
264  * congestion.
265  */
266 static int
267 __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
268 			pgoff_t offset, unsigned long nr_to_read)
269 {
270 	struct inode *inode = mapping->host;
271 	struct page *page;
272 	unsigned long end_index;	/* The last page we want to read */
273 	LIST_HEAD(page_pool);
274 	int page_idx;
275 	int ret = 0;
276 	loff_t isize = i_size_read(inode);
277 
278 	if (isize == 0)
279 		goto out;
280 
281  	end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
282 
283 	/*
284 	 * Preallocate as many pages as we will need.
285 	 */
286 	read_lock_irq(&mapping->tree_lock);
287 	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
288 		pgoff_t page_offset = offset + page_idx;
289 
290 		if (page_offset > end_index)
291 			break;
292 
293 		page = radix_tree_lookup(&mapping->page_tree, page_offset);
294 		if (page)
295 			continue;
296 
297 		read_unlock_irq(&mapping->tree_lock);
298 		page = page_cache_alloc_cold(mapping);
299 		read_lock_irq(&mapping->tree_lock);
300 		if (!page)
301 			break;
302 		page->index = page_offset;
303 		list_add(&page->lru, &page_pool);
304 		ret++;
305 	}
306 	read_unlock_irq(&mapping->tree_lock);
307 
308 	/*
309 	 * Now start the IO.  We ignore I/O errors - if the page is not
310 	 * uptodate then the caller will launch readpage again, and
311 	 * will then handle the error.
312 	 */
313 	if (ret)
314 		read_pages(mapping, filp, &page_pool, ret);
315 	BUG_ON(!list_empty(&page_pool));
316 out:
317 	return ret;
318 }
319 
320 /*
321  * Chunk the readahead into 2 megabyte units, so that we don't pin too much
322  * memory at once.
323  */
324 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
325 		pgoff_t offset, unsigned long nr_to_read)
326 {
327 	int ret = 0;
328 
329 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
330 		return -EINVAL;
331 
332 	while (nr_to_read) {
333 		int err;
334 
335 		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
336 
337 		if (this_chunk > nr_to_read)
338 			this_chunk = nr_to_read;
339 		err = __do_page_cache_readahead(mapping, filp,
340 						offset, this_chunk);
341 		if (err < 0) {
342 			ret = err;
343 			break;
344 		}
345 		ret += err;
346 		offset += this_chunk;
347 		nr_to_read -= this_chunk;
348 	}
349 	return ret;
350 }
351 
352 /*
353  * Check how effective readahead is being.  If the amount of started IO is
354  * less than expected then the file is partly or fully in pagecache and
355  * readahead isn't helping.
356  *
357  */
358 static inline int check_ra_success(struct file_ra_state *ra,
359 			unsigned long nr_to_read, unsigned long actual)
360 {
361 	if (actual == 0) {
362 		ra->cache_hit += nr_to_read;
363 		if (ra->cache_hit >= VM_MAX_CACHE_HIT) {
364 			ra_off(ra);
365 			ra->flags |= RA_FLAG_INCACHE;
366 			return 0;
367 		}
368 	} else {
369 		ra->cache_hit=0;
370 	}
371 	return 1;
372 }
373 
374 /*
375  * This version skips the IO if the queue is read-congested, and will tell the
376  * block layer to abandon the readahead if request allocation would block.
377  *
378  * force_page_cache_readahead() will ignore queue congestion and will block on
379  * request queues.
380  */
381 int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
382 			pgoff_t offset, unsigned long nr_to_read)
383 {
384 	if (bdi_read_congested(mapping->backing_dev_info))
385 		return -1;
386 
387 	return __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
388 }
389 
390 /*
391  * Read 'nr_to_read' pages starting at page 'offset'. If the flag 'block'
392  * is set wait till the read completes.  Otherwise attempt to read without
393  * blocking.
394  * Returns 1 meaning 'success' if read is successful without switching off
395  * readahead mode. Otherwise return failure.
396  */
397 static int
398 blockable_page_cache_readahead(struct address_space *mapping, struct file *filp,
399 			pgoff_t offset, unsigned long nr_to_read,
400 			struct file_ra_state *ra, int block)
401 {
402 	int actual;
403 
404 	if (!block && bdi_read_congested(mapping->backing_dev_info))
405 		return 0;
406 
407 	actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
408 
409 	return check_ra_success(ra, nr_to_read, actual);
410 }
411 
412 static int make_ahead_window(struct address_space *mapping, struct file *filp,
413 				struct file_ra_state *ra, int force)
414 {
415 	int block, ret;
416 
417 	ra->ahead_size = get_next_ra_size(ra);
418 	ra->ahead_start = ra->start + ra->size;
419 
420 	block = force || (ra->prev_index >= ra->ahead_start);
421 	ret = blockable_page_cache_readahead(mapping, filp,
422 			ra->ahead_start, ra->ahead_size, ra, block);
423 
424 	if (!ret && !force) {
425 		/* A read failure in blocking mode, implies pages are
426 		 * all cached. So we can safely assume we have taken
427 		 * care of all the pages requested in this call.
428 		 * A read failure in non-blocking mode, implies we are
429 		 * reading more pages than requested in this call.  So
430 		 * we safely assume we have taken care of all the pages
431 		 * requested in this call.
