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 <linux/cleancache.h>
 33#include "internal.h"
 34
 35/*
 36 * I/O completion handler for multipage BIOs.
 37 *
 38 * The mpage code never puts partial pages into a BIO (except for end-of-file).
 39 * If a page does not map to a contiguous run of blocks then it simply falls
 40 * back to block_read_full_page().
 41 *
 42 * Why is this?  If a page's completion depends on a number of different BIOs
 43 * which can complete in any order (or at the same time) then determining the
 44 * status of that page is hard.  See end_buffer_async_read() for the details.
 45 * There is no point in duplicating all that complexity.
 46 */
 47static void mpage_end_io(struct bio *bio)
 48{
 49	struct bio_vec *bv;
 50	struct bvec_iter_all iter_all;
 51
 52	bio_for_each_segment_all(bv, bio, iter_all) {
 53		struct page *page = bv->bv_page;
 54		page_endio(page, bio_op(bio),
 55			   blk_status_to_errno(bio->bi_status));
 56	}
 57
 58	bio_put(bio);
 59}
 60
 61static struct bio *mpage_bio_submit(int op, int op_flags, struct bio *bio)
 62{
 63	bio->bi_end_io = mpage_end_io;
 64	bio_set_op_attrs(bio, op, op_flags);
 65	guard_bio_eod(bio);
 66	submit_bio(bio);
 67	return NULL;
 68}
 69
 70static struct bio *
 71mpage_alloc(struct block_device *bdev,
 72		sector_t first_sector, int nr_vecs,
 73		gfp_t gfp_flags)
 74{
 75	struct bio *bio;
 76
 77	/* Restrict the given (page cache) mask for slab allocations */
 78	gfp_flags &= GFP_KERNEL;
 79	bio = bio_alloc(gfp_flags, nr_vecs);
 80
 81	if (bio == NULL && (current->flags & PF_MEMALLOC)) {
 82		while (!bio && (nr_vecs /= 2))
 83			bio = bio_alloc(gfp_flags, nr_vecs);
 84	}
 85
 86	if (bio) {
 87		bio_set_dev(bio, bdev);
 88		bio->bi_iter.bi_sector = first_sector;
 89	}
 90	return bio;
 91}
 92
 93/*
 94 * support function for mpage_readahead.  The fs supplied get_block might
 95 * return an up to date buffer.  This is used to map that buffer into
 96 * the page, which allows readpage to avoid triggering a duplicate call
 97 * to get_block.
 98 *
 99 * The idea is to avoid adding buffers to pages that don't already have
100 * them.  So when the buffer is up to date and the page size == block size,
101 * this marks the page up to date instead of adding new buffers.
102 */
103static void 
104map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block) 
105{
106	struct inode *inode = page->mapping->host;
107	struct buffer_head *page_bh, *head;
108	int block = 0;
109
110	if (!page_has_buffers(page)) {
111		/*
112		 * don't make any buffers if there is only one buffer on
113		 * the page and the page just needs to be set up to date
114		 */
115		if (inode->i_blkbits == PAGE_SHIFT &&
116		    buffer_uptodate(bh)) {
117			SetPageUptodate(page);    
118			return;
119		}
120		create_empty_buffers(page, i_blocksize(inode), 0);
121	}
122	head = page_buffers(page);
123	page_bh = head;
124	do {
125		if (block == page_block) {
126			page_bh->b_state = bh->b_state;
127			page_bh->b_bdev = bh->b_bdev;
128			page_bh->b_blocknr = bh->b_blocknr;
129			break;
130		}
131		page_bh = page_bh->b_this_page;
132		block++;
133	} while (page_bh != head);
134}
135
136struct mpage_readpage_args {
137	struct bio *bio;
138	struct page *page;
139	unsigned int nr_pages;
140	bool is_readahead;
141	sector_t last_block_in_bio;
142	struct buffer_head map_bh;
143	unsigned long first_logical_block;
144	get_block_t *get_block;
145};
146
147/*
148 * This is the worker routine which does all the work of mapping the disk
149 * blocks and constructs largest possible bios, submits them for IO if the
150 * blocks are not contiguous on the disk.
151 *
152 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
153 * represent the validity of its disk mapping and to decide when to do the next
154 * get_block() call.
155 */
156static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
157{
158	struct page *page = args->page;
159	struct inode *inode = page->mapping->host;
160	const unsigned blkbits = inode->i_blkbits;
161	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
162	const unsigned blocksize = 1 << blkbits;
163	struct buffer_head *map_bh = &args->map_bh;
164	sector_t block_in_file;
165	sector_t last_block;
166	sector_t last_block_in_file;
167	sector_t blocks[MAX_BUF_PER_PAGE];
168	unsigned page_block;
169	unsigned first_hole = blocks_per_page;
170	struct block_device *bdev = NULL;
171	int length;
172	int fully_mapped = 1;
173	int op_flags;
174	unsigned nblocks;
175	unsigned relative_block;
176	gfp_t gfp;
177
178	if (args->is_readahead) {
179		op_flags = REQ_RAHEAD;
180		gfp = readahead_gfp_mask(page->mapping);
181	} else {
182		op_flags = 0;
183		gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
184	}
185
186	if (page_has_buffers(page))
187		goto confused;
188
189	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
190	last_block = block_in_file + args->nr_pages * blocks_per_page;
191	last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
192	if (last_block > last_block_in_file)
193		last_block = last_block_in_file;
194	page_block = 0;
195
196	/*
197	 * Map blocks using the result from the previous get_blocks call first.
