1// SPDX-License-Identifier: GPL-2.0
2/*
3 * fs/mpage.c
4 *
5 * Copyright (C) 2002, Linus Torvalds.
6 *
7 * Contains functions related to preparing and submitting BIOs which contain
8 * multiple pagecache pages.
9 *
10 * 15May2002 Andrew Morton
11 * Initial version
12 * 27Jun2002 axboe@suse.de
13 * use bio_add_page() to build bio's just the right size
14 */
15
16#include <linux/kernel.h>
17#include <linux/export.h>
18#include <linux/mm.h>
19#include <linux/kdev_t.h>
20#include <linux/gfp.h>
21#include <linux/bio.h>
22#include <linux/fs.h>
23#include <linux/buffer_head.h>
24#include <linux/blkdev.h>
25#include <linux/highmem.h>
26#include <linux/prefetch.h>
27#include <linux/mpage.h>
28#include <linux/mm_inline.h>
29#include <linux/writeback.h>
30#include <linux/backing-dev.h>
31#include <linux/pagevec.h>
32#include "internal.h"
33
34/*
35 * I/O completion handler for multipage BIOs.
36 *
37 * The mpage code never puts partial pages into a BIO (except for end-of-file).
38 * If a page does not map to a contiguous run of blocks then it simply falls
39 * back to block_read_full_folio().
40 *
41 * Why is this? If a page's completion depends on a number of different BIOs
42 * which can complete in any order (or at the same time) then determining the
43 * status of that page is hard. See end_buffer_async_read() for the details.
44 * There is no point in duplicating all that complexity.
45 */
46static void mpage_read_end_io(struct bio *bio)
47{
48 struct folio_iter fi;
49 int err = blk_status_to_errno(status: bio->bi_status);
50
51 bio_for_each_folio_all(fi, bio) {
52 if (err)
53 folio_set_error(folio: fi.folio);
54 else
55 folio_mark_uptodate(folio: fi.folio);
56 folio_unlock(folio: fi.folio);
57 }
58
59 bio_put(bio);
60}
61
62static void mpage_write_end_io(struct bio *bio)
63{
64 struct folio_iter fi;
65 int err = blk_status_to_errno(status: bio->bi_status);
66
67 bio_for_each_folio_all(fi, bio) {
68 if (err) {
69 folio_set_error(folio: fi.folio);
70 mapping_set_error(mapping: fi.folio->mapping, error: err);
71 }
72 folio_end_writeback(folio: fi.folio);
73 }
74
75 bio_put(bio);
76}
77
78static struct bio *mpage_bio_submit_read(struct bio *bio)
79{
80 bio->bi_end_io = mpage_read_end_io;
81 guard_bio_eod(bio);
82 submit_bio(bio);
83 return NULL;
84}
85
86static struct bio *mpage_bio_submit_write(struct bio *bio)
87{
88 bio->bi_end_io = mpage_write_end_io;
89 guard_bio_eod(bio);
90 submit_bio(bio);
91 return NULL;
92}
93
94/*
95 * support function for mpage_readahead. The fs supplied get_block might
96 * return an up to date buffer. This is used to map that buffer into
97 * the page, which allows read_folio to avoid triggering a duplicate call
98 * to get_block.
99 *
100 * The idea is to avoid adding buffers to pages that don't already have
101 * them. So when the buffer is up to date and the page size == block size,
102 * this marks the page up to date instead of adding new buffers.
