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TOMOYO Linux Cross Reference
Linux/fs/mpage.c

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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 <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  */
 47 static void mpage_end_io(struct bio *bio)
 48 {
 49         struct bio_vec *bv;
 50         int i;
 51 
 52         bio_for_each_segment_all(bv, bio, i) {
 53                 struct page *page = bv->bv_page;
 54                 page_endio(page, op_is_write(bio_op(bio)),
 55                                 blk_status_to_errno(bio->bi_status));
 56         }
 57 
 58         bio_put(bio);
 59 }
 60 
 61 static 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(op, bio);
 66         submit_bio(bio);
 67         return NULL;
 68 }
 69 
 70 static struct bio *
 71 mpage_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_readpages.  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  */
103 static void 
104 map_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 
136 /*
137  * This is the worker routine which does all the work of mapping the disk
138  * blocks and constructs largest possible bios, submits them for IO if the
139  * blocks are not contiguous on the disk.
140  *
141  * We pass a buffer_head back and forth and use its buffer_mapped() flag to
142  * represent the validity of its disk mapping and to decide when to do the next
143  * get_block() call.
144  */
145 static struct bio *
146 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
147                 sector_t *last_block_in_bio, struct buffer_head *map_bh,
148                 unsigned long *first_logical_block, get_block_t get_block,
149                 gfp_t gfp)
150 {
151         struct inode *inode = page->mapping->host;
152         const unsigned blkbits = inode->i_blkbits;
153         const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
154         const unsigned blocksize = 1 << blkbits;
155         sector_t block_in_file;
156         sector_t last_block;
157         sector_t last_block_in_file;
158         sector_t blocks[MAX_BUF_PER_PAGE];
159         unsigned page_block;
160         unsigned first_hole = blocks_per_page;
161         struct block_device *bdev = NULL;
162         int length;
163         int fully_mapped = 1;
164         unsigned nblocks;
165         unsigned relative_block;
166 
167         if (page_has_buffers(page))
168                 goto confused;
169 
170         block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
171         last_block = block_in_file + nr_pages * blocks_per_page;
172         last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
173         if (last_block > last_block_in_file)
174                 last_block = last_block_in_file;
175         page_block = 0;
176 
177         /*
178          * Map blocks using the result from the previous get_blocks call first.
179          */
180         nblocks = map_bh->b_size >> blkbits;
181         if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
182                         block_in_file < (*first_logical_block + nblocks)) {
183                 unsigned map_offset = block_in_file - *first_logical_block;
184                 unsigned last = nblocks - map_offset;
185 
186                 for (relative_block = 0; ; relative_block++) {
187                         if (relative_block == last) {
188                                 clear_buffer_mapped(map_bh);
189                                 break;
190                         }
191                         if (page_block == blocks_per_page)
192                                 break;
193                         blocks[page_block] = map_bh->b_blocknr + map_offset +
194                                                 relative_block;
195                         page_block++;
196                         block_in_file++;
197                 }
198                 bdev = map_bh->b_bdev;
199         }
200 
201         /*
202          * Then do more get_blocks calls until we are done with this page.