198	 */
199	nblocks = map_bh->b_size >> blkbits;
200	if (buffer_mapped(map_bh) &&
201			block_in_file > args->first_logical_block &&
202			block_in_file < (args->first_logical_block + nblocks)) {
203		unsigned map_offset = block_in_file - args->first_logical_block;
204		unsigned last = nblocks - map_offset;
205
206		for (relative_block = 0; ; relative_block++) {
207			if (relative_block == last) {
208				clear_buffer_mapped(map_bh);
209				break;
210			}
211			if (page_block == blocks_per_page)
212				break;
213			blocks[page_block] = map_bh->b_blocknr + map_offset +
214						relative_block;
215			page_block++;
216			block_in_file++;
217		}
218		bdev = map_bh->b_bdev;
219	}
220
221	/*
222	 * Then do more get_blocks calls until we are done with this page.
223	 */
224	map_bh->b_page = page;
225	while (page_block < blocks_per_page) {
226		map_bh->b_state = 0;
227		map_bh->b_size = 0;
228
229		if (block_in_file < last_block) {
230			map_bh->b_size = (last_block-block_in_file) << blkbits;
231			if (args->get_block(inode, block_in_file, map_bh, 0))
232				goto confused;
233			args->first_logical_block = block_in_file;
234		}
235
236		if (!buffer_mapped(map_bh)) {
237			fully_mapped = 0;
238			if (first_hole == blocks_per_page)
239				first_hole = page_block;
240			page_block++;
241			block_in_file++;
242			continue;
243		}
244
245		/* some filesystems will copy data into the page during
246		 * the get_block call, in which case we don't want to
247		 * read it again.  map_buffer_to_page copies the data
248		 * we just collected from get_block into the page's buffers
249		 * so readpage doesn't have to repeat the get_block call
250		 */
251		if (buffer_uptodate(map_bh)) {
252			map_buffer_to_page(page, map_bh, page_block);
253			goto confused;
254		}
255	
256		if (first_hole != blocks_per_page)
257			goto confused;		/* hole -> non-hole */
258
259		/* Contiguous blocks? */
260		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
261			goto confused;
262		nblocks = map_bh->b_size >> blkbits;
263		for (relative_block = 0; ; relative_block++) {
264			if (relative_block == nblocks) {
265				clear_buffer_mapped(map_bh);
266				break;
267			} else if (page_block == blocks_per_page)
268				break;
269			blocks[page_block] = map_bh->b_blocknr+relative_block;
270			page_block++;
271			block_in_file++;
272		}
273		bdev = map_bh->b_bdev;
274	}
275
276	if (first_hole != blocks_per_page) {
277		zero_user_segment(page, first_hole << blkbits, PAGE_SIZE);
278		if (first_hole == 0) {
279			SetPageUptodate(page);
280			unlock_page(page);
281			goto out;
282		}
283	} else if (fully_mapped) {
284		SetPageMappedToDisk(page);
285	}
286
287	if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
288	    cleancache_get_page(page) == 0) {
289		SetPageUptodate(page);
290		goto confused;
291	}
292
293	/*
294	 * This page will go to BIO.  Do we need to send this BIO off first?
295	 */
296	if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
297		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
298
299alloc_new:
300	if (args->bio == NULL) {
301		if (first_hole == blocks_per_page) {
302			if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
303								page))
304				goto out;
305		}
306		args->bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
307					min_t(int, args->nr_pages,
308					      BIO_MAX_PAGES),
309					gfp);
310		if (args->bio == NULL)
311			goto confused;
312	}
313
314	length = first_hole << blkbits;
315	if (bio_add_page(args->bio, page, length, 0) < length) {
316		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
317		goto alloc_new;
318	}
319
320	relative_block = block_in_file - args->first_logical_block;
321	nblocks = map_bh->b_size >> blkbits;
322	if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
323	    (first_hole != blocks_per_page))
324		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
325	else
326		args->last_block_in_bio = blocks[blocks_per_page - 1];
327out:
328	return args->bio;
329
330confused:
331	if (args->bio)
332		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
333	if (!PageUptodate(page))
334		block_read_full_page(page, args->get_block);
335	else
336		unlock_page(page);
337	goto out;
338}
339
340/**
341 * mpage_readahead - start reads against pages
342 * @rac: Describes which pages to read.
343 * @get_block: The filesystem's block mapper function.
344 *
345 * This function walks the pages and the blocks within each page, building and
346 * emitting large BIOs.
347 *
348 * If anything unusual happens, such as:
349 *
350 * - encountering a page which has buffers
351 * - encountering a page which has a non-hole after a hole
352 * - encountering a page with non-contiguous blocks
353 *
354 * then this code just gives up and calls the buffer_head-based read function.
355 * It does handle a page which has holes at the end - that is a common case:
356 * the end-of-file on blocksize < PAGE_SIZE setups.
357 *
358 * BH_Boundary explanation:
359 *
360 * There is a problem.  The mpage read code assembles several pages, gets all
361 * their disk mappings, and then submits them all.  That's fine, but obtaining
362 * the disk mappings may require I/O.  Reads of indirect blocks, for example.
363 *
364 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
365 * submitted in the following order:
366 *
367 * 	12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
368 *
369 * because the indirect block has to be read to get the mappings of blocks
370 * 13,14,15,16.  Obviously, this impacts performance.
371 *
372 * So what we do it to allow the filesystem's get_block() function to set
373 * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
374 * after this one will require I/O against a block which is probably close to
375 * this one.  So you should push what I/O you have currently accumulated.
376 *
377 * This all causes the disk requests to be issued in the correct order.
