xref: /openbmc/linux/fs/mpage.c (revision b04b4f78)
1 /*
2  * fs/mpage.c
3  *
4  * Copyright (C) 2002, Linus Torvalds.
5  *
6  * Contains functions related to preparing and submitting BIOs which contain
7  * multiple pagecache pages.
8  *
9  * 15May2002	Andrew Morton
10  *		Initial version
11  * 27Jun2002	axboe@suse.de
12  *		use bio_add_page() to build bio's just the right size
13  */
14 
15 #include <linux/kernel.h>
16 #include <linux/module.h>
17 #include <linux/mm.h>
18 #include <linux/kdev_t.h>
19 #include <linux/bio.h>
20 #include <linux/fs.h>
21 #include <linux/buffer_head.h>
22 #include <linux/blkdev.h>
23 #include <linux/highmem.h>
24 #include <linux/prefetch.h>
25 #include <linux/mpage.h>
26 #include <linux/writeback.h>
27 #include <linux/backing-dev.h>
28 #include <linux/pagevec.h>
29 
30 /*
31  * I/O completion handler for multipage BIOs.
32  *
33  * The mpage code never puts partial pages into a BIO (except for end-of-file).
34  * If a page does not map to a contiguous run of blocks then it simply falls
35  * back to block_read_full_page().
36  *
37  * Why is this?  If a page's completion depends on a number of different BIOs
38  * which can complete in any order (or at the same time) then determining the
39  * status of that page is hard.  See end_buffer_async_read() for the details.
40  * There is no point in duplicating all that complexity.
41  */
42 static void mpage_end_io_read(struct bio *bio, int err)
43 {
44 	const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
45 	struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;
46 
47 	do {
48 		struct page *page = bvec->bv_page;
49 
50 		if (--bvec >= bio->bi_io_vec)
51 			prefetchw(&bvec->bv_page->flags);
52 
53 		if (uptodate) {
54 			SetPageUptodate(page);
55 		} else {
56 			ClearPageUptodate(page);
57 			SetPageError(page);
58 		}
59 		unlock_page(page);
60 	} while (bvec >= bio->bi_io_vec);
61 	bio_put(bio);
62 }
63 
64 static void mpage_end_io_write(struct bio *bio, int err)
65 {
66 	const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
67 	struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;
68 
69 	do {
70 		struct page *page = bvec->bv_page;
71 
72 		if (--bvec >= bio->bi_io_vec)
73 			prefetchw(&bvec->bv_page->flags);
74 
75 		if (!uptodate){
76 			SetPageError(page);
77 			if (page->mapping)
78 				set_bit(AS_EIO, &page->mapping->flags);
79 		}
80 		end_page_writeback(page);
81 	} while (bvec >= bio->bi_io_vec);
82 	bio_put(bio);
83 }
84 
85 static struct bio *mpage_bio_submit(int rw, struct bio *bio)
86 {
87 	bio->bi_end_io = mpage_end_io_read;
88 	if (rw == WRITE)
89 		bio->bi_end_io = mpage_end_io_write;
90 	submit_bio(rw, bio);
91 	return NULL;
92 }
93 
94 static struct bio *
95 mpage_alloc(struct block_device *bdev,
96 		sector_t first_sector, int nr_vecs,
97 		gfp_t gfp_flags)
98 {
99 	struct bio *bio;
100 
101 	bio = bio_alloc(gfp_flags, nr_vecs);
102 
103 	if (bio == NULL && (current->flags & PF_MEMALLOC)) {
104 		while (!bio && (nr_vecs /= 2))
105 			bio = bio_alloc(gfp_flags, nr_vecs);
106 	}
107 
108 	if (bio) {
109 		bio->bi_bdev = bdev;
110 		bio->bi_sector = first_sector;
111 	}
112 	return bio;
113 }
114 
115 /*
116  * support function for mpage_readpages.  The fs supplied get_block might
117  * return an up to date buffer.  This is used to map that buffer into
118  * the page, which allows readpage to avoid triggering a duplicate call
119  * to get_block.
