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    akpm@zip.com.au
  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 int mpage_end_io_read(struct bio *bio, unsigned int bytes_done, 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         if (bio->bi_size)
  48                 return 1;
  49
  50         do {
  51                 struct page *page = bvec->bv_page;
  52
  53                 if (--bvec >= bio->bi_io_vec)
  54                         prefetchw(&bvec->bv_page->flags);
  55
  56                 if (uptodate) {
  57                         SetPageUptodate(page);
  58                 } else {
  59                         ClearPageUptodate(page);
  60                         SetPageError(page);
  61                 }
  62                 unlock_page(page);
  63         } while (bvec >= bio->bi_io_vec);
  64         bio_put(bio);
  65         return 0;
  66 }
  67
  68 static int mpage_end_io_write(struct bio *bio, unsigned int bytes_done, int err)
  69 {
  70         const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
  71         struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;
  72
  73         if (bio->bi_size)
  74                 return 1;
  75
  76         do {
  77                 struct page *page = bvec->bv_page;
  78
  79                 if (--bvec >= bio->bi_io_vec)
  80                         prefetchw(&bvec->bv_page->flags);
  81
  82                 if (!uptodate)
  83                         SetPageError(page);
  84                 end_page_writeback(page);
  85         } while (bvec >= bio->bi_io_vec);
  86         bio_put(bio);
  87         return 0;
  88 }
  89
  90 struct bio *mpage_bio_submit(int rw, struct bio *bio)
  91 {
  92         bio->bi_end_io = mpage_end_io_read;
  93         if (rw == WRITE)
  94                 bio->bi_end_io = mpage_end_io_write;
  95         submit_bio(rw, bio);
  96         return NULL;
  97 }
  98
  99 static struct bio *
 100 mpage_alloc(struct block_device *bdev,
 101                 sector_t first_sector, int nr_vecs, int gfp_flags)
 102 {
 103         struct bio *bio;
 104
 105         bio = bio_alloc(gfp_flags, nr_vecs);
 106
 107         if (bio == NULL && (current->flags & PF_MEMALLOC)) {
 108                 while (!bio && (nr_vecs /= 2))
 109                         bio = bio_alloc(gfp_flags, nr_vecs);
 110         }
 111
 112         if (bio) {
 113                 bio->bi_bdev = bdev;
 114                 bio->bi_sector = first_sector;
 115         }
 116         return bio;
 117 }
 118
 119 /*
 120  * support function for mpage_readpages.  The fs supplied get_block might
 121  * return an up to date buffer.  This is used to map that buffer into
 122  * the page, which allows readpage to avoid triggering a duplicate call
 123  * to get_block.
 124  *
 125  * The idea is to avoid adding buffers to pages that don't already have
 126