378 */
379void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
380{
381	struct page *page;
382	struct mpage_readpage_args args = {
383		.get_block = get_block,
384		.is_readahead = true,
385	};
386
387	while ((page = readahead_page(rac))) {
388		prefetchw(&page->flags);
389		args.page = page;
390		args.nr_pages = readahead_count(rac);
391		args.bio = do_mpage_readpage(&args);
392		put_page(page);
393	}
394	if (args.bio)
395		mpage_bio_submit(REQ_OP_READ, REQ_RAHEAD, args.bio);
396}
397EXPORT_SYMBOL(mpage_readahead);
398
399/*
400 * This isn't called much at all
401 */
402int mpage_readpage(struct page *page, get_block_t get_block)
403{
404	struct mpage_readpage_args args = {
405		.page = page,
406		.nr_pages = 1,
407		.get_block = get_block,
408	};
409
410	args.bio = do_mpage_readpage(&args);
411	if (args.bio)
412		mpage_bio_submit(REQ_OP_READ, 0, args.bio);
413	return 0;
414}
415EXPORT_SYMBOL(mpage_readpage);
416
417/*
418 * Writing is not so simple.
419 *
420 * If the page has buffers then they will be used for obtaining the disk
421 * mapping.  We only support pages which are fully mapped-and-dirty, with a
422 * special case for pages which are unmapped at the end: end-of-file.
423 *
424 * If the page has no buffers (preferred) then the page is mapped here.
425 *
426 * If all blocks are found to be contiguous then the page can go into the
427 * BIO.  Otherwise fall back to the mapping's writepage().
428 * 
429 * FIXME: This code wants an estimate of how many pages are still to be
430 * written, so it can intelligently allocate a suitably-sized BIO.  For now,
431 * just allocate full-size (16-page) BIOs.
432 */
433
434struct mpage_data {
435	struct bio *bio;
436	sector_t last_block_in_bio;
437	get_block_t *get_block;
438	unsigned use_writepage;
439};
440
441/*
442 * We have our BIO, so we can now mark the buffers clean.  Make
443 * sure to only clean buffers which we know we'll be writing.
444 */
445static void clean_buffers(struct page *page, unsigned first_unmapped)
446{
447	unsigned buffer_counter = 0;
448	struct buffer_head *bh, *head;
449	if (!page_has_buffers(page))
450		return;
451	head = page_buffers(page);
452	bh = head;
453
454	do {
455		if (buffer_counter++ == first_unmapped)
456			break;
457		clear_buffer_dirty(bh);
458		bh = bh->b_this_page;
459	} while (bh != head);
460
461	/*
462	 * we cannot drop the bh if the page is not uptodate or a concurrent
463	 * readpage would fail to serialize with the bh and it would read from
464	 * disk before we reach the platter.
465	 */
466	if (buffer_heads_over_limit && PageUptodate(page))
467		try_to_free_buffers(page);
468}
469
470/*
471 * For situations where we want to clean all buffers attached to a page.
472 * We don't need to calculate how many buffers are attached to the page,
473 * we just need to specify a number larger than the maximum number of buffers.
474 */
475void clean_page_buffers(struct page *page)
476{
477	clean_buffers(page, ~0U);
478}
479
480static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
481		      void *data)
482{
483	struct mpage_data *mpd = data;
484	struct bio *bio = mpd->bio;
485	struct address_space *mapping = page->mapping;
486	struct inode *inode = page->mapping->host;
487	const unsigned blkbits = inode->i_blkbits;
488	unsigned long end_index;
489	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
490	sector_t last_block;
491	sector_t block_in_file;
492	sector_t blocks[MAX_BUF_PER_PAGE];
493	unsigned page_block;
494	unsigned first_unmapped = blocks_per_page;
495	struct block_device *bdev = NULL;
496	int boundary = 0;
497	sector_t boundary_block = 0;
498	struct block_device *boundary_bdev = NULL;
499	int length;
500	struct buffer_head map_bh;
501	loff_t i_size = i_size_read(inode);
502	int ret = 0;
503	int op_flags = wbc_to_write_flags(wbc);
504
505	if (page_has_buffers(page)) {
506		struct buffer_head *head = page_buffers(page);
507		struct buffer_head *bh = head;
508
509		/* If they're all mapped and dirty, do it */
510		page_block = 0;
511		do {
512			BUG_ON(buffer_locked(bh));
513			if (!buffer_mapped(bh)) {
514				/*
515				 * unmapped dirty buffers are created by
516				 * __set_page_dirty_buffers -> mmapped data
517				 */
518				if (buffer_dirty(bh))
519					goto confused;
520				if (first_unmapped == blocks_per_page)
521					first_unmapped = page_block;
522				continue;
523			}
524
525			if (first_unmapped != blocks_per_page)
526				goto confused;	/* hole -> non-hole */
527
528			if (!buffer_dirty(bh) || !buffer_uptodate(bh))
529				goto confused;
530			if (page_block) {
531				if (bh->b_blocknr != blocks[page_block-1] + 1)
532					goto confused;
533			}
534			blocks[page_block++] = bh->b_blocknr;
535			boundary = buffer_boundary(bh);
536			if (boundary) {
537				boundary_block = bh->b_blocknr;
538				boundary_bdev = bh->b_bdev;
539			}
540			bdev = bh->b_bdev;
541		} while ((bh = bh->b_this_page) != head);
542
543		if (first_unmapped)
544			goto page_is_mapped;
545
546		/*
547		 * Page has buffers, but they are all unmapped. The page was
548		 * created by pagein or read over a hole which was handled by
549		 * block_read_full_page().  If this address_space is also
550		 * using mpage_readahead then this can rarely happen.
