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