203          */
204         map_bh->b_page = page;
205         while (page_block < blocks_per_page) {
206                 map_bh->b_state = 0;
207                 map_bh->b_size = 0;
208 
209                 if (block_in_file < last_block) {
210                         map_bh->b_size = (last_block-block_in_file) << blkbits;
211                         if (get_block(inode, block_in_file, map_bh, 0))
212                                 goto confused;
213                         *first_logical_block = block_in_file;
214                 }
215 
216                 if (!buffer_mapped(map_bh)) {
217                         fully_mapped = 0;
218                         if (first_hole == blocks_per_page)
219                                 first_hole = page_block;
220                         page_block++;
221                         block_in_file++;
222                         continue;
223                 }
224 
225                 /* some filesystems will copy data into the page during
226                  * the get_block call, in which case we don't want to
227                  * read it again.  map_buffer_to_page copies the data
228                  * we just collected from get_block into the page's buffers
229                  * so readpage doesn't have to repeat the get_block call
230                  */
231                 if (buffer_uptodate(map_bh)) {
232                         map_buffer_to_page(page, map_bh, page_block);
233                         goto confused;
234                 }
235         
236                 if (first_hole != blocks_per_page)
237                         goto confused;          /* hole -> non-hole */
238 
239                 /* Contiguous blocks? */
240                 if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
241                         goto confused;
242                 nblocks = map_bh->b_size >> blkbits;
243                 for (relative_block = 0; ; relative_block++) {
244                         if (relative_block == nblocks) {
245                                 clear_buffer_mapped(map_bh);
246                                 break;
247                         } else if (page_block == blocks_per_page)
248                                 break;
249                         blocks[page_block] = map_bh->b_blocknr+relative_block;
250                         page_block++;
251                         block_in_file++;
252                 }
253                 bdev = map_bh->b_bdev;
254         }
255 
256         if (first_hole != blocks_per_page) {
257                 zero_user_segment(page, first_hole << blkbits, PAGE_SIZE);
258                 if (first_hole == 0) {
259                         SetPageUptodate(page);
260                         unlock_page(page);
261                         goto out;
262                 }
263         } else if (fully_mapped) {
264                 SetPageMappedToDisk(page);
265         }
266 
267         if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
268             cleancache_get_page(page) == 0) {
269                 SetPageUptodate(page);
270                 goto confused;
271         }
272 
273         /*
274          * This page will go to BIO.  Do we need to send this BIO off first?
275          */
276         if (bio && (*last_block_in_bio != blocks[0] - 1))
277                 bio = mpage_bio_submit(REQ_OP_READ, 0, bio);
278 
279 alloc_new:
280         if (bio == NULL) {
281                 if (first_hole == blocks_per_page) {
282                         if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
283                                                                 page))
284                                 goto out;
285                 }
286                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
287                                 min_t(int, nr_pages, BIO_MAX_PAGES), gfp);
288                 if (bio == NULL)
289                         goto confused;
290         }
291 
292         length = first_hole << blkbits;
293         if (bio_add_page(bio, page, length, 0) < length) {
294                 bio = mpage_bio_submit(REQ_OP_READ, 0, bio);
295                 goto alloc_new;
296         }
297 
298         relative_block = block_in_file - *first_logical_block;
299         nblocks = map_bh->b_size >> blkbits;
300         if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
301             (first_hole != blocks_per_page))
302                 bio = mpage_bio_submit(REQ_OP_READ, 0, bio);
303         else
304                 *last_block_in_bio = blocks[blocks_per_page - 1];
305 out:
306         return bio;
307 
308 confused:
309         if (bio)
310                 bio = mpage_bio_submit(REQ_OP_READ, 0, bio);
311         if (!PageUptodate(page))
312                 block_read_full_page(page, get_block);
313         else
314                 unlock_page(page);
315         goto out;
316 }
317 
318 /**
319  * mpage_readpages - populate an address space with some pages & start reads against them
320  * @mapping: the address_space
321  * @pages: The address of a list_head which contains the target pages.  These
322  *   pages have their ->index populated and are otherwise uninitialised.
323  *   The page at @pages->prev has the lowest file offset, and reads should be
324  *   issued in @pages->prev to @pages->next order.