120  *
121  * The idea is to avoid adding buffers to pages that don't already have
122  * them.  So when the buffer is up to date and the page size == block size,
123  * this marks the page up to date instead of adding new buffers.
124  */
125 static void
126 map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
127 {
128 	struct inode *inode = page->mapping->host;
129 	struct buffer_head *page_bh, *head;
130 	int block = 0;
131 
132 	if (!page_has_buffers(page)) {
133 		/*
134 		 * don't make any buffers if there is only one buffer on
135 		 * the page and the page just needs to be set up to date
136 		 */
137 		if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
138 		    buffer_uptodate(bh)) {
139 			SetPageUptodate(page);
140 			return;
141 		}
142 		create_empty_buffers(page, 1 << inode->i_blkbits, 0);
143 	}
144 	head = page_buffers(page);
145 	page_bh = head;
146 	do {
147 		if (block == page_block) {
148 			page_bh->b_state = bh->b_state;
149 			page_bh->b_bdev = bh->b_bdev;
150 			page_bh->b_blocknr = bh->b_blocknr;
151 			break;
152 		}
153 		page_bh = page_bh->b_this_page;
154 		block++;
155 	} while (page_bh != head);
156 }
157 
158 /*
159  * This is the worker routine which does all the work of mapping the disk
160  * blocks and constructs largest possible bios, submits them for IO if the
161  * blocks are not contiguous on the disk.
162  *
163  * We pass a buffer_head back and forth and use its buffer_mapped() flag to
164  * represent the validity of its disk mapping and to decide when to do the next
165  * get_block() call.
166  */
167 static struct bio *
168 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
169 		sector_t *last_block_in_bio, struct buffer_head *map_bh,
170 		unsigned long *first_logical_block, get_block_t get_block)
171 {
172 	struct inode *inode = page->mapping->host;
173 	const unsigned blkbits = inode->i_blkbits;
174 	const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
175 	const unsigned blocksize = 1 << blkbits;
176 	sector_t block_in_file;
177 	sector_t last_block;
178 	sector_t last_block_in_file;
179 	sector_t blocks[MAX_BUF_PER_PAGE];
180 	unsigned page_block;
181 	unsigned first_hole = blocks_per_page;
182 	struct block_device *bdev = NULL;
183 	int length;
184 	int fully_mapped = 1;
185 	unsigned nblocks;
186 	unsigned relative_block;
187 
188 	if (page_has_buffers(page))
189 		goto confused;
190 
191 	block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
192 	last_block = block_in_file + 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(map_bh) && block_in_file > *first_logical_block &&
203 			block_in_file < (*first_logical_block + nblocks)) {
204 		unsigned map_offset = block_in_file - *first_logical_block;
205 		unsigned last = nblocks - map_offset;
206 
207 		for (relative_block = 0; ; relative_block++) {
208 			if (relative_block == last) {
209 				clear_buffer_mapped(map_bh);
210 				break;
211 			}
212 			if (page_block == blocks_per_page)
213 				break;
214 			blocks[page_block] = map_bh->b_blocknr + map_offset +
215 						relative_block;
216 			page_block++;
217 			block_in_file++;
218 		}
219 		bdev = map_bh->b_bdev;
220 	}
221 
222 	/*
223 	 * Then do more get_blocks calls until we are done with this page.
