fs/mpage.c

   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/export.h>
  17 #include <linux/mm.h>
  18 #include <linux/kdev_t.h>
  19 #include <linux/gfp.h>
  20 #include <linux/bio.h>
  21 #include <linux/fs.h>
  22 #include <linux/buffer_head.h>
  23 #include <linux/blkdev.h>
  24 #include <linux/highmem.h>
  25 #include <linux/prefetch.h>
  26 #include <linux/mpage.h>
  27 #include <linux/writeback.h>
  28 #include <linux/backing-dev.h>
  29 #include <linux/pagevec.h>
  30 #include <linux/cleancache.h>
  31 #include "internal.h"
  32
  33 /*
  34  * I/O completion handler for multipage BIOs.
  35  *
  36  * The mpage code never puts partial pages into a BIO (except for end-of-file).
  37  * If a page does not map to a contiguous run of blocks then it simply falls
  38  * back to block_read_full_page().
  39  *
  40  * Why is this?  If a page's completion depends on a number of different BIOs
  41  * which can complete in any order (or at the same time) then determining the
  42  * status of that page is hard.  See end_buffer_async_read() for the details.
  43  * There is no point in duplicating all that complexity.
  44  */
  45 static void mpage_end_io(struct bio *bio, int err)
  46 {
  47         struct bio_vec *bv;
  48         int i;
  49
  50         bio_for_each_segment_all(bv, bio, i) {
  51                 struct page *page = bv->bv_page;
  52                 page_endio(page, bio_data_dir(bio), err);
  53         }
  54
  55         bio_put(bio);
  56 }
  57
  58 static struct bio *mpage_bio_submit(int rw, struct bio *bio)
  59 {
  60         bio->bi_end_io = mpage_end_io;
  61         guard_bio_eod(rw, bio);
  62         submit_bio(rw, bio);
  63         return NULL;
  64 }
  65
  66 static struct bio *
  67 mpage_alloc(struct block_device *bdev,
  68                 sector_t first_sector, int nr_vecs,
  69                 gfp_t gfp_flags)
  70 {
  71         struct bio *bio;
  72
  73         bio = bio_alloc(gfp_flags, nr_vecs);
  74
  75         if (bio == NULL && (current->flags & PF_MEMALLOC)) {
  76                 while (!bio && (nr_vecs /= 2))
  77                         bio = bio_alloc(gfp_flags, nr_vecs);
  78         }
  79
  80         if (bio) {
  81                 bio->bi_bdev = bdev;
  82                 bio->bi_iter.bi_sector = first_sector;
  83         }
  84         return bio;
  85 }
  86
  87 /*
  88  * support function for mpage_readpages.  The fs supplied get_block might
  89  * return an up to date buffer.  This is used to map that buffer into
  90  * the page, which allows readpage to avoid triggering a duplicate call
  91  * to get_block.
  92  *
  93  * The idea is to avoid adding buffers to pages that don't already have
  94  * them.  So when the buffer is up to date and the page size == block size,
  95  * this marks the page up to date instead of adding new buffers.
  96  */
  97 static void
  98 map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
  99 {
 100         struct inode *inode = page->mapping->host;
 101         struct buffer_head *page_bh, *head;
 102         int block = 0;
 103
 104         if (!page_has_buffers(page)) {
 105                 /*
 106                  * don't make any buffers if there is only one buffer on
 107                  * the page and the page just needs to be set up to date
 108                  */
 109                 if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
 110                     buffer_uptodate(bh)) {
 111                         SetPageUptodate(page);
 112                         return;
 113                 }
 114                 create_empty_buffers(page, 1 << inode->i_blkbits, 0);
 115         }
 116         head = page_buffers(page);
 117         page_bh = head;
 118         do {
 119                 if (block == page_block) {
 120                         page_bh->b_state = bh->b_state;
 121                         page_bh->b_bdev = bh->b_bdev;
 122                         page_bh->b_blocknr = bh->b_blocknr;
 123                         break;
 124                 }
 125                 page_bh = page_bh->b_this_page;
 126                 block++;
 127         } while (page_bh != head);
 128 }
 129
 130 /*
 131  * This is the worker routine which does all the work of mapping the disk
 132  * blocks and constructs largest possible bios, submits them for IO if the
 133  * blocks are not contiguous on the disk.