325  * @nr_pages: The number of pages at *@pages
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  */
362 int
363 mpage_readpages(struct address_space *mapping, struct list_head *pages,
364                                 unsigned nr_pages, get_block_t get_block)
365 {
366         struct bio *bio = NULL;
367         unsigned page_idx;
368         sector_t last_block_in_bio = 0;
369         struct buffer_head map_bh;
370         unsigned long first_logical_block = 0;
371         gfp_t gfp = readahead_gfp_mask(mapping);
372 
373         map_bh.b_state = 0;
374         map_bh.b_size = 0;
375         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
376                 struct page *page = lru_to_page(pages);
377 
378                 prefetchw(&page->flags);
379                 list_del(&page->lru);
380                 if (!add_to_page_cache_lru(page, mapping,
381                                         page->index,
382                                         gfp)) {
383                         bio = do_mpage_readpage(bio, page,
384                                         nr_pages - page_idx,
385                                         &last_block_in_bio, &map_bh,
386                                         &first_logical_block,
387                                         get_block, gfp);
388                 }
389                 put_page(page);
390         }
391         BUG_ON(!list_empty(pages));
392         if (bio)
393                 mpage_bio_submit(REQ_OP_READ, 0, bio);
394         return 0;
395 }
396 EXPORT_SYMBOL(mpage_readpages);
397 
398 /*
399  * This isn't called much at all
400  */
401 int mpage_readpage(struct page *page, get_block_t get_block)
402 {
403         struct bio *bio = NULL;
404         sector_t last_block_in_bio = 0;
405         struct buffer_head map_bh;
406         unsigned long first_logical_block = 0;
407         gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
408 
409         map_bh.b_state = 0;
410         map_bh.b_size = 0;
411         bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
412                         &map_bh, &first_logical_block, get_block, gfp);
413         if (bio)
414                 mpage_bio_submit(REQ_OP_READ, 0, bio);
415         return 0;
416 }
417 EXPORT_SYMBOL(mpage_readpage);
418 
419 /*
420  * Writing is not so simple.
421  *
422  * If the page has buffers then they will be used for obtaining the disk
423  * mapping.  We only support pages which are fully mapped-and-dirty, with a
424  * special case for pages which are unmapped at the end: end-of-file.
425  *
426  * If the page has no buffers (preferred) then the page is mapped here.
427  *
428  * If all blocks are found to be contiguous then the page can go into the
429  * BIO.  Otherwise fall back to the mapping's writepage().
430  * 
431  * FIXME: This code wants an estimate of how many pages are still to be
432  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
433  * just allocate full-size (16-page) BIOs.
434  */
435 
436 struct mpage_data {
437         struct bio *bio;
438         sector_t last_block_in_bio;
439         get_block_t *get_block;
440         unsigned use_writepage;
441 };
442 
443 /*
444  * We have our BIO, so we can now mark the buffers clean.  Make
445  * sure to only clean buffers which we know we'll be writing.
446  */
447 static void clean_buffers(struct page *page, unsigned first_unmapped)
448 {
449         unsigned buffer_counter = 0;
450         struct buffer_head *bh, *head;
451         if (!page_has_buffers(page))
452                 return;
453         head = page_buffers(page);
454         bh = head;
455 
456         do {
457                 if (buffer_counter++ == first_unmapped)
458                         break;
459                 clear_buffer_dirty(bh);
460                 bh = bh->b_this_page;
461         } while (bh != head);
462 
463         /*
464          * we cannot drop the bh if the page is not uptodate or a concurrent
465          * readpage would fail to serialize with the bh and it would read from
466          * disk before we reach the platter.
467          */
468         if (buffer_heads_over_limit && PageUptodate(page))
469                 try_to_free_buffers(page);
470 }
471 
472 /*
473  * For situations where we want to clean all buffers attached to a page.
474  * We don't need to calculate how many buffers are attached to the page,
475  * we just need to specify a number larger than the maximum number of buffers.