224 	 */
225 	map_bh->b_page = page;
226 	while (page_block < blocks_per_page) {
227 		map_bh->b_state = 0;
228 		map_bh->b_size = 0;
229 
230 		if (block_in_file < last_block) {
231 			map_bh->b_size = (last_block-block_in_file) << blkbits;
232 			if (get_block(inode, block_in_file, map_bh, 0))
233 				goto confused;
234 			*first_logical_block = block_in_file;
235 		}
236 
237 		if (!buffer_mapped(map_bh)) {
238 			fully_mapped = 0;
239 			if (first_hole == blocks_per_page)
240 				first_hole = page_block;
241 			page_block++;
242 			block_in_file++;
243 			continue;
244 		}
245 
246 		/* some filesystems will copy data into the page during
247 		 * the get_block call, in which case we don't want to
248 		 * read it again.  map_buffer_to_page copies the data
249 		 * we just collected from get_block into the page's buffers
250 		 * so readpage doesn't have to repeat the get_block call
251 		 */
252 		if (buffer_uptodate(map_bh)) {
253 			map_buffer_to_page(page, map_bh, page_block);
254 			goto confused;
255 		}
256 
257 		if (first_hole != blocks_per_page)
258 			goto confused;		/* hole -> non-hole */
259 
260 		/* Contiguous blocks? */
261 		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
262 			goto confused;
263 		nblocks = map_bh->b_size >> blkbits;
264 		for (relative_block = 0; ; relative_block++) {
265 			if (relative_block == nblocks) {
266 				clear_buffer_mapped(map_bh);
267 				break;
268 			} else if (page_block == blocks_per_page)
269 				break;
270 			blocks[page_block] = map_bh->b_blocknr+relative_block;
271 			page_block++;
272 			block_in_file++;
273 		}
274 		bdev = map_bh->b_bdev;
275 	}
276 
277 	if (first_hole != blocks_per_page) {
278 		zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
279 		if (first_hole == 0) {
280 			SetPageUptodate(page);
281 			unlock_page(page);
282 			goto out;
283 		}
284 	} else if (fully_mapped) {
285 		SetPageMappedToDisk(page);
286 	}
287 
288 	/*
289 	 * This page will go to BIO.  Do we need to send this BIO off first?
290 	 */
291 	if (bio && (*last_block_in_bio != blocks[0] - 1))
292 		bio = mpage_bio_submit(READ, bio);
293 
294 alloc_new:
295 	if (bio == NULL) {
296 		bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
297 			  	min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
298 				GFP_KERNEL);
299 		if (bio == NULL)
300 			goto confused;
301 	}
302 
303 	length = first_hole << blkbits;
304 	if (bio_add_page(bio, page, length, 0) < length) {
305 		bio = mpage_bio_submit(READ, bio);
306 		goto alloc_new;
307 	}
308 
309 	relative_block = block_in_file - *first_logical_block;
310 	nblocks = map_bh->b_size >> blkbits;
311 	if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
312 	    (first_hole != blocks_per_page))
313 		bio = mpage_bio_submit(READ, bio);
314 	else
315 		*last_block_in_bio = blocks[blocks_per_page - 1];
316 out:
317 	return bio;
318 
319 confused:
320 	if (bio)
321 		bio = mpage_bio_submit(READ, bio);
322 	if (!PageUptodate(page))
323 	        block_read_full_page(page, get_block);
324 	else
325 		unlock_page(page);
326 	goto out;
327 }
328 
329 /**
330  * mpage_readpages - populate an address space with some pages & start reads against them
331  * @mapping: the address_space
332  * @pages: The address of a list_head which contains the target pages.  These
333  *   pages have their ->index populated and are otherwise uninitialised.
334  *   The page at @pages->prev has the lowest file offset, and reads should be
335  *   issued in @pages->prev to @pages->next order.
336  * @nr_pages: The number of pages at *@pages
337  * @get_block: The filesystem's block mapper function.
338  *
339  * This function walks the pages and the blocks within each page, building and
340  * emitting large BIOs.
341  *
342  * If anything unusual happens, such as:
343  *
344  * - encountering a page which has buffers
345  * - encountering a page which has a non-hole after a hole
346  * - encountering a page with non-contiguous blocks
347  *
348  * then this code just gives up and calls the buffer_head-based read function.
349  * It does handle a page which has holes at the end - that is a common case:
350  * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
351  *
352  * BH_Boundary explanation:
353  *
354  * There is a problem.  The mpage read code assembles several pages, gets all
355  * their disk mappings, and then submits them all.  That's fine, but obtaining
356  * the disk mappings may require I/O.  Reads of indirect blocks, for example.
357  *
358  * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
359  * submitted in the following order:
360  * 	12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
361  *
362  * because the indirect block has to be read to get the mappings of blocks
363  * 13,14,15,16.  Obviously, this impacts performance.