 134  *
 135  * We pass a buffer_head back and forth and use its buffer_mapped() flag to
 136  * represent the validity of its disk mapping and to decide when to do the next
 137  * get_block() call.
 138  */
 139 static struct bio *
 140 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 141                 sector_t *last_block_in_bio, struct buffer_head *map_bh,
 142                 unsigned long *first_logical_block, get_block_t get_block)
 143 {
 144         struct inode *inode = page->mapping->host;
 145         const unsigned blkbits = inode->i_blkbits;
 146         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 147         const unsigned blocksize = 1 << blkbits;
 148         sector_t block_in_file;
 149         sector_t last_block;
 150         sector_t last_block_in_file;
 151         sector_t blocks[MAX_BUF_PER_PAGE];
 152         unsigned page_block;
 153         unsigned first_hole = blocks_per_page;
 154         struct block_device *bdev = NULL;
 155         int length;
 156         int fully_mapped = 1;
 157         unsigned nblocks;
 158         unsigned relative_block;
 159
 160         if (page_has_buffers(page))
 161                 goto confused;
 162
 163         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 164         last_block = block_in_file + nr_pages * blocks_per_page;
 165         last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
 166         if (last_block > last_block_in_file)
 167                 last_block = last_block_in_file;
 168         page_block = 0;
 169
 170         /*
 171          * Map blocks using the result from the previous get_blocks call first.
 172          */
 173         nblocks = map_bh->b_size >> blkbits;
 174         if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
 175                         block_in_file < (*first_logical_block + nblocks)) {
 176                 unsigned map_offset = block_in_file - *first_logical_block;
 177                 unsigned last = nblocks - map_offset;
 178
 179                 for (relative_block = 0; ; relative_block++) {
 180                         if (relative_block == last) {
 181                                 clear_buffer_mapped(map_bh);
 182                                 break;
 183                         }
 184                         if (page_block == blocks_per_page)
 185                                 break;
 186                         blocks[page_block] = map_bh->b_blocknr + map_offset +
 187                                                 relative_block;
 188                         page_block++;
 189                         block_in_file++;
 190                 }
 191                 bdev = map_bh->b_bdev;
 192         }
 193
 194         /*
 195          * Then do more get_blocks calls until we are done with this page.
 196          */
 197         map_bh->b_page = page;
 198         while (page_block < blocks_per_page) {
 199                 map_bh->b_state = 0;
 200                 map_bh->b_size = 0;
 201
 202                 if (block_in_file < last_block) {
 203                         map_bh->b_size = (last_block-block_in_file) << blkbits;
 204                         if (get_block(inode, block_in_file, map_bh, 0))
 205                                 goto confused;
 206                         *first_logical_block = block_in_file;
 207                 }
 208
 209                 if (!buffer_mapped(map_bh)) {
 210                         fully_mapped = 0;
 211                         if (first_hole == blocks_per_page)
 212                                 first_hole = page_block;
 213                         page_block++;
 214                         block_in_file++;
 215                         continue;
 216                 }
 217
 218                 /* some filesystems will copy data into the page during
 219                  * the get_block call, in which case we don't want to
 220                  * read it again.  map_buffer_to_page copies the data
 221                  * we just collected from get_block into the page's buffers
 222                  * so readpage doesn't have to repeat the get_block call
 223                  */
 224                 if (buffer_uptodate(map_bh)) {
 225                         map_buffer_to_page(page, map_bh, page_block);
 226                         goto confused;
 227                 }
 228
 229                 if (first_hole != blocks_per_page)
 230                         goto confused;          /* hole -> non-hole */
 231
 232                 /* Contiguous blocks? */
 233                 if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
 234                         goto confused;
 235                 nblocks = map_bh->b_size >> blkbits;
 236                 for (relative_block = 0; ; relative_block++) {
 237                         if (relative_block == nblocks) {
 238                                 clear_buffer_mapped(map_bh);
 239                                 break;
 240                         } else if (page_block == blocks_per_page)
 241                                 break;
 242                         blocks[page_block] = map_bh->b_blocknr+relative_block;
 243                         page_block++;
 244                         block_in_file++;
 245                 }
 246                 bdev = map_bh->b_bdev;
 247         }
 248
 249         if (first_hole != blocks_per_page) {
 250                 zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
 251                 if (first_hole == 0) {
 252                         SetPageUptodate(page);
 253                         unlock_page(page);
 254                         goto out;
 255                 }
 256         } else if (fully_mapped) {
 257                 SetPageMappedToDisk(page);
 258         }
 259
 260         if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
 261             cleancache_get_page(page) == 0) {
 262                 SetPageUptodate(page);
 263                 goto confused;
 264         }
 265
 266         /*
 267          * This page will go to BIO.  Do we need to send this BIO off first?