476  */
477 void clean_page_buffers(struct page *page)
478 {
479         clean_buffers(page, ~0U);
480 }
481 
482 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
483                       void *data)
484 {
485         struct mpage_data *mpd = data;
486         struct bio *bio = mpd->bio;
487         struct address_space *mapping = page->mapping;
488         struct inode *inode = page->mapping->host;
489         const unsigned blkbits = inode->i_blkbits;
490         unsigned long end_index;
491         const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
492         sector_t last_block;
493         sector_t block_in_file;
494         sector_t blocks[MAX_BUF_PER_PAGE];
495         unsigned page_block;
496         unsigned first_unmapped = blocks_per_page;
497         struct block_device *bdev = NULL;
498         int boundary = 0;
499         sector_t boundary_block = 0;
500         struct block_device *boundary_bdev = NULL;
501         int length;
502         struct buffer_head map_bh;
503         loff_t i_size = i_size_read(inode);
504         int ret = 0;
505         int op_flags = wbc_to_write_flags(wbc);
506 
507         if (page_has_buffers(page)) {
508                 struct buffer_head *head = page_buffers(page);
509                 struct buffer_head *bh = head;
510 
511                 /* If they're all mapped and dirty, do it */
512                 page_block = 0;
513                 do {
514                         BUG_ON(buffer_locked(bh));
515                         if (!buffer_mapped(bh)) {
516                                 /*
517                                  * unmapped dirty buffers are created by
518                                  * __set_page_dirty_buffers -> mmapped data
519                                  */
520                                 if (buffer_dirty(bh))
521                                         goto confused;
522                                 if (first_unmapped == blocks_per_page)
523                                         first_unmapped = page_block;
524                                 continue;
525                         }
526 
527                         if (first_unmapped != blocks_per_page)
528                                 goto confused;  /* hole -> non-hole */
529 
530                         if (!buffer_dirty(bh) || !buffer_uptodate(bh))
531                                 goto confused;
532                         if (page_block) {
533                                 if (bh->b_blocknr != blocks[page_block-1] + 1)
534                                         goto confused;
535                         }
536                         blocks[page_block++] = bh->b_blocknr;
537                         boundary = buffer_boundary(bh);
538                         if (boundary) {
539                                 boundary_block = bh->b_blocknr;
540                                 boundary_bdev = bh->b_bdev;
541                         }
542                         bdev = bh->b_bdev;
543                 } while ((bh = bh->b_this_page) != head);
544 
545                 if (first_unmapped)
546                         goto page_is_mapped;
547 
548                 /*
549                  * Page has buffers, but they are all unmapped. The page was
550                  * created by pagein or read over a hole which was handled by
551                  * block_read_full_page().  If this address_space is also
552                  * using mpage_readpages then this can rarely happen.
553                  */
554                 goto confused;
555         }
556 
557         /*
558          * The page has no buffers: map it to disk
559          */
560         BUG_ON(!PageUptodate(page));
561         block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
562         last_block = (i_size - 1) >> blkbits;
563         map_bh.b_page = page;
564         for (page_block = 0; page_block < blocks_per_page; ) {
565 
566                 map_bh.b_state = 0;
567                 map_bh.b_size = 1 << blkbits;
568                 if (mpd->get_block(inode, block_in_file, &map_bh, 1))
569                         goto confused;
570                 if (buffer_new(&map_bh))
571                         clean_bdev_bh_alias(&map_bh);
572                 if (buffer_boundary(&map_bh)) {
573                         boundary_block = map_bh.b_blocknr;
574                         boundary_bdev = map_bh.b_bdev;
575                 }
576                 if (page_block) {
577                         if (map_bh.b_blocknr != blocks[page_block-1] + 1)
578                                 goto confused;
579                 }
580                 blocks[page_block++] = map_bh.b_blocknr;
581                 boundary = buffer_boundary(&map_bh);
582                 bdev = map_bh.b_bdev;
583                 if (block_in_file == last_block)
584                         break;
585                 block_in_file++;
586         }
587         BUG_ON(page_block == 0);
588 
589         first_unmapped = page_block;
590 
591 page_is_mapped:
592         end_index = i_size >> PAGE_SHIFT;
593         if (page->index >= end_index) {
594                 /*
595                  * The page straddles i_size.  It must be zeroed out on each
596                  * and every writepage invocation because it may be mmapped.
597                  * "A file is mapped in multiples of the page size.  For a file
598                  * that is not a multiple of the page size, the remaining memory
599                  * is zeroed when mapped, and writes to that region are not
600                  * written out to the file."
601                  */
602                 unsigned offset = i_size & (PAGE_SIZE - 1);
603 
604                 if (page->index > end_index || !offset)
605                         goto confused;
606                 zero_user_segment(page, offset, PAGE_SIZE);
607         }
608 
609         /*
610          * This page will go to BIO.  Do we need to send this BIO off first?