364  *
365  * So what we do it to allow the filesystem's get_block() function to set
366  * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
367  * after this one will require I/O against a block which is probably close to
368  * this one.  So you should push what I/O you have currently accumulated.
369  *
370  * This all causes the disk requests to be issued in the correct order.
371  */
372 int
373 mpage_readpages(struct address_space *mapping, struct list_head *pages,
374 				unsigned nr_pages, get_block_t get_block)
375 {
376 	struct bio *bio = NULL;
377 	unsigned page_idx;
378 	sector_t last_block_in_bio = 0;
379 	struct buffer_head map_bh;
380 	unsigned long first_logical_block = 0;
381 
382 	clear_buffer_mapped(&map_bh);
383 	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
384 		struct page *page = list_entry(pages->prev, struct page, lru);
385 
386 		prefetchw(&page->flags);
387 		list_del(&page->lru);
388 		if (!add_to_page_cache_lru(page, mapping,
389 					page->index, GFP_KERNEL)) {
390 			bio = do_mpage_readpage(bio, page,
391 					nr_pages - page_idx,
392 					&last_block_in_bio, &map_bh,
393 					&first_logical_block,
394 					get_block);
395 		}
396 		page_cache_release(page);
397 	}
398 	BUG_ON(!list_empty(pages));
399 	if (bio)
400 		mpage_bio_submit(READ, bio);
401 	return 0;
402 }
403 EXPORT_SYMBOL(mpage_readpages);
404 
405 /*
406  * This isn't called much at all
407  */
408 int mpage_readpage(struct page *page, get_block_t get_block)
409 {
410 	struct bio *bio = NULL;
411 	sector_t last_block_in_bio = 0;
412 	struct buffer_head map_bh;
413 	unsigned long first_logical_block = 0;
414 
415 	clear_buffer_mapped(&map_bh);
416 	bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
417 			&map_bh, &first_logical_block, get_block);
418 	if (bio)
419 		mpage_bio_submit(READ, bio);
420 	return 0;
421 }
422 EXPORT_SYMBOL(mpage_readpage);
423 
424 /*
425  * Writing is not so simple.
426  *
427  * If the page has buffers then they will be used for obtaining the disk
428  * mapping.  We only support pages which are fully mapped-and-dirty, with a
429  * special case for pages which are unmapped at the end: end-of-file.
430  *
431  * If the page has no buffers (preferred) then the page is mapped here.
432  *
433  * If all blocks are found to be contiguous then the page can go into the
434  * BIO.  Otherwise fall back to the mapping's writepage().
435  *
436  * FIXME: This code wants an estimate of how many pages are still to be
437  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
438  * just allocate full-size (16-page) BIOs.
439  */
440 
441 struct mpage_data {
442 	struct bio *bio;
443 	sector_t last_block_in_bio;
444 	get_block_t *get_block;
445 	unsigned use_writepage;
446 };
447 
448 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
449 		      void *data)
450 {
451 	struct mpage_data *mpd = data;
452 	struct bio *bio = mpd->bio;
453 	struct address_space *mapping = page->mapping;
454 	struct inode *inode = page->mapping->host;
455 	const unsigned blkbits = inode->i_blkbits;
456 	unsigned long end_index;
457 	const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
458 	sector_t last_block;
459 	sector_t block_in_file;
460 	sector_t blocks[MAX_BUF_PER_PAGE];
461 	unsigned page_block;
462 	unsigned first_unmapped = blocks_per_page;
463 	struct block_device *bdev = NULL;
464 	int boundary = 0;
465 	sector_t boundary_block = 0;
466 	struct block_device *boundary_bdev = NULL;