103 */
104static void map_buffer_to_folio(struct folio *folio, struct buffer_head *bh,
105 int page_block)
106{
107 struct inode *inode = folio->mapping->host;
108 struct buffer_head *page_bh, *head;
109 int block = 0;
110
111 head = folio_buffers(folio);
112 if (!head) {
113 /*
114 * don't make any buffers if there is only one buffer on
115 * the folio and the folio just needs to be set up to date
116 */
117 if (inode->i_blkbits == PAGE_SHIFT &&
118 buffer_uptodate(bh)) {
119 folio_mark_uptodate(folio);
120 return;
121 }
122 head = create_empty_buffers(folio, blocksize: i_blocksize(node: inode), b_state: 0);
123 }
124
125 page_bh = head;
126 do {
127 if (block == page_block) {
128 page_bh->b_state = bh->b_state;
129 page_bh->b_bdev = bh->b_bdev;
130 page_bh->b_blocknr = bh->b_blocknr;
131 break;
132 }
133 page_bh = page_bh->b_this_page;
134 block++;
135 } while (page_bh != head);
136}
137
138struct mpage_readpage_args {
139 struct bio *bio;
140 struct folio *folio;
141 unsigned int nr_pages;
142 bool is_readahead;
143 sector_t last_block_in_bio;
144 struct buffer_head map_bh;
145 unsigned long first_logical_block;
146 get_block_t *get_block;
147};
148
149/*
150 * This is the worker routine which does all the work of mapping the disk
151 * blocks and constructs largest possible bios, submits them for IO if the
152 * blocks are not contiguous on the disk.
153 *
154 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
155 * represent the validity of its disk mapping and to decide when to do the next
156 * get_block() call.
157 */
158static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
159{
160 struct folio *folio = args->folio;
161 struct inode *inode = folio->mapping->host;
162 const unsigned blkbits = inode->i_blkbits;
163 const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
164 const unsigned blocksize = 1 << blkbits;
165 struct buffer_head *map_bh = &args->map_bh;
166 sector_t block_in_file;
167 sector_t last_block;
168 sector_t last_block_in_file;
169 sector_t blocks[MAX_BUF_PER_PAGE];
170 unsigned page_block;
171 unsigned first_hole = blocks_per_page;
172 struct block_device *bdev = NULL;
173 int length;
174 int fully_mapped = 1;
175 blk_opf_t opf = REQ_OP_READ;
176 unsigned nblocks;
177 unsigned relative_block;
178 gfp_t gfp = mapping_gfp_constraint(mapping: folio->mapping, GFP_KERNEL);
179
180 /* MAX_BUF_PER_PAGE, for example */
181 VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
182
183 if (args->is_readahead) {
184 opf |= REQ_RAHEAD;
185 gfp |= __GFP_NORETRY | __GFP_NOWARN;
186 }
187
188 if (folio_buffers(folio))
189 goto confused;
190
191 block_in_file = (sector_t)folio->index << (PAGE_SHIFT - blkbits);
192 last_block = block_in_file + args->nr_pages * blocks_per_page;
193 last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
194 if (last_block > last_block_in_file)
195 last_block = last_block_in_file;
196 page_block = 0;
197
198 /*
199 * Map blocks using the result from the previous get_blocks call first.
200 */
201 nblocks = map_bh->b_size >> blkbits;
202 if (buffer_mapped(bh: map_bh) &&
203 block_in_file > args->first_logical_block &&
204 block_in_file < (args->first_logical_block + nblocks)) {
205 unsigned map_offset = block_in_file - args->first_logical_block;
206 unsigned last = nblocks - map_offset;
207
208 for (relative_block = 0; ; relative_block++) {
209 if (relative_block == last) {
210 clear_buffer_mapped(bh: map_bh);
211 break;
212 }
213 if (page_block == blocks_per_page)
214 break;
215 blocks[page_block] = map_bh->b_blocknr + map_offset +
216 relative_block;
217 page_block++;
218 block_in_file++;
219 }
220 bdev = map_bh->b_bdev;
221 }
222
223 /*
224 * Then do more get_blocks calls until we are done with this folio.
