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