551		 */
552		goto confused;
553	}
554
555	/*
556	 * The page has no buffers: map it to disk
557	 */
558	BUG_ON(!PageUptodate(page));
559	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
560	last_block = (i_size - 1) >> blkbits;
561	map_bh.b_page = page;
562	for (page_block = 0; page_block < blocks_per_page; ) {
563
564		map_bh.b_state = 0;
565		map_bh.b_size = 1 << blkbits;
566		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
567			goto confused;
568		if (buffer_new(&map_bh))
569			clean_bdev_bh_alias(&map_bh);
570		if (buffer_boundary(&map_bh)) {
571			boundary_block = map_bh.b_blocknr;
572			boundary_bdev = map_bh.b_bdev;
573		}
574		if (page_block) {
575			if (map_bh.b_blocknr != blocks[page_block-1] + 1)
576				goto confused;
577		}
578		blocks[page_block++] = map_bh.b_blocknr;
579		boundary = buffer_boundary(&map_bh);
580		bdev = map_bh.b_bdev;
581		if (block_in_file == last_block)
582			break;
583		block_in_file++;
584	}
585	BUG_ON(page_block == 0);
586
587	first_unmapped = page_block;
588
589page_is_mapped:
590	end_index = i_size >> PAGE_SHIFT;
591	if (page->index >= end_index) {
592		/*
593		 * The page straddles i_size.  It must be zeroed out on each
594		 * and every writepage invocation because it may be mmapped.
595		 * "A file is mapped in multiples of the page size.  For a file
596		 * that is not a multiple of the page size, the remaining memory
597		 * is zeroed when mapped, and writes to that region are not
598		 * written out to the file."
599		 */
600		unsigned offset = i_size & (PAGE_SIZE - 1);
601
602		if (page->index > end_index || !offset)
603			goto confused;
604		zero_user_segment(page, offset, PAGE_SIZE);
605	}
606
607	/*
608	 * This page will go to BIO.  Do we need to send this BIO off first?
609	 */
610	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
611		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
612
613alloc_new:
614	if (bio == NULL) {
615		if (first_unmapped == blocks_per_page) {
616			if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
617								page, wbc))
618				goto out;
619		}
620		bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
621				BIO_MAX_PAGES, GFP_NOFS|__GFP_HIGH);
622		if (bio == NULL)
623			goto confused;
624
625		wbc_init_bio(wbc, bio);
626		bio->bi_write_hint = inode->i_write_hint;
627	}
628
629	/*
630	 * Must try to add the page before marking the buffer clean or
631	 * the confused fail path above (OOM) will be very confused when
632	 * it finds all bh marked clean (i.e. it will not write anything)
633	 */
634	wbc_account_cgroup_owner(wbc, page, PAGE_SIZE);
635	length = first_unmapped << blkbits;
636	if (bio_add_page(bio, page, length, 0) < length) {
637		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
638		goto alloc_new;
639	}
640
641	clean_buffers(page, first_unmapped);
642
643	BUG_ON(PageWriteback(page));
644	set_page_writeback(page);
645	unlock_page(page);
646	if (boundary || (first_unmapped != blocks_per_page)) {
647		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
648		if (boundary_block) {
649			write_boundary_block(boundary_bdev,
650					boundary_block, 1 << blkbits);
651		}
652	} else {
653		mpd->last_block_in_bio = blocks[blocks_per_page - 1];
654	}
655	goto out;
656
657confused:
658	if (bio)
659		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
660
661	if (mpd->use_writepage) {
662		ret = mapping->a_ops->writepage(page, wbc);
663	} else {
664		ret = -EAGAIN;
665		goto out;
666	}
667	/*
668	 * The caller has a ref on the inode, so *mapping is stable
669	 */
670	mapping_set_error(mapping, ret);
671out:
672	mpd->bio = bio;
673	return ret;
674}
675
676/**
677 * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
678 * @mapping: address space structure to write
679 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
680 * @get_block: the filesystem's block mapper function.
681 *             If this is NULL then use a_ops->writepage.  Otherwise, go
682 *             direct-to-BIO.
683 *
684 * This is a library function, which implements the writepages()
685 * address_space_operation.
686 *
687 * If a page is already under I/O, generic_writepages() skips it, even
688 * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
689 * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
690 * and msync() need to guarantee that all the data which was dirty at the time
691 * the call was made get new I/O started against them.  If wbc->sync_mode is
692 * WB_SYNC_ALL then we were called for data integrity and we must wait for
693 * existing IO to complete.
694 */
695int
696mpage_writepages(struct address_space *mapping,
697		struct writeback_control *wbc, get_block_t get_block)
698{
699	struct blk_plug plug;
700	int ret;
701
702	blk_start_plug(&plug);
703
704	if (!get_block)
705		ret = generic_writepages(mapping, wbc);
706	else {
707		struct mpage_data mpd = {
708			.bio = NULL,
709			.last_block_in_bio = 0,
710			.get_block = get_block,
711			.use_writepage = 1,
712		};
713
714		ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
715		if (mpd.bio) {
716			int op_flags = (wbc->sync_mode == WB_SYNC_ALL ?
717				  REQ_SYNC : 0);
718			mpage_bio_submit(REQ_OP_WRITE, op_flags, mpd.bio);
719		}
720	}
721	blk_finish_plug(&plug);
722	return ret;
723}
724EXPORT_SYMBOL(mpage_writepages);
725
726int mpage_writepage(struct page *page, get_block_t get_block,
727	struct writeback_control *wbc)
728{
729	struct mpage_data mpd = {
730		.bio = NULL,
731		.last_block_in_bio = 0,
732		.get_block = get_block,
733		.use_writepage = 0,
734	};
735	int ret = __mpage_writepage(page, wbc, &mpd);
736	if (mpd.bio) {
737		int op_flags = (wbc->sync_mode == WB_SYNC_ALL ?
738			  REQ_SYNC : 0);
739		mpage_bio_submit(REQ_OP_WRITE, op_flags, mpd.bio);
740	}
741	return ret;
742}
743EXPORT_SYMBOL(mpage_writepage);

  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 <linux/cleancache.h>
 33#include "internal.h"
 34
 35/*
 36 * I/O completion handler for multipage BIOs.
