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