225 */
226 map_bh->b_folio = folio;
227 while (page_block < blocks_per_page) {
228 map_bh->b_state = 0;
229 map_bh->b_size = 0;
230
231 if (block_in_file < last_block) {
232 map_bh->b_size = (last_block-block_in_file) << blkbits;
233 if (args->get_block(inode, block_in_file, map_bh, 0))
234 goto confused;
235 args->first_logical_block = block_in_file;
236 }
237
238 if (!buffer_mapped(bh: map_bh)) {
239 fully_mapped = 0;
240 if (first_hole == blocks_per_page)
241 first_hole = page_block;
242 page_block++;
243 block_in_file++;
244 continue;
245 }
246
247 /* some filesystems will copy data into the page during
248 * the get_block call, in which case we don't want to
249 * read it again. map_buffer_to_folio copies the data
250 * we just collected from get_block into the folio's buffers
251 * so read_folio doesn't have to repeat the get_block call
252 */
253 if (buffer_uptodate(bh: map_bh)) {
254 map_buffer_to_folio(folio, bh: map_bh, page_block);
255 goto confused;
256 }
257
258 if (first_hole != blocks_per_page)
259 goto confused; /* hole -> non-hole */
260
261 /* Contiguous blocks? */
262 if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
263 goto confused;
264 nblocks = map_bh->b_size >> blkbits;
265 for (relative_block = 0; ; relative_block++) {
266 if (relative_block == nblocks) {
267 clear_buffer_mapped(bh: map_bh);
268 break;
269 } else if (page_block == blocks_per_page)
270 break;
271 blocks[page_block] = map_bh->b_blocknr+relative_block;
272 page_block++;
273 block_in_file++;
274 }
275 bdev = map_bh->b_bdev;
276 }
277
278 if (first_hole != blocks_per_page) {
279 folio_zero_segment(folio, start: first_hole << blkbits, PAGE_SIZE);
280 if (first_hole == 0) {
281 folio_mark_uptodate(folio);
282 folio_unlock(folio);
283 goto out;
284 }
285 } else if (fully_mapped) {
286 folio_set_mappedtodisk(folio);
287 }
288
289 /*
290 * This folio will go to BIO. Do we need to send this BIO off first?
291 */
292 if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
293 args->bio = mpage_bio_submit_read(bio: args->bio);
294
295alloc_new:
296 if (args->bio == NULL) {
297 args->bio = bio_alloc(bdev, nr_vecs: bio_max_segs(nr_segs: args->nr_pages), opf,
298 gfp_mask: gfp);
299 if (args->bio == NULL)
300 goto confused;
301 args->bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
302 }
303
304 length = first_hole << blkbits;
305 if (!bio_add_folio(bio: args->bio, folio, len: length, off: 0)) {
306 args->bio = mpage_bio_submit_read(bio: args->bio);
307 goto alloc_new;
308 }
309
310 relative_block = block_in_file - args->first_logical_block;
311 nblocks = map_bh->b_size >> blkbits;
312 if ((buffer_boundary(bh: map_bh) && relative_block == nblocks) ||
313 (first_hole != blocks_per_page))
314 args->bio = mpage_bio_submit_read(bio: args->bio);
315 else
316 args->last_block_in_bio = blocks[blocks_per_page - 1];
317out:
318 return args->bio;
319
320confused:
321 if (args->bio)
322 args->bio = mpage_bio_submit_read(bio: args->bio);
323 if (!folio_test_uptodate(folio))
324 block_read_full_folio(folio, args->get_block);
325 else
326 folio_unlock(folio);
327 goto out;
328}
329
330/**
331 * mpage_readahead - start reads against pages
332 * @rac: Describes which pages to read.
333 * @get_block: The filesystem's block mapper function.
334 *
335 * This function walks the pages and the blocks within each page, building and
336 * emitting large BIOs.
337 *
338 * If anything unusual happens, such as:
339 *
340 * - encountering a page which has buffers
341 * - encountering a page which has a non-hole after a hole
342 * - encountering a page with non-contiguous blocks
343 *
344 * then this code just gives up and calls the buffer_head-based read function.
345 * It does handle a page which has holes at the end - that is a common case:
346 * the end-of-file on blocksize < PAGE_SIZE setups.