 37 *
 38 * The mpage code never puts partial pages into a BIO (except for end-of-file).
 39 * If a page does not map to a contiguous run of blocks then it simply falls
 40 * back to block_read_full_page().
 41 *
 42 * Why is this?  If a page's completion depends on a number of different BIOs
 43 * which can complete in any order (or at the same time) then determining the
 44 * status of that page is hard.  See end_buffer_async_read() for the details.
 45 * There is no point in duplicating all that complexity.
 46 */
 47static void mpage_end_io(struct bio *bio)
 48{
 49	struct bio_vec *bv;
 50	struct bvec_iter_all iter_all;
 51
 52	bio_for_each_segment_all(bv, bio, iter_all) {
 53		struct page *page = bv->bv_page;
 54		page_endio(page, bio_op(bio),
 55			   blk_status_to_errno(bio->bi_status));
 56	}
 57
 58	bio_put(bio);
 59}
 60
 61static struct bio *mpage_bio_submit(int op, int op_flags, struct bio *bio)
 62{
 63	bio->bi_end_io = mpage_end_io;
 64	bio_set_op_attrs(bio, op, op_flags);
 65	guard_bio_eod(bio);
 66	submit_bio(bio);
 67	return NULL;
 68}
 69
 70static struct bio *
 71mpage_alloc(struct block_device *bdev,
 72		sector_t first_sector, int nr_vecs,
 73		gfp_t gfp_flags)
 74{
 75	struct bio *bio;
 76
 77	/* Restrict the given (page cache) mask for slab allocations */
 78	gfp_flags &= GFP_KERNEL;
 79	bio = bio_alloc(gfp_flags, nr_vecs);
 80
 81	if (bio == NULL && (current->flags & PF_MEMALLOC)) {
 82		while (!bio && (nr_vecs /= 2))
 83			bio = bio_alloc(gfp_flags, nr_vecs);
 84	}
 85
 86	if (bio) {
 87		bio_set_dev(bio, bdev);
 88		bio->bi_iter.bi_sector = first_sector;
 89	}
 90	return bio;
 91}
 92
 93/*
 94 * support function for mpage_readahead.  The fs supplied get_block might
 95 * return an up to date buffer.  This is used to map that buffer into
 96 * the page, which allows readpage to avoid triggering a duplicate call
 97 * to get_block.
 98 *
 99 * The idea is to avoid adding buffers to pages that don't already have
100 * them.  So when the buffer is up to date and the page size == block size,
101 * this marks the page up to date instead of adding new buffers.
102 */
103static void 
104map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block) 
105{
106	struct inode *inode = page->mapping->host;
107	struct buffer_head *page_bh, *head;
108	int block = 0;
109
110	if (!page_has_buffers(page)) {
111		/*
112		 * don't make any buffers if there is only one buffer on
113		 * the page and the page just needs to be set up to date
114		 */
115		if (inode->i_blkbits == PAGE_SHIFT &&
116		    buffer_uptodate(bh)) {
117			SetPageUptodate(page);    
118			return;
119		}
120		create_empty_buffers(page, i_blocksize(inode), 0);
121	}
122	head = page_buffers(page);
123	page_bh = head;
124	do {
125		if (block == page_block) {
126			page_bh->b_state = bh->b_state;
127			page_bh->b_bdev = bh->b_bdev;
128			page_bh->b_blocknr = bh->b_blocknr;
129			break;
130		}
131		page_bh = page_bh->b_this_page;
132		block++;
133	} while (page_bh != head);
134}
135
136struct mpage_readpage_args {
137	struct bio *bio;
138	struct page *page;
139	unsigned int nr_pages;
140	bool is_readahead;
141	sector_t last_block_in_bio;
142	struct buffer_head map_bh;
143	unsigned long first_logical_block;
144	get_block_t *get_block;
145};
146
147/*
148 * This is the worker routine which does all the work of mapping the disk
149 * blocks and constructs largest possible bios, submits them for IO if the
150 * blocks are not contiguous on the disk.
151 *
152 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
153 * represent the validity of its disk mapping and to decide when to do the next
154 * get_block() call.
155 */
156static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
157{
158	struct page *page = args->page;
159	struct inode *inode = page->mapping->host;
160	const unsigned blkbits = inode->i_blkbits;
161	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
162	const unsigned blocksize = 1 << blkbits;
163	struct buffer_head *map_bh = &args->map_bh;
164	sector_t block_in_file;
165	sector_t last_block;
166	sector_t last_block_in_file;
167	sector_t blocks[MAX_BUF_PER_PAGE];
168	unsigned page_block;
169	unsigned first_hole = blocks_per_page;
170	struct block_device *bdev = NULL;
171	int length;
172	int fully_mapped = 1;
173	int op_flags;
174	unsigned nblocks;
175	unsigned relative_block;
176	gfp_t gfp;
177
178	if (args->is_readahead) {
179		op_flags = REQ_RAHEAD;
180		gfp = readahead_gfp_mask(page->mapping);
181	} else {
182		op_flags = 0;
183		gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
184	}
185
186	if (page_has_buffers(page))
187		goto confused;
188
189	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
190	last_block = block_in_file + args->nr_pages * blocks_per_page;
191	last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
192	if (last_block > last_block_in_file)
193		last_block = last_block_in_file;
194	page_block = 0;
195
196	/*
197	 * Map blocks using the result from the previous get_blocks call first.