432 		 *
433 		 * Just reset the ahead window in case we failed due to
434 		 * congestion.  The ahead window will any way be closed
435 		 * in case we failed due to excessive page cache hits.
436 		 */
437 		reset_ahead_window(ra);
438 	}
439 
440 	return ret;
441 }
442 
443 /**
444  * page_cache_readahead - generic adaptive readahead
445  * @mapping: address_space which holds the pagecache and I/O vectors
446  * @ra: file_ra_state which holds the readahead state
447  * @filp: passed on to ->readpage() and ->readpages()
448  * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
449  * @req_size: hint: total size of the read which the caller is performing in
450  *            PAGE_CACHE_SIZE units
451  *
452  * page_cache_readahead() is the main function.  If performs the adaptive
453  * readahead window size management and submits the readahead I/O.
454  *
455  * Note that @filp is purely used for passing on to the ->readpage[s]()
456  * handler: it may refer to a different file from @mapping (so we may not use
457  * @filp->f_mapping or @filp->f_path.dentry->d_inode here).
458  * Also, @ra may not be equal to &@filp->f_ra.
459  *
460  */
461 unsigned long
462 page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
463 		     struct file *filp, pgoff_t offset, unsigned long req_size)
464 {
465 	unsigned long max, newsize;
466 	int sequential;
467 
468 	/*
469 	 * We avoid doing extra work and bogusly perturbing the readahead
470 	 * window expansion logic.
471 	 */
472 	if (offset == ra->prev_index && --req_size)
473 		++offset;
474 
475 	/* Note that prev_index == -1 if it is a first read */
476 	sequential = (offset == ra->prev_index + 1);
477 	ra->prev_index = offset;
478 	ra->prev_offset = 0;
479 
480 	max = get_max_readahead(ra);
481 	newsize = min(req_size, max);
482 
483 	/* No readahead or sub-page sized read or file already in cache */
484 	if (newsize == 0 || (ra->flags & RA_FLAG_INCACHE))
485 		goto out;
486 
487 	ra->prev_index += newsize - 1;
488 
489 	/*
490 	 * Special case - first read at start of file. We'll assume it's
491 	 * a whole-file read and grow the window fast.  Or detect first
492 	 * sequential access
493 	 */
494 	if (sequential && ra->size == 0) {
495 		ra->size = get_init_ra_size(newsize, max);
496 		ra->start = offset;
497 		if (!blockable_page_cache_readahead(mapping, filp, offset,
498 							 ra->size, ra, 1))
499 			goto out;
500 
501 		/*
502 		 * If the request size is larger than our max readahead, we
503 		 * at least want to be sure that we get 2 IOs in flight and
504 		 * we know that we will definitly need the new I/O.
505 		 * once we do this, subsequent calls should be able to overlap
506 		 * IOs,* thus preventing stalls. so issue the ahead window
507 		 * immediately.
508 		 */
509 		if (req_size >= max)
510 			make_ahead_window(mapping, filp, ra, 1);
511 
512 		goto out;
513 	}
514 
515 	/*
516 	 * Now handle the random case:
517 	 * partial page reads and first access were handled above,
518 	 * so this must be the next page otherwise it is random
519 	 */
520 	if (!sequential) {
521 		ra_off(ra);
522 		blockable_page_cache_readahead(mapping, filp, offset,
523 				 newsize, ra, 1);
524 		goto out;
525 	}
526 
527 	/*
528 	 * If we get here we are doing sequential IO and this was not the first
529 	 * occurence (ie we have an existing window)
530 	 */
531 	if (ra->ahead_start == 0) {	 /* no ahead window yet */
532 		if (!make_ahead_window(mapping, filp, ra, 0))
533 			goto recheck;
534 	}
535 
536 	/*
537 	 * Already have an ahead window, check if we crossed into it.
538 	 * If so, shift windows and issue a new ahead window.
539 	 * Only return the #pages that are in the current window, so that
540 	 * we get called back on the first page of the ahead window which
541 	 * will allow us to submit more IO.
542 	 */
543 	if (ra->prev_index >= ra->ahead_start) {
544 		ra->start = ra->ahead_start;
545 		ra->size = ra->ahead_size;
546 		make_ahead_window(mapping, filp, ra, 0);
547 recheck:
548 		/* prev_index shouldn't overrun the ahead window */
549 		ra->prev_index = min(ra->prev_index,
550 			ra->ahead_start + ra->ahead_size - 1);
551 	}
552 
553 out:
554 	return ra->prev_index + 1;
555 }
556 EXPORT_SYMBOL_GPL(page_cache_readahead);
557 
558 /*
559  * handle_ra_miss() is called when it is known that a page which should have
560  * been present in the pagecache (we just did some readahead there) was in fact
561  * not found.  This will happen if it was evicted by the VM (readahead
562  * thrashing)
563  *
564  * Turn on the cache miss flag in the RA struct, this will cause the RA code
565  * to reduce the RA size on the next read.
566  */
567 void handle_ra_miss(struct address_space *mapping,
568 		struct file_ra_state *ra, pgoff_t offset)
569 {
570 	ra->flags |= RA_FLAG_MISS;
571 	ra->flags &= ~RA_FLAG_INCACHE;
572 	ra->cache_hit = 0;
573 }
574 
575 /*
576  * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
577  * sensible upper limit.
578  */
579 unsigned long max_sane_readahead(unsigned long nr)
580 {
581 	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE)
582 		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
583 }
584