 268          */
 269         if (bio && (*last_block_in_bio != blocks[0] - 1))
 270                 bio = mpage_bio_submit(READ, bio);
 271
 272 alloc_new:
 273         if (bio == NULL) {
 274                 if (first_hole == blocks_per_page) {
 275                         if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
 276                                                                 page))
 277                                 goto out;
 278                 }
 279                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 280                                 min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
 281                                 GFP_KERNEL);
 282                 if (bio == NULL)
 283                         goto confused;
 284         }
 285
 286         length = first_hole << blkbits;
 287         if (bio_add_page(bio, page, length, 0) < length) {
 288                 bio = mpage_bio_submit(READ, bio);
 289                 goto alloc_new;
 290         }
 291
 292         relative_block = block_in_file - *first_logical_block;
 293         nblocks = map_bh->b_size >> blkbits;
 294         if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
 295             (first_hole != blocks_per_page))
 296                 bio = mpage_bio_submit(READ, bio);
 297         else
 298                 *last_block_in_bio = blocks[blocks_per_page - 1];
 299 out:
 300         return bio;
 301
 302 confused:
 303         if (bio)
 304                 bio = mpage_bio_submit(READ, bio);
 305         if (!PageUptodate(page))
 306                 block_read_full_page(page, get_block);
 307         else
 308                 unlock_page(page);
 309         goto out;
 310 }
 311
 312 /**
 313  * mpage_readpages - populate an address space with some pages & start reads against them
 314  * @mapping: the address_space
 315  * @pages: The address of a list_head which contains the target pages.  These
 316  *   pages have their ->index populated and are otherwise uninitialised.
 317  *   The page at @pages->prev has the lowest file offset, and reads should be
 318  *   issued in @pages->prev to @pages->next order.
 319  * @nr_pages: The number of pages at *@pages
 320  * @get_block: The filesystem's block mapper function.
 321  *
 322  * This function walks the pages and the blocks within each page, building and
 323  * emitting large BIOs.
 324  *
 325  * If anything unusual happens, such as:
 326  *
 327  * - encountering a page which has buffers
 328  * - encountering a page which has a non-hole after a hole
 329  * - encountering a page with non-contiguous blocks
 330  *
 331  * then this code just gives up and calls the buffer_head-based read function.
 332  * It does handle a page which has holes at the end - that is a common case:
 333  * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
 334  *
 335  * BH_Boundary explanation:
 336  *
 337  * There is a problem.  The mpage read code assembles several pages, gets all
 338  * their disk mappings, and then submits them all.  That's fine, but obtaining
 339  * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 340  *
 341  * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 342  * submitted in the following order:
 343  *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 344  *
 345  * because the indirect block has to be read to get the mappings of blocks
 346  * 13,14,15,16.  Obviously, this impacts performance.
 347  *
 348  * So what we do it to allow the filesystem's get_block() function to set
 349  * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 350  * after this one will require I/O against a block which is probably close to
 351  * this one.  So you should push what I/O you have currently accumulated.
 352  *
 353  * This all causes the disk requests to be issued in the correct order.