198	 */
199	nblocks = map_bh->b_size >> blkbits;
200	if (buffer_mapped(map_bh) &&
201			block_in_file > args->first_logical_block &&
202			block_in_file < (args->first_logical_block + nblocks)) {
203		unsigned map_offset = block_in_file - args->first_logical_block;
204		unsigned last = nblocks - map_offset;
205
206		for (relative_block = 0; ; relative_block++) {
207			if (relative_block == last) {
208				clear_buffer_mapped(map_bh);
209				break;
210			}
211			if (page_block == blocks_per_page)
212				break;
213			blocks[page_block] = map_bh->b_blocknr + map_offset +
214						relative_block;
215			page_block++;
216			block_in_file++;
217		}
218		bdev = map_bh->b_bdev;
219	}
220
221	/*
222	 * Then do more get_blocks calls until we are done with this page.
223	 */
224	map_bh->b_page = page;
225	while (page_block < blocks_per_page) {
226		map_bh->b_state = 0;
227		map_bh->b_size = 0;
228
229		if (block_in_file < last_block) {
230			map_bh->b_size = (last_block-block_in_file) << blkbits;
231			if (args->get_block(inode, block_in_file, map_bh, 0))
232				goto confused;
233			args->first_logical_block = block_in_file;
234		}
235
236		if (!buffer_mapped(map_bh)) {
237			fully_mapped = 0;
238			if (first_hole == blocks_per_page)
239				first_hole = page_block;
240			page_block++;
241			block_in_file++;
242			continue;
243		}
244
245		/* some filesystems will copy data into the page during
246		 * the get_block call, in which case we don't want to
247		 * read it again.  map_buffer_to_page copies the data
248		 * we just collected from get_block into the page's buffers
249		 * so readpage doesn't have to repeat the get_block call
250		 */
251		if (buffer_uptodate(map_bh)) {
252			map_buffer_to_page(page, map_bh, page_block);
253			goto confused;
254		}
255	
256		if (first_hole != blocks_per_page)
257			goto confused;		/* hole -> non-hole */
258
259		/* Contiguous blocks? */
260		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
261			goto confused;
262		nblocks = map_bh->b_size >> blkbits;
263		for (relative_block = 0; ; relative_block++) {
264			if (relative_block == nblocks) {
265				clear_buffer_mapped(map_bh);
266				break;
267			} else if (page_block == blocks_per_page)
268				break;
269			blocks[page_block] = map_bh->b_blocknr+relative_block;
270			page_block++;
271			block_in_file++;
272		}
273		bdev = map_bh->b_bdev;
274	}
275
276	if (first_hole != blocks_per_page) {
277		zero_user_segment(page, first_hole << blkbits, PAGE_SIZE);
278		if (first_hole == 0) {
279			SetPageUptodate(page);
280			unlock_page(page);
281			goto out;
282		}
283	} else if (fully_mapped) {
284		SetPageMappedToDisk(page);
285	}
286
287	if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
288	    cleancache_get_page(page) == 0) {
289		SetPageUptodate(page);
290		goto confused;
291	}
292
293	/*
294	 * This page will go to BIO.  Do we need to send this BIO off first?
295	 */
296	if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
297		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
298
299alloc_new:
300	if (args->bio == NULL) {
301		if (first_hole == blocks_per_page) {
302			if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
303								page))
304				goto out;
305		}
306		args->bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
307					bio_max_segs(args->nr_pages), gfp);
 
 
308		if (args->bio == NULL)
309			goto confused;
310	}
311
312	length = first_hole << blkbits;
313	if (bio_add_page(args->bio, page, length, 0) < length) {
314		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
315		goto alloc_new;
316	}
317
318	relative_block = block_in_file - args->first_logical_block;
319	nblocks = map_bh->b_size >> blkbits;
320	if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
321	    (first_hole != blocks_per_page))
322		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
323	else
324		args->last_block_in_bio = blocks[blocks_per_page - 1];
325out:
326	return args->bio;
327
328confused:
329	if (args->bio)
330		args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
331	if (!PageUptodate(page))
332		block_read_full_page(page, args->get_block);
333	else
334		unlock_page(page);
335	goto out;
336}
337
338/**
339 * mpage_readahead - start reads against pages
340 * @rac: Describes which pages to read.
341 * @get_block: The filesystem's block mapper function.
342 *
343 * This function walks the pages and the blocks within each page, building and
344 * emitting large BIOs.
345 *
346 * If anything unusual happens, such as:
347 *
348 * - encountering a page which has buffers
349 * - encountering a page which has a non-hole after a hole
350 * - encountering a page with non-contiguous blocks
351 *
352 * then this code just gives up and calls the buffer_head-based read function.
353 * It does handle a page which has holes at the end - that is a common case:
354 * the end-of-file on blocksize < PAGE_SIZE setups.
355 *
356 * BH_Boundary explanation:
357 *
358 * There is a problem.  The mpage read code assembles several pages, gets all
359 * their disk mappings, and then submits them all.  That's fine, but obtaining
360 * the disk mappings may require I/O.  Reads of indirect blocks, for example.
361 *
362 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
363 * submitted in the following order:
364 *
365 * 	12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
366 *
367 * because the indirect block has to be read to get the mappings of blocks
368 * 13,14,15,16.  Obviously, this impacts performance.
369 *
370 * So what we do it to allow the filesystem's get_block() function to set
371 * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
372 * after this one will require I/O against a block which is probably close to
373 * this one.  So you should push what I/O you have currently accumulated.
374 *
375 * This all causes the disk requests to be issued in the correct order.