347 *
348 * BH_Boundary explanation:
349 *
350 * There is a problem. The mpage read code assembles several pages, gets all
351 * their disk mappings, and then submits them all. That's fine, but obtaining
352 * the disk mappings may require I/O. Reads of indirect blocks, for example.
353 *
354 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
355 * submitted in the following order:
356 *
357 * 12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
358 *
359 * because the indirect block has to be read to get the mappings of blocks
360 * 13,14,15,16. Obviously, this impacts performance.
361 *
362 * So what we do it to allow the filesystem's get_block() function to set
363 * BH_Boundary when it maps block 11. BH_Boundary says: mapping of the block
364 * after this one will require I/O against a block which is probably close to
365 * this one. So you should push what I/O you have currently accumulated.
366 *
367 * This all causes the disk requests to be issued in the correct order.
368 */
369void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
370{
371 struct folio *folio;
372 struct mpage_readpage_args args = {
373 .get_block = get_block,
374 .is_readahead = true,
375 };
376
377 while ((folio = readahead_folio(ractl: rac))) {
378 prefetchw(x: &folio->flags);
379 args.folio = folio;
380 args.nr_pages = readahead_count(rac);
381 args.bio = do_mpage_readpage(args: &args);
382 }
383 if (args.bio)
384 mpage_bio_submit_read(bio: args.bio);
385}
386EXPORT_SYMBOL(mpage_readahead);
387
388/*
389 * This isn't called much at all
390 */
391int mpage_read_folio(struct folio *folio, get_block_t get_block)
392{
393 struct mpage_readpage_args args = {
394 .folio = folio,
395 .nr_pages = 1,
396 .get_block = get_block,
397 };
398
399 args.bio = do_mpage_readpage(args: &args);
400 if (args.bio)
401 mpage_bio_submit_read(bio: args.bio);
402 return 0;
403}
404EXPORT_SYMBOL(mpage_read_folio);
405
406/*
407 * Writing is not so simple.
408 *
409 * If the page has buffers then they will be used for obtaining the disk
410 * mapping. We only support pages which are fully mapped-and-dirty, with a
411 * special case for pages which are unmapped at the end: end-of-file.
412 *
413 * If the page has no buffers (preferred) then the page is mapped here.
414 *
415 * If all blocks are found to be contiguous then the page can go into the
416 * BIO. Otherwise fall back to the mapping's writepage().
417 *
418 * FIXME: This code wants an estimate of how many pages are still to be
419 * written, so it can intelligently allocate a suitably-sized BIO. For now,
420 * just allocate full-size (16-page) BIOs.
421 */
422
423struct mpage_data {
424 struct bio *bio;
425 sector_t last_block_in_bio;
426 get_block_t *get_block;
427};
428
429/*
430 * We have our BIO, so we can now mark the buffers clean. Make
431 * sure to only clean buffers which we know we'll be writing.
432 */
433static void clean_buffers(struct page *page, unsigned first_unmapped)
434{
435 unsigned buffer_counter = 0;
436 struct buffer_head *bh, *head;
437 if (!page_has_buffers(page))
438 return;
439 head = page_buffers(page);
440 bh = head;
441
442 do {
443 if (buffer_counter++ == first_unmapped)
444 break;
445 clear_buffer_dirty(bh);
446 bh = bh->b_this_page;
447 } while (bh != head);
448
449 /*
450 * we cannot drop the bh if the page is not uptodate or a concurrent
451 * read_folio would fail to serialize with the bh and it would read from
452 * disk before we reach the platter.
453 */
454 if (buffer_heads_over_limit && PageUptodate(page))
455 try_to_free_buffers(page_folio(page));
456}
457
458/*
459 * For situations where we want to clean all buffers attached to a page.
460 * We don't need to calculate how many buffers are attached to the page,
461 * we just need to specify a number larger than the maximum number of buffers.