611          */
612         if (bio && mpd->last_block_in_bio != blocks[0] - 1)
613                 bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
614 
615 alloc_new:
616         if (bio == NULL) {
617                 if (first_unmapped == blocks_per_page) {
618                         if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
619                                                                 page, wbc))
620                                 goto out;
621                 }
622                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
623                                 BIO_MAX_PAGES, GFP_NOFS|__GFP_HIGH);
624                 if (bio == NULL)
625                         goto confused;
626 
627                 wbc_init_bio(wbc, bio);
628                 bio->bi_write_hint = inode->i_write_hint;
629         }
630 
631         /*
632          * Must try to add the page before marking the buffer clean or
633          * the confused fail path above (OOM) will be very confused when
634          * it finds all bh marked clean (i.e. it will not write anything)
635          */
636         wbc_account_io(wbc, page, PAGE_SIZE);
637         length = first_unmapped << blkbits;
638         if (bio_add_page(bio, page, length, 0) < length) {
639                 bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
640                 goto alloc_new;
641         }
642 
643         clean_buffers(page, first_unmapped);
644 
645         BUG_ON(PageWriteback(page));
646         set_page_writeback(page);
647         unlock_page(page);
648         if (boundary || (first_unmapped != blocks_per_page)) {
649                 bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
650                 if (boundary_block) {
651                         write_boundary_block(boundary_bdev,
652                                         boundary_block, 1 << blkbits);
653                 }
654         } else {
655                 mpd->last_block_in_bio = blocks[blocks_per_page - 1];
656         }
657         goto out;
658 
659 confused:
660         if (bio)
661                 bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
662 
663         if (mpd->use_writepage) {
664                 ret = mapping->a_ops->writepage(page, wbc);
665         } else {
666                 ret = -EAGAIN;
667                 goto out;
668         }
669         /*
670          * The caller has a ref on the inode, so *mapping is stable
671          */
672         mapping_set_error(mapping, ret);
673 out:
674         mpd->bio = bio;
675         return ret;
676 }
677 
678 /**
679  * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
680  * @mapping: address space structure to write
681  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
682  * @get_block: the filesystem's block mapper function.
683  *             If this is NULL then use a_ops->writepage.  Otherwise, go
684  *             direct-to-BIO.
685  *
686  * This is a library function, which implements the writepages()
687  * address_space_operation.
688  *
689  * If a page is already under I/O, generic_writepages() skips it, even
690  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
691  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
692  * and msync() need to guarantee that all the data which was dirty at the time
693  * the call was made get new I/O started against them.  If wbc->sync_mode is
694  * WB_SYNC_ALL then we were called for data integrity and we must wait for
695  * existing IO to complete.
696  */
697 int
698 mpage_writepages(struct address_space *mapping,
699                 struct writeback_control *wbc, get_block_t get_block)
700 {
701         struct blk_plug plug;
702         int ret;
703 
704         blk_start_plug(&plug);
705 
706         if (!get_block)
707                 ret = generic_writepages(mapping, wbc);
708         else {
709                 struct mpage_data mpd = {
710                         .bio = NULL,
711                         .last_block_in_bio = 0,
712                         .get_block = get_block,
713                         .use_writepage = 1,
714                 };
715 
716                 ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
717                 if (mpd.bio) {
718                         int op_flags = (wbc->sync_mode == WB_SYNC_ALL ?
719                                   REQ_SYNC : 0);
720                         mpage_bio_submit(REQ_OP_WRITE, op_flags, mpd.bio);
721                 }
722         }
723         blk_finish_plug(&plug);
724         return ret;
725 }
726 EXPORT_SYMBOL(mpage_writepages);
727 
728 int mpage_writepage(struct page *page, get_block_t get_block,
729         struct writeback_control *wbc)
730 {
731         struct mpage_data mpd = {
732                 .bio = NULL,
733                 .last_block_in_bio = 0,
734                 .get_block = get_block,
735                 .use_writepage = 0,
736         };
737         int ret = __mpage_writepage(page, wbc, &mpd);
738         if (mpd.bio) {
739                 int op_flags = (wbc->sync_mode == WB_SYNC_ALL ?
740                           REQ_SYNC : 0);
741                 mpage_bio_submit(REQ_OP_WRITE, op_flags, mpd.bio);
742         }
743         return ret;
744 }
745 EXPORT_SYMBOL(mpage_writepage);
746 

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