376 */
377void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
378{
379	struct page *page;
380	struct mpage_readpage_args args = {
381		.get_block = get_block,
382		.is_readahead = true,
383	};
384
385	while ((page = readahead_page(rac))) {
386		prefetchw(&page->flags);
387		args.page = page;
388		args.nr_pages = readahead_count(rac);
389		args.bio = do_mpage_readpage(&args);
390		put_page(page);
391	}
392	if (args.bio)
393		mpage_bio_submit(REQ_OP_READ, REQ_RAHEAD, args.bio);
394}
395EXPORT_SYMBOL(mpage_readahead);
396
397/*
398 * This isn't called much at all
399 */
400int mpage_readpage(struct page *page, get_block_t get_block)
401{
402	struct mpage_readpage_args args = {
403		.page = page,
404		.nr_pages = 1,
405		.get_block = get_block,
406	};
407
408	args.bio = do_mpage_readpage(&args);
409	if (args.bio)
410		mpage_bio_submit(REQ_OP_READ, 0, args.bio);
411	return 0;
412}
413EXPORT_SYMBOL(mpage_readpage);
414
415/*
416 * Writing is not so simple.
417 *
418 * If the page has buffers then they will be used for obtaining the disk
419 * mapping.  We only support pages which are fully mapped-and-dirty, with a
420 * special case for pages which are unmapped at the end: end-of-file.
421 *
422 * If the page has no buffers (preferred) then the page is mapped here.
423 *
424 * If all blocks are found to be contiguous then the page can go into the
425 * BIO.  Otherwise fall back to the mapping's writepage().
426 * 
427 * FIXME: This code wants an estimate of how many pages are still to be
428 * written, so it can intelligently allocate a suitably-sized BIO.  For now,
429 * just allocate full-size (16-page) BIOs.
430 */
431
432struct mpage_data {
433	struct bio *bio;
434	sector_t last_block_in_bio;
435	get_block_t *get_block;
436	unsigned use_writepage;
437};
438
439/*
440 * We have our BIO, so we can now mark the buffers clean.  Make
441 * sure to only clean buffers which we know we'll be writing.
442 */
443static void clean_buffers(struct page *page, unsigned first_unmapped)
444{
445	unsigned buffer_counter = 0;
446	struct buffer_head *bh, *head;
447	if (!page_has_buffers(page))
448		return;
449	head = page_buffers(page);
450	bh = head;
451
452	do {
453		if (buffer_counter++ == first_unmapped)
454			break;
455		clear_buffer_dirty(bh);
456		bh = bh->b_this_page;
457	} while (bh != head);
458
459	/*
460	 * we cannot drop the bh if the page is not uptodate or a concurrent
461	 * readpage would fail to serialize with the bh and it would read from
462	 * disk before we reach the platter.
463	 */
464	if (buffer_heads_over_limit && PageUptodate(page))
465		try_to_free_buffers(page);
466}
467
468/*
469 * For situations where we want to clean all buffers attached to a page.
470 * We don't need to calculate how many buffers are attached to the page,
471 * we just need to specify a number larger than the maximum number of buffers.
472 */
473void clean_page_buffers(struct page *page)
474{
475	clean_buffers(page, ~0U);
476}
477
478static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
479		      void *data)
480{
481	struct mpage_data *mpd = data;
482	struct bio *bio = mpd->bio;
483	struct address_space *mapping = page->mapping;
484	struct inode *inode = page->mapping->host;
485	const unsigned blkbits = inode->i_blkbits;
486	unsigned long end_index;
487	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
488	sector_t last_block;
489	sector_t block_in_file;
490	sector_t blocks[MAX_BUF_PER_PAGE];
491	unsigned page_block;
492	unsigned first_unmapped = blocks_per_page;
493	struct block_device *bdev = NULL;
494	int boundary = 0;
495	sector_t boundary_block = 0;
496	struct block_device *boundary_bdev = NULL;
497	int length;
498	struct buffer_head map_bh;
499	loff_t i_size = i_size_read(inode);
500	int ret = 0;
501	int op_flags = wbc_to_write_flags(wbc);
502
503	if (page_has_buffers(page)) {
504		struct buffer_head *head = page_buffers(page);
505		struct buffer_head *bh = head;
506
507		/* If they're all mapped and dirty, do it */
508		page_block = 0;
509		do {
510			BUG_ON(buffer_locked(bh));
511			if (!buffer_mapped(bh)) {
512				/*
513				 * unmapped dirty buffers are created by
514				 * __set_page_dirty_buffers -> mmapped data
515				 */
516				if (buffer_dirty(bh))
517					goto confused;
518				if (first_unmapped == blocks_per_page)
519					first_unmapped = page_block;
520				continue;
521			}
522
523			if (first_unmapped != blocks_per_page)
524				goto confused;	/* hole -> non-hole */
525
526			if (!buffer_dirty(bh) || !buffer_uptodate(bh))
527				goto confused;
528			if (page_block) {
529				if (bh->b_blocknr != blocks[page_block-1] + 1)
530					goto confused;
531			}
532			blocks[page_block++] = bh->b_blocknr;
533			boundary = buffer_boundary(bh);
534			if (boundary) {
535				boundary_block = bh->b_blocknr;
536				boundary_bdev = bh->b_bdev;
537			}
538			bdev = bh->b_bdev;
539		} while ((bh = bh->b_this_page) != head);
540
541		if (first_unmapped)
542			goto page_is_mapped;
543
544		/*
545		 * Page has buffers, but they are all unmapped. The page was
546		 * created by pagein or read over a hole which was handled by
547		 * block_read_full_page().  If this address_space is also
548		 * using mpage_readahead then this can rarely happen.
