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/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         map_bh.b_state = 0;
 383         map_bh.b_size = 0;
 384         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 385                 struct page *page = list_entry(pages->prev, struct page, lru);
 386
 387                 prefetchw(&page->flags);
 388                 list_del(&page->lru);
 389                 if (!add_to_page_cache_lru(page, mapping,
 390                                         page->index, GFP_KERNEL)) {
 391                         bio = do_mpage_readpage(bio, page,
 392                                         nr_pages - page_idx,
 393                                         &last_block_in_bio, &map_bh,
 394                                         &first_logical_block,
 395                                         get_block);
 396                 }
 397                 page_cache_release(page);
 398         }
 399         BUG_ON(!list_empty(pages));
 400         if (bio)
 401                 mpage_bio_submit(READ, bio);
 402         return 0;
 403 }
 404 EXPORT_SYMBOL(mpage_readpages);
 405
 406 /*
 407  * This isn't called much at all
 408  */
 409 int mpage_readpage(struct page *page, get_block_t get_block)
 410 {
 411         struct bio *bio = NULL;
 412         sector_t last_block_in_bio = 0;
 413         struct buffer_head map_bh;
 414         unsigned long first_logical_block = 0;
 415
 416         map_bh.b_state = 0;
 417         map_bh.b_size = 0;
 418         bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
 419                         &map_bh, &first_logical_block, get_block);
 420         if (bio)
 421                 mpage_bio_submit(READ, bio);
 422         return 0;
 423 }
 424 EXPORT_SYMBOL(mpage_readpage);
 425
 426 /*
 427  * Writing is not so simple.
 428  *
 429  * If the page has buffers then they will be used for obtaining the disk
 430  * mapping.  We only support pages which are fully mapped-and-dirty, with a
 431  * special case for pages which are unmapped at the end: end-of-file.
 432  *
 433  * If the page has no buffers (preferred) then the page is mapped here.
 434  *
 435  * If all blocks are found to be contiguous then the page can go into the
 436  * BIO.  Otherwise fall back to the mapping's writepage().
 437  *
 438  * FIXME: This code wants an estimate of how many pages are still to be
 439  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 440  * just allocate full-size (16-page) BIOs.
 441  */
 442
 443 struct mpage_data {
 444         struct bio *bio;
 445         sector_t last_block_in_bio;
 446         get_block_t *get_block;
 447         unsigned use_writepage;
 448 };
 449
 450 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 451                       void *data)
 452 {
 453         struct mpage_data *mpd = data;
 454         struct bio *bio = mpd->bio;
 455         struct address_space *mapping = page->mapping;
 456         struct inode *inode = page->mapping->host;
 457         const unsigned blkbits = inode->i_blkbits;
 458         unsigned long end_index;
 459         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 460         sector_t last_block;
 461         sector_t block_in_file;
 462         sector_t blocks[MAX_BUF_PER_PAGE];
 463         unsigned page_block;
 464         unsigned first_unmapped = blocks_per_page;
 465         struct block_device *bdev = NULL;
 466         int boundary = 0;
 467         sector_t boundary_block = 0;
 468         struct block_device *boundary_bdev = NULL;
 469         int length;
 470         struct buffer_head map_bh;
 471         loff_t i_size = i_size_read(inode);
 472         int ret = 0;
 473
 474         if (page_has_buffers(page)) {
 475                 struct buffer_head *head = page_buffers(page);
 476                 struct buffer_head *bh = head;
 477
 478                 /* If they're all mapped and dirty, do it */
 479                 page_block = 0;
 480                 do {
 481                         BUG_ON(buffer_locked(bh));
 482                         if (!buffer_mapped(bh)) {
 483                                 /*
 484                                  * unmapped dirty buffers are created by
 485                                  * __set_page_dirty_buffers -> mmapped data
 486                                  */
 487                                 if (buffer_dirty(bh))
 488                                         goto confused;
 489                                 if (first_unmapped == blocks_per_page)
 490                                         first_unmapped = page_block;
 491                                 continue;
 492                         }
 493
 494                         if (first_unmapped != blocks_per_page)
 495                                 goto confused;  /* hole -> non-hole */
 496
 497                         if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 498                                 goto confused;
 499                         if (page_block) {
 500                                 if (bh->b_blocknr != blocks[page_block-1] + 1)
 501                                         goto confused;
 502                         }
 503                         blocks[page_block++] = bh->b_blocknr;
 504                         boundary = buffer_boundary(bh);
 505                         if (boundary) {
 506                                 boundary_block = bh->b_blocknr;
 507                                 boundary_bdev = bh->b_bdev;
 508                         }
 509                         bdev = bh->b_bdev;
 510                 } while ((bh = bh->b_this_page) != head);
 511
 512                 if (first_unmapped)
 513                         goto page_is_mapped;
 514
 515                 /*
 516                  * Page has buffers, but they are all unmapped. The page was
 517                  * created by pagein or read over a hole which was handled by
 518                  * block_read_full_page().  If this address_space is also
 519                  * using mpage_readpages then this can rarely happen.