 354  */
 355 int
 356 mpage_readpages(struct address_space *mapping, struct list_head *pages,
 357                                 unsigned nr_pages, get_block_t get_block)
 358 {
 359         struct bio *bio = NULL;
 360         unsigned page_idx;
 361         sector_t last_block_in_bio = 0;
 362         struct buffer_head map_bh;
 363         unsigned long first_logical_block = 0;
 364
 365         map_bh.b_state = 0;
 366         map_bh.b_size = 0;
 367         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 368                 struct page *page = list_entry(pages->prev, struct page, lru);
 369
 370                 prefetchw(&page->flags);
 371                 list_del(&page->lru);
 372                 if (!add_to_page_cache_lru(page, mapping,
 373                                         page->index, GFP_KERNEL)) {
 374                         bio = do_mpage_readpage(bio, page,
 375                                         nr_pages - page_idx,
 376                                         &last_block_in_bio, &map_bh,
 377                                         &first_logical_block,
 378                                         get_block);
 379                 }
 380                 page_cache_release(page);
 381         }
 382         BUG_ON(!list_empty(pages));
 383         if (bio)
 384                 mpage_bio_submit(READ, bio);
 385         return 0;
 386 }
 387 EXPORT_SYMBOL(mpage_readpages);
 388
 389 /*
 390  * This isn't called much at all
 391  */
 392 int mpage_readpage(struct page *page, get_block_t get_block)
 393 {
 394         struct bio *bio = NULL;
 395         sector_t last_block_in_bio = 0;
 396         struct buffer_head map_bh;
 397         unsigned long first_logical_block = 0;
 398
 399         map_bh.b_state = 0;
 400         map_bh.b_size = 0;
 401         bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
 402                         &map_bh, &first_logical_block, get_block);
 403         if (bio)
 404                 mpage_bio_submit(READ, bio);
 405         return 0;
 406 }
 407 EXPORT_SYMBOL(mpage_readpage);
 408
 409 /*
 410  * Writing is not so simple.
 411  *
 412  * If the page has buffers then they will be used for obtaining the disk
 413  * mapping.  We only support pages which are fully mapped-and-dirty, with a
 414  * special case for pages which are unmapped at the end: end-of-file.
 415  *
 416  * If the page has no buffers (preferred) then the page is mapped here.
 417  *
 418  * If all blocks are found to be contiguous then the page can go into the
 419  * BIO.  Otherwise fall back to the mapping's writepage().
 420  *
 421  * FIXME: This code wants an estimate of how many pages are still to be
 422  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 423  * just allocate full-size (16-page) BIOs.
 424  */
 425
 426 struct mpage_data {
 427         struct bio *bio;
 428         sector_t last_block_in_bio;
 429         get_block_t *get_block;
 430         unsigned use_writepage;
 431 };
 432
 433 /*
 434  * We have our BIO, so we can now mark the buffers clean.  Make
 435  * sure to only clean buffers which we know we'll be writing.
 436  */
 437 static void clean_buffers(struct page *page, unsigned first_unmapped)
 438 {
 439         unsigned buffer_counter = 0;
 440         struct buffer_head *bh, *head;
 441         if (!page_has_buffers(page))
 442                 return;
 443         head = page_buffers(page);
 444         bh = head;
 445
 446         do {
 447                 if (buffer_counter++ == first_unmapped)
 448                         break;
 449                 clear_buffer_dirty(bh);
 450                 bh = bh->b_this_page;
 451         } while (bh != head);
 452
 453         /*
 454          * we cannot drop the bh if the page is not uptodate or a concurrent
 455          * readpage would fail to serialize with the bh and it would read from
 456          * disk before we reach the platter.