462 */
463void clean_page_buffers(struct page *page)
464{
465 clean_buffers(page, first_unmapped: ~0U);
466}
467
468static int __mpage_writepage(struct folio *folio, struct writeback_control *wbc,
469 void *data)
470{
471 struct mpage_data *mpd = data;
472 struct bio *bio = mpd->bio;
473 struct address_space *mapping = folio->mapping;
474 struct inode *inode = mapping->host;
475 const unsigned blkbits = inode->i_blkbits;
476 const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
477 sector_t last_block;
478 sector_t block_in_file;
479 sector_t blocks[MAX_BUF_PER_PAGE];
480 unsigned page_block;
481 unsigned first_unmapped = blocks_per_page;
482 struct block_device *bdev = NULL;
483 int boundary = 0;
484 sector_t boundary_block = 0;
485 struct block_device *boundary_bdev = NULL;
486 size_t length;
487 struct buffer_head map_bh;
488 loff_t i_size = i_size_read(inode);
489 int ret = 0;
490 struct buffer_head *head = folio_buffers(folio);
491
492 if (head) {
493 struct buffer_head *bh = head;
494
495 /* If they're all mapped and dirty, do it */
496 page_block = 0;
497 do {
498 BUG_ON(buffer_locked(bh));
499 if (!buffer_mapped(bh)) {
500 /*
501 * unmapped dirty buffers are created by
502 * block_dirty_folio -> mmapped data
503 */
504 if (buffer_dirty(bh))
505 goto confused;
506 if (first_unmapped == blocks_per_page)
507 first_unmapped = page_block;
508 continue;
509 }
510
511 if (first_unmapped != blocks_per_page)
512 goto confused; /* hole -> non-hole */
513
514 if (!buffer_dirty(bh) || !buffer_uptodate(bh))
515 goto confused;
516 if (page_block) {
517 if (bh->b_blocknr != blocks[page_block-1] + 1)
518 goto confused;
519 }
520 blocks[page_block++] = bh->b_blocknr;
521 boundary = buffer_boundary(bh);
522 if (boundary) {
523 boundary_block = bh->b_blocknr;
524 boundary_bdev = bh->b_bdev;
525 }
526 bdev = bh->b_bdev;
527 } while ((bh = bh->b_this_page) != head);
528
529 if (first_unmapped)
530 goto page_is_mapped;
531
532 /*
533 * Page has buffers, but they are all unmapped. The page was
534 * created by pagein or read over a hole which was handled by
535 * block_read_full_folio(). If this address_space is also
536 * using mpage_readahead then this can rarely happen.
537 */
538 goto confused;
539 }
540
541 /*
542 * The page has no buffers: map it to disk
543 */
544 BUG_ON(!folio_test_uptodate(folio));
545 block_in_file = (sector_t)folio->index << (PAGE_SHIFT - blkbits);
546 /*
547 * Whole page beyond EOF? Skip allocating blocks to avoid leaking
548 * space.
549 */
550 if (block_in_file >= (i_size + (1 << blkbits) - 1) >> blkbits)
551 goto page_is_mapped;
552 last_block = (i_size - 1) >> blkbits;
553 map_bh.b_folio = folio;
554 for (page_block = 0; page_block < blocks_per_page; ) {
555
556 map_bh.b_state = 0;
557 map_bh.b_size = 1 << blkbits;
558 if (mpd->get_block(inode, block_in_file, &map_bh, 1))
559 goto confused;
560 if (!buffer_mapped(bh: &map_bh))
561 goto confused;
562 if (buffer_new(bh: &map_bh))
563 clean_bdev_bh_alias(bh: &map_bh);
564 if (buffer_boundary(bh: &map_bh)) {
565 boundary_block = map_bh.b_blocknr;
566 boundary_bdev = map_bh.b_bdev;
567 }
568 if (page_block) {
569 if (map_bh.b_blocknr != blocks[page_block-1] + 1)
570 goto confused;
571 }
572 blocks[page_block++] = map_bh.b_blocknr;
573 boundary = buffer_boundary(bh: &map_bh);
574 bdev = map_bh.b_bdev;
575 if (block_in_file == last_block)
576 break;
577 block_in_file++;
578 }
579 BUG_ON(page_block == 0);
580
581 first_unmapped = page_block;
582
583page_is_mapped:
584 /* Don't bother writing beyond EOF, truncate will discard the folio */
585 if (folio_pos(folio) >= i_size)
586 goto confused;
587 length = folio_size(folio);
588 if (folio_pos(folio) + length > i_size) {
589 /*
590 * The page straddles i_size. It must be zeroed out on each
591 * and every writepage invocation because it may be mmapped.