 520                  */
 521                 goto confused;
 522         }
 523
 524         /*
 525          * The page has no buffers: map it to disk
 526          */
 527         BUG_ON(!PageUptodate(page));
 528         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 529         last_block = (i_size - 1) >> blkbits;
 530         map_bh.b_page = page;
 531         for (page_block = 0; page_block < blocks_per_page; ) {
 532
 533                 map_bh.b_state = 0;
 534                 map_bh.b_size = 1 << blkbits;
 535                 if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 536                         goto confused;
 537                 if (buffer_new(&map_bh))
 538                         unmap_underlying_metadata(map_bh.b_bdev,
 539                                                 map_bh.b_blocknr);
 540                 if (buffer_boundary(&map_bh)) {
 541                         boundary_block = map_bh.b_blocknr;
 542                         boundary_bdev = map_bh.b_bdev;
 543                 }
 544                 if (page_block) {
 545                         if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 546                                 goto confused;
 547                 }
 548                 blocks[page_block++] = map_bh.b_blocknr;
 549                 boundary = buffer_boundary(&map_bh);
 550                 bdev = map_bh.b_bdev;
 551                 if (block_in_file == last_block)
 552                         break;
 553                 block_in_file++;
 554         }
 555         BUG_ON(page_block == 0);
 556
 557         first_unmapped = page_block;
 558
 559 page_is_mapped:
 560         end_index = i_size >> PAGE_CACHE_SHIFT;
 561         if (page->index >= end_index) {
 562                 /*
 563                  * The page straddles i_size.  It must be zeroed out on each
 564                  * and every writepage invocation because it may be mmapped.
 565                  * "A file is mapped in multiples of the page size.  For a file
 566                  * that is not a multiple of the page size, the remaining memory
 567                  * is zeroed when mapped, and writes to that region are not
 568                  * written out to the file."
 569                  */
 570                 unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
 571
 572                 if (page->index > end_index || !offset)
 573                         goto confused;
 574                 zero_user_segment(page, offset, PAGE_CACHE_SIZE);
 575         }
 576
 577         /*
 578          * This page will go to BIO.  Do we need to send this BIO off first?
 579          */
 580         if (bio && mpd->last_block_in_bio != blocks[0] - 1)
 581                 bio = mpage_bio_submit(WRITE, bio);
 582
 583 alloc_new:
 584         if (bio == NULL) {
 585                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 586                                 bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
 587                 if (bio == NULL)
 588                         goto confused;
 589         }
 590
 591         /*
 592          * Must try to add the page before marking the buffer clean or
 593          * the confused fail path above (OOM) will be very confused when
 594          * it finds all bh marked clean (i.e. it will not write anything)
 595          */
 596         length = first_unmapped << blkbits;
 597         if (bio_add_page(bio, page, length, 0) < length) {
 598                 bio = mpage_bio_submit(WRITE, bio);
 599                 goto alloc_new;
 600         }
 601
 602         /*
 603          * OK, we have our BIO, so we can now mark the buffers clean.  Make
 604          * sure to only clean buffers which we know we'll be writing.