 457          */
 458         if (buffer_heads_over_limit && PageUptodate(page))
 459                 try_to_free_buffers(page);
 460 }
 461
 462 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 463                       void *data)
 464 {
 465         struct mpage_data *mpd = data;
 466         struct bio *bio = mpd->bio;
 467         struct address_space *mapping = page->mapping;
 468         struct inode *inode = page->mapping->host;
 469         const unsigned blkbits = inode->i_blkbits;
 470         unsigned long end_index;
 471         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 472         sector_t last_block;
 473         sector_t block_in_file;
 474         sector_t blocks[MAX_BUF_PER_PAGE];
 475         unsigned page_block;
 476         unsigned first_unmapped = blocks_per_page;
 477         struct block_device *bdev = NULL;
 478         int boundary = 0;
 479         sector_t boundary_block = 0;
 480         struct block_device *boundary_bdev = NULL;
 481         int length;
 482         struct buffer_head map_bh;
 483         loff_t i_size = i_size_read(inode);
 484         int ret = 0;
 485
 486         if (page_has_buffers(page)) {
 487                 struct buffer_head *head = page_buffers(page);
 488                 struct buffer_head *bh = head;
 489
 490                 /* If they're all mapped and dirty, do it */
 491                 page_block = 0;
 492                 do {
 493                         BUG_ON(buffer_locked(bh));
 494                         if (!buffer_mapped(bh)) {
 495                                 /*
 496                                  * unmapped dirty buffers are created by
 497                                  * __set_page_dirty_buffers -> mmapped data
 498                                  */
 499                                 if (buffer_dirty(bh))
 500                                         goto confused;
 501                                 if (first_unmapped == blocks_per_page)
 502                                         first_unmapped = page_block;
 503                                 continue;
 504                         }
 505
 506                         if (first_unmapped != blocks_per_page)
 507                                 goto confused;  /* hole -> non-hole */
 508
 509                         if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 510                                 goto confused;
 511                         if (page_block) {
 512                                 if (bh->b_blocknr != blocks[page_block-1] + 1)
 513                                         goto confused;
 514                         }
 515                         blocks[page_block++] = bh->b_blocknr;
 516                         boundary = buffer_boundary(bh);
 517                         if (boundary) {
 518                                 boundary_block = bh->b_blocknr;
 519                                 boundary_bdev = bh->b_bdev;
 520                         }
 521                         bdev = bh->b_bdev;
 522                 } while ((bh = bh->b_this_page) != head);
 523
 524                 if (first_unmapped)
 525                         goto page_is_mapped;
 526
 527                 /*
 528                  * Page has buffers, but they are all unmapped. The page was
 529                  * created by pagein or read over a hole which was handled by
 530                  * block_read_full_page().  If this address_space is also
 531                  * using mpage_readpages then this can rarely happen.
 532                  */
 533                 goto confused;
 534         }
 535
 536         /*
 537          * The page has no buffers: map it to disk
 538          */
 539         BUG_ON(!PageUptodate(page));
 540         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 541         last_block = (i_size - 1) >> blkbits;
 542         map_bh.b_page = page;
 543         for (page_block = 0; page_block < blocks_per_page; ) {
 544
 545                 map_bh.b_state = 0;
 546                 map_bh.b_size = 1 << blkbits;
 547                 if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 548                         goto confused;
 549                 if (buffer_new(&map_bh))
 550                         unmap_underlying_metadata(map_bh.b_bdev,
 551                                                 map_bh.b_blocknr);
 552                 if (buffer_boundary(&map_bh)) {
 553                         boundary_block = map_bh.b_blocknr;
 554                         boundary_bdev = map_bh.b_bdev;
 555                 }
 556                 if (page_block) {
 557                         if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 558                                 goto confused;
 559                 }
 560                 blocks[page_block++] = map_bh.b_blocknr;
 561                 boundary = buffer_boundary(&map_bh);
 562                 bdev = map_bh.b_bdev;
 563                 if (block_in_file == last_block)
 564                         break;
 565                 block_in_file++;
 566         }
 567         BUG_ON(page_block == 0);
 568
 569         first_unmapped = page_block;
 570
 571 page_is_mapped:
 572         end_index = i_size >> PAGE_CACHE_SHIFT;
 573         if (page->index >= end_index) {
 574                 /*
 575                  * The page straddles i_size.  It must be zeroed out on each
 576                  * and every writepage invocation because it may be mmapped.
 577                  * "A file is mapped in multiples of the page size.  For a file
 578                  * that is not a multiple of the page size, the remaining memory
 579                  * is zeroed when mapped, and writes to that region are not
 580                  * written out to the file."
 581                  */
 582                 unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
 583
 584                 if (page->index > end_index || !offset)
 585                         goto confused;
 586                 zero_user_segment(page, offset, PAGE_CACHE_SIZE);
 587         }
 588
 589         /*
 590          * This page will go to BIO.  Do we need to send this BIO off first?