592 * "A file is mapped in multiples of the page size. For a file
593 * that is not a multiple of the page size, the remaining memory
594 * is zeroed when mapped, and writes to that region are not
595 * written out to the file."
596 */
597 length = i_size - folio_pos(folio);
598 folio_zero_segment(folio, start: length, xend: folio_size(folio));
599 }
600
601 /*
602 * This page will go to BIO. Do we need to send this BIO off first?
603 */
604 if (bio && mpd->last_block_in_bio != blocks[0] - 1)
605 bio = mpage_bio_submit_write(bio);
606
607alloc_new:
608 if (bio == NULL) {
609 bio = bio_alloc(bdev, BIO_MAX_VECS,
610 opf: REQ_OP_WRITE | wbc_to_write_flags(wbc),
611 GFP_NOFS);
612 bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
613 wbc_init_bio(wbc, bio);
614 }
615
616 /*
617 * Must try to add the page before marking the buffer clean or
618 * the confused fail path above (OOM) will be very confused when
619 * it finds all bh marked clean (i.e. it will not write anything)
620 */
621 wbc_account_cgroup_owner(wbc, page: &folio->page, bytes: folio_size(folio));
622 length = first_unmapped << blkbits;
623 if (!bio_add_folio(bio, folio, len: length, off: 0)) {
624 bio = mpage_bio_submit_write(bio);
625 goto alloc_new;
626 }
627
628 clean_buffers(page: &folio->page, first_unmapped);
629
630 BUG_ON(folio_test_writeback(folio));
631 folio_start_writeback(folio);
632 folio_unlock(folio);
633 if (boundary || (first_unmapped != blocks_per_page)) {
634 bio = mpage_bio_submit_write(bio);
635 if (boundary_block) {
636 write_boundary_block(bdev: boundary_bdev,
637 bblock: boundary_block, blocksize: 1 << blkbits);
638 }
639 } else {
640 mpd->last_block_in_bio = blocks[blocks_per_page - 1];
641 }
642 goto out;
643
644confused:
645 if (bio)
646 bio = mpage_bio_submit_write(bio);
647
648 /*
649 * The caller has a ref on the inode, so *mapping is stable
650 */
651 ret = block_write_full_page(page: &folio->page, get_block: mpd->get_block, wbc);
652 mapping_set_error(mapping, error: ret);
653out:
654 mpd->bio = bio;
655 return ret;
656}
657
658/**
659 * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
660 * @mapping: address space structure to write
661 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
662 * @get_block: the filesystem's block mapper function.
663 *
664 * This is a library function, which implements the writepages()
665 * address_space_operation.
666 */
667int
668mpage_writepages(struct address_space *mapping,
669 struct writeback_control *wbc, get_block_t get_block)
670{
671 struct mpage_data mpd = {
672 .get_block = get_block,
673 };
674 struct blk_plug plug;
675 int ret;
676
677 blk_start_plug(&plug);
678 ret = write_cache_pages(mapping, wbc, writepage: __mpage_writepage, data: &mpd);
679 if (mpd.bio)
680 mpage_bio_submit_write(bio: mpd.bio);
681 blk_finish_plug(&plug);
682 return ret;
683}
684EXPORT_SYMBOL(mpage_writepages);
685

source code of linux/fs/mpage.c