 605          */
 606         if (page_has_buffers(page)) {
 607                 struct buffer_head *head = page_buffers(page);
 608                 struct buffer_head *bh = head;
 609                 unsigned buffer_counter = 0;
 610
 611                 do {
 612                         if (buffer_counter++ == first_unmapped)
 613                                 break;
 614                         clear_buffer_dirty(bh);
 615                         bh = bh->b_this_page;
 616                 } while (bh != head);
 617
 618                 /*
 619                  * we cannot drop the bh if the page is not uptodate
 620                  * or a concurrent readpage would fail to serialize with the bh
 621                  * and it would read from disk before we reach the platter.
 622                  */
 623                 if (buffer_heads_over_limit && PageUptodate(page))
 624                         try_to_free_buffers(page);
 625         }
 626
 627         BUG_ON(PageWriteback(page));
 628         set_page_writeback(page);
 629         unlock_page(page);
 630         if (boundary || (first_unmapped != blocks_per_page)) {
 631                 bio = mpage_bio_submit(WRITE, bio);
 632                 if (boundary_block) {
 633                         write_boundary_block(boundary_bdev,
 634                                         boundary_block, 1 << blkbits);
 635                 }
 636         } else {
 637                 mpd->last_block_in_bio = blocks[blocks_per_page - 1];
 638         }
 639         goto out;
 640
 641 confused:
 642         if (bio)
 643                 bio = mpage_bio_submit(WRITE, bio);
 644
 645         if (mpd->use_writepage) {
 646                 ret = mapping->a_ops->writepage(page, wbc);
 647         } else {
 648                 ret = -EAGAIN;
 649                 goto out;
 650         }
 651         /*
 652          * The caller has a ref on the inode, so *mapping is stable
 653          */
 654         mapping_set_error(mapping, ret);
 655 out:
 656         mpd->bio = bio;
 657         return ret;
 658 }
 659
 660 /**
 661  * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
 662  * @mapping: address space structure to write
 663  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 664  * @get_block: the filesystem's block mapper function.
 665  *             If this is NULL then use a_ops->writepage.  Otherwise, go
 666  *             direct-to-BIO.
 667  *
 668  * This is a library function, which implements the writepages()
 669  * address_space_operation.
 670  *
 671  * If a page is already under I/O, generic_writepages() skips it, even
 672  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 673  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 674  * and msync() need to guarantee that all the data which was dirty at the time
 675  * the call was made get new I/O started against them.  If wbc->sync_mode is
 676  * WB_SYNC_ALL then we were called for data integrity and we must wait for
 677  * existing IO to complete.
 678  */
 679 int
 680 mpage_writepages(struct address_space *mapping,
 681                 struct writeback_control *wbc, get_block_t get_block)
 682 {
 683         int ret;
 684
 685         if (!get_block)
 686                 ret = generic_writepages(mapping, wbc);
 687         else {
 688                 struct mpage_data mpd = {
 689                         .bio = NULL,
 690                         .last_block_in_bio = 0,
 691                         .get_block = get_block,
 692                         .use_writepage = 1,
 693                 };
 694
 695                 ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
 696                 if (mpd.bio)
 697                         mpage_bio_submit(WRITE, mpd.bio);
 698         }
 699         return ret;
 700 }
 701 EXPORT_SYMBOL(mpage_writepages);
 702
 703 int mpage_writepage(struct page *page, get_block_t get_block,
 704         struct writeback_control *wbc)
 705 {
 706         struct mpage_data mpd = {
 707                 .bio = NULL,
 708                 .last_block_in_bio = 0,
 709                 .get_block = get_block,
 710                 .use_writepage = 0,
 711         };
 712         int ret = __mpage_writepage(page, wbc, &mpd);
 713         if (mpd.bio)
 714                 mpage_bio_submit(WRITE, mpd.bio);
 715         return ret;
 716 }
 717 EXPORT_SYMBOL(mpage_writepage);