 591          */
 592         if (bio && mpd->last_block_in_bio != blocks[0] - 1)
 593                 bio = mpage_bio_submit(WRITE, bio);
 594
 595 alloc_new:
 596         if (bio == NULL) {
 597                 if (first_unmapped == blocks_per_page) {
 598                         if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
 599                                                                 page, wbc)) {
 600                                 clean_buffers(page, first_unmapped);
 601                                 goto out;
 602                         }
 603                 }
 604                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 605                                 bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
 606                 if (bio == NULL)
 607                         goto confused;
 608
 609                 wbc_init_bio(wbc, bio);
 610         }
 611
 612         /*
 613          * Must try to add the page before marking the buffer clean or
 614          * the confused fail path above (OOM) will be very confused when
 615          * it finds all bh marked clean (i.e. it will not write anything)
 616          */
 617         wbc_account_io(wbc, page, PAGE_SIZE);
 618         length = first_unmapped << blkbits;
 619         if (bio_add_page(bio, page, length, 0) < length) {
 620                 bio = mpage_bio_submit(WRITE, bio);
 621                 goto alloc_new;
 622         }
 623
 624         clean_buffers(page, first_unmapped);
 625
 626         BUG_ON(PageWriteback(page));
 627         set_page_writeback(page);
 628         unlock_page(page);
 629         if (boundary || (first_unmapped != blocks_per_page)) {
 630                 bio = mpage_bio_submit(WRITE, bio);
 631                 if (boundary_block) {
 632                         write_boundary_block(boundary_bdev,
 633                                         boundary_block, 1 << blkbits);
 634                 }
 635         } else {
 636                 mpd->last_block_in_bio = blocks[blocks_per_page - 1];
 637         }
 638         goto out;
 639
 640 confused:
 641         if (bio)
 642                 bio = mpage_bio_submit(WRITE, bio);
 643
 644         if (mpd->use_writepage) {
 645                 ret = mapping->a_ops->writepage(page, wbc);
 646         } else {
 647                 ret = -EAGAIN;
 648                 goto out;
 649         }
 650         /*
 651          * The caller has a ref on the inode, so *mapping is stable
 652          */
 653         mapping_set_error(mapping, ret);
 654 out:
 655         mpd->bio = bio;
 656         return ret;
 657 }
 658
 659 /**
 660  * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
 661  * @mapping: address space structure to write
 662  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 663  * @get_block: the filesystem's block mapper function.
 664  *             If this is NULL then use a_ops->writepage.  Otherwise, go
 665  *             direct-to-BIO.
 666  *
 667  * This is a library function, which implements the writepages()
 668  * address_space_operation.
 669  *
 670  * If a page is already under I/O, generic_writepages() skips it, even
 671  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 672  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 673  * and msync() need to guarantee that all the data which was dirty at the time
 674  * the call was made get new I/O started against them.  If wbc->sync_mode is
 675  * WB_SYNC_ALL then we were called for data integrity and we must wait for
 676  * existing IO to complete.
 677  */
 678 int
 679 mpage_writepages(struct address_space *mapping,
 680                 struct writeback_control *wbc, get_block_t get_block)
 681 {
 682         struct blk_plug plug;
 683         int ret;
 684
 685         blk_start_plug(&plug);
 686
 687         if (!get_block)
 688                 ret = generic_writepages(mapping, wbc);
 689         else {
 690                 struct mpage_data mpd = {
 691                         .bio = NULL,
 692                         .last_block_in_bio = 0,
 693                         .get_block = get_block,
 694                         .use_writepage = 1,
 695                 };
 696
 697                 ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
 698                 if (mpd.bio)
 699                         mpage_bio_submit(WRITE, mpd.bio);
 700         }
 701         blk_finish_plug(&plug);
 702         return ret;
 703 }
 704 EXPORT_SYMBOL(mpage_writepages);
 705
 706 int mpage_writepage(struct page *page, get_block_t get_block,
 707         struct writeback_control *wbc)
 708 {
 709         struct mpage_data mpd = {
 710                 .bio = NULL,
 711                 .last_block_in_bio = 0,
 712                 .get_block = get_block,
 713                 .use_writepage = 0,
 714         };
 715         int ret = __mpage_writepage(page, wbc, &mpd);
 716         if (mpd.bio)
 717                 mpage_bio_submit(WRITE, mpd.bio);
 718         return ret;
 719 }
 720 EXPORT_SYMBOL(mpage_writepage);