549		 */
550		goto confused;
551	}
552
553	/*
554	 * The page has no buffers: map it to disk
555	 */
556	BUG_ON(!PageUptodate(page));
557	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
558	last_block = (i_size - 1) >> blkbits;
559	map_bh.b_page = page;
560	for (page_block = 0; page_block < blocks_per_page; ) {
561
562		map_bh.b_state = 0;
563		map_bh.b_size = 1 << blkbits;
564		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
565			goto confused;
566		if (buffer_new(&map_bh))
567			clean_bdev_bh_alias(&map_bh);
568		if (buffer_boundary(&map_bh)) {
569			boundary_block = map_bh.b_blocknr;
570			boundary_bdev = map_bh.b_bdev;
571		}
572		if (page_block) {
573			if (map_bh.b_blocknr != blocks[page_block-1] + 1)
574				goto confused;
575		}
576		blocks[page_block++] = map_bh.b_blocknr;
577		boundary = buffer_boundary(&map_bh);
578		bdev = map_bh.b_bdev;
579		if (block_in_file == last_block)
580			break;
581		block_in_file++;
582	}
583	BUG_ON(page_block == 0);
584
585	first_unmapped = page_block;
586
587page_is_mapped:
588	end_index = i_size >> PAGE_SHIFT;
589	if (page->index >= end_index) {
590		/*
591		 * The page straddles i_size.  It must be zeroed out on each
592		 * and every writepage invocation because it may be mmapped.
593		 * "A file is mapped in multiples of the page size.  For a file
594		 * that is not a multiple of the page size, the remaining memory
595		 * is zeroed when mapped, and writes to that region are not
596		 * written out to the file."
597		 */
598		unsigned offset = i_size & (PAGE_SIZE - 1);
599
600		if (page->index > end_index || !offset)
601			goto confused;
602		zero_user_segment(page, offset, PAGE_SIZE);
603	}
604
605	/*
606	 * This page will go to BIO.  Do we need to send this BIO off first?
607	 */
608	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
609		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
610
611alloc_new:
612	if (bio == NULL) {
613		if (first_unmapped == blocks_per_page) {
614			if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
615								page, wbc))
616				goto out;
617		}
618		bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
619				BIO_MAX_VECS, GFP_NOFS|__GFP_HIGH);
620		if (bio == NULL)
621			goto confused;
622
623		wbc_init_bio(wbc, bio);
624		bio->bi_write_hint = inode->i_write_hint;
625	}
626
627	/*
628	 * Must try to add the page before marking the buffer clean or
629	 * the confused fail path above (OOM) will be very confused when
630	 * it finds all bh marked clean (i.e. it will not write anything)
631	 */
632	wbc_account_cgroup_owner(wbc, page, PAGE_SIZE);
633	length = first_unmapped << blkbits;
634	if (bio_add_page(bio, page, length, 0) < length) {
635		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
636		goto alloc_new;
637	}
638
639	clean_buffers(page, first_unmapped);
640
641	BUG_ON(PageWriteback(page));
642	set_page_writeback(page);
643	unlock_page(page);
644	if (boundary || (first_unmapped != blocks_per_page)) {
645		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
646		if (boundary_block) {
647			write_boundary_block(boundary_bdev,
648					boundary_block, 1 << blkbits);
649		}
650	} else {
651		mpd->last_block_in_bio = blocks[blocks_per_page - 1];
652	}
653	goto out;
654
655confused:
656	if (bio)
657		bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
658
659	if (mpd->use_writepage) {
660		ret = mapping->a_ops->writepage(page, wbc);
661	} else {
662		ret = -EAGAIN;
663		goto out;
664	}
665	/*
666	 * The caller has a ref on the inode, so *mapping is stable
667	 */
668	mapping_set_error(mapping, ret);
669out:
670	mpd->bio = bio;
671	return ret;
672}
673
674/**
675 * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
676 * @mapping: address space structure to write
677 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
678 * @get_block: the filesystem's block mapper function.
679 *             If this is NULL then use a_ops->writepage.  Otherwise, go
680 *             direct-to-BIO.
681 *
682 * This is a library function, which implements the writepages()
683 * address_space_operation.
684 *
685 * If a page is already under I/O, generic_writepages() skips it, even
686 * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
687 * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
688 * and msync() need to guarantee that all the data which was dirty at the time
689 * the call was made get new I/O started against them.  If wbc->sync_mode is
690 * WB_SYNC_ALL then we were called for data integrity and we must wait for
691 * existing IO to complete.
692 */
693int
694mpage_writepages(struct address_space *mapping,
695		struct writeback_control *wbc, get_block_t get_block)
696{
697	struct blk_plug plug;
698	int ret;
699
700	blk_start_plug(&plug);
701
702	if (!get_block)
703		ret = generic_writepages(mapping, wbc);
704	else {
705		struct mpage_data mpd = {
706			.bio = NULL,
707			.last_block_in_bio = 0,
708			.get_block = get_block,
709			.use_writepage = 1,
710		};
711
712		ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
713		if (mpd.bio) {
714			int op_flags = (wbc->sync_mode == WB_SYNC_ALL ?
715				  REQ_SYNC : 0);
716			mpage_bio_submit(REQ_OP_WRITE, op_flags, mpd.bio);
717		}
718	}
719	blk_finish_plug(&plug);
720	return ret;
721}
722EXPORT_SYMBOL(mpage_writepages);
723
724int mpage_writepage(struct page *page, get_block_t get_block,
725	struct writeback_control *wbc)
726{
727	struct mpage_data mpd = {
728		.bio = NULL,
729		.last_block_in_bio = 0,
730		.get_block = get_block,
731		.use_writepage = 0,
732	};
733	int ret = __mpage_writepage(page, wbc, &mpd);
734	if (mpd.bio) {
735		int op_flags = (wbc->sync_mode == WB_SYNC_ALL ?
736			  REQ_SYNC : 0);
737		mpage_bio_submit(REQ_OP_WRITE, op_flags, mpd.bio);
738	}
739	return ret;
740}
741EXPORT_SYMBOL(mpage_writepage);