467 	int length;
468 	struct buffer_head map_bh;
469 	loff_t i_size = i_size_read(inode);
470 	int ret = 0;
471 
472 	if (page_has_buffers(page)) {
473 		struct buffer_head *head = page_buffers(page);
474 		struct buffer_head *bh = head;
475 
476 		/* If they're all mapped and dirty, do it */
477 		page_block = 0;
478 		do {
479 			BUG_ON(buffer_locked(bh));
480 			if (!buffer_mapped(bh)) {
481 				/*
482 				 * unmapped dirty buffers are created by
483 				 * __set_page_dirty_buffers -> mmapped data
484 				 */
485 				if (buffer_dirty(bh))
486 					goto confused;
487 				if (first_unmapped == blocks_per_page)
488 					first_unmapped = page_block;
489 				continue;
490 			}
491 
492 			if (first_unmapped != blocks_per_page)
493 				goto confused;	/* hole -> non-hole */
494 
495 			if (!buffer_dirty(bh) || !buffer_uptodate(bh))
496 				goto confused;
497 			if (page_block) {
498 				if (bh->b_blocknr != blocks[page_block-1] + 1)
499 					goto confused;
500 			}
501 			blocks[page_block++] = bh->b_blocknr;
502 			boundary = buffer_boundary(bh);
503 			if (boundary) {
504 				boundary_block = bh->b_blocknr;
505 				boundary_bdev = bh->b_bdev;
506 			}
507 			bdev = bh->b_bdev;
508 		} while ((bh = bh->b_this_page) != head);
509 
510 		if (first_unmapped)
511 			goto page_is_mapped;
512 
513 		/*
514 		 * Page has buffers, but they are all unmapped. The page was
515 		 * created by pagein or read over a hole which was handled by
516 		 * block_read_full_page().  If this address_space is also
517 		 * using mpage_readpages then this can rarely happen.
518 		 */
519 		goto confused;
520 	}
521 
522 	/*
523 	 * The page has no buffers: map it to disk
524 	 */
525 	BUG_ON(!PageUptodate(page));
526 	block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
527 	last_block = (i_size - 1) >> blkbits;
528 	map_bh.b_page = page;
529 	for (page_block = 0; page_block < blocks_per_page; ) {
530 
531 		map_bh.b_state = 0;
532 		map_bh.b_size = 1 << blkbits;
533 		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
534 			goto confused;
535 		if (buffer_new(&map_bh))
536 			unmap_underlying_metadata(map_bh.b_bdev,
537 						map_bh.b_blocknr);
538 		if (buffer_boundary(&map_bh)) {
539 			boundary_block = map_bh.b_blocknr;
540 			boundary_bdev = map_bh.b_bdev;
541 		}
542 		if (page_block) {
543 			if (map_bh.b_blocknr != blocks[page_block-1] + 1)
544 				goto confused;
545 		}
546 		blocks[page_block++] = map_bh.b_blocknr;
547 		boundary = buffer_boundary(&map_bh);
548 		bdev = map_bh.b_bdev;
549 		if (block_in_file == last_block)
550 			break;
551 		block_in_file++;
552 	}
553 	BUG_ON(page_block == 0);
554 
555 	first_unmapped = page_block;
556 
557 page_is_mapped:
558 	end_index = i_size >> PAGE_CACHE_SHIFT;
559 	if (page->index >= end_index) {
560 		/*
561 		 * The page straddles i_size.  It must be zeroed out on each
562 		 * and every writepage invokation because it may be mmapped.
563 		 * "A file is mapped in multiples of the page size.  For a file
564 		 * that is not a multiple of the page size, the remaining memory
565 		 * is zeroed when mapped, and writes to that region are not
566 		 * written out to the file."
567 		 */
568 		unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
569 
570 		if (page->index > end_index || !offset)
571 			goto confused;
572 		zero_user_segment(page, offset, PAGE_CACHE_SIZE);
573 	}
574 
575 	/*
576 	 * This page will go to BIO.  Do we need to send this BIO off first?
577 	 */
578 	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
579 		bio = mpage_bio_submit(WRITE, bio);
580 
581 alloc_new:
582 	if (bio == NULL) {
583 		bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
584 				bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
585 		if (bio == NULL)
586 			goto confused;
587 	}
588 
589 	/*
590 	 * Must try to add the page before marking the buffer clean or
591 	 * the confused fail path above (OOM) will be very confused when
592 	 * it finds all bh marked clean (i.e. it will not write anything)
593 	 */
594 	length = first_unmapped << blkbits;
595 	if (bio_add_page(bio, page, length, 0) < length) {
596 		bio = mpage_bio_submit(WRITE, bio);
597 		goto alloc_new;
598 	}
599 
600 	/*
601 	 * OK, we have our BIO, so we can now mark the buffers clean.  Make
602 	 * sure to only clean buffers which we know we'll be writing.
603 	 */
604 	if (page_has_buffers(page)) {
605 		struct buffer_head *head = page_buffers(page);
606 		struct buffer_head *bh = head;
607 		unsigned buffer_counter = 0;
608 
609 		do {
610 			if (buffer_counter++ == first_unmapped)
611 				break;
612 			clear_buffer_dirty(bh);
613 			bh = bh->b_this_page;
614 		} while (bh != head);
615 
616 		/*
617 		 * we cannot drop the bh if the page is not uptodate
618 		 * or a concurrent readpage would fail to serialize with the bh
619 		 * and it would read from disk before we reach the platter.
620 		 */
621 		if (buffer_heads_over_limit && PageUptodate(page))
622 			try_to_free_buffers(page);
623 	}
624 
625 	BUG_ON(PageWriteback(page));
626 	set_page_writeback(page);
627 	unlock_page(page);
628 	if (boundary || (first_unmapped != blocks_per_page)) {
629 		bio = mpage_bio_submit(WRITE, bio);
630 		if (boundary_block) {
631 			write_boundary_block(boundary_bdev,
632 					boundary_block, 1 << blkbits);
633 		}
634 	} else {
635 		mpd->last_block_in_bio = blocks[blocks_per_page - 1];
636 	}
637 	goto out;
638 
639 confused:
640 	if (bio)
641 		bio = mpage_bio_submit(WRITE, bio);
642 
643 	if (mpd->use_writepage) {
644 		ret = mapping->a_ops->writepage(page, wbc);
645 	} else {
646 		ret = -EAGAIN;
647 		goto out;
648 	}
649 	/*
650 	 * The caller has a ref on the inode, so *mapping is stable
651 	 */
652 	mapping_set_error(mapping, ret);
653 out:
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  *             If this is NULL then use a_ops->writepage.  Otherwise, go
664  *             direct-to-BIO.
665  *
666  * This is a library function, which implements the writepages()
667  * address_space_operation.
668  *
669  * If a page is already under I/O, generic_writepages() skips it, even
670  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
671  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
672  * and msync() need to guarantee that all the data which was dirty at the time
673  * the call was made get new I/O started against them.  If wbc->sync_mode is
674  * WB_SYNC_ALL then we were called for data integrity and we must wait for
675  * existing IO to complete.
676  */
677 int
678 mpage_writepages(struct address_space *mapping,
679 		struct writeback_control *wbc, get_block_t get_block)
680 {
681 	int ret;
682 
683 	if (!get_block)
684 		ret = generic_writepages(mapping, wbc);
685 	else {
686 		struct mpage_data mpd = {
687 			.bio = NULL,
688 			.last_block_in_bio = 0,
689 			.get_block = get_block,
690 			.use_writepage = 1,
691 		};
692 
693 		ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
694 		if (mpd.bio)
695 			mpage_bio_submit(WRITE, mpd.bio);
696 	}
697 	return ret;
698 }
699 EXPORT_SYMBOL(mpage_writepages);
700 
701 int mpage_writepage(struct page *page, get_block_t get_block,
702 	struct writeback_control *wbc)
703 {
704 	struct mpage_data mpd = {
705 		.bio = NULL,
706 		.last_block_in_bio = 0,
707 		.get_block = get_block,
708 		.use_writepage = 0,
709 	};
710 	int ret = __mpage_writepage(page, wbc, &mpd);
711 	if (mpd.bio)
712 		mpage_bio_submit(WRITE, mpd.bio);
713 	return ret;
714 }
715 EXPORT_SYMBOL(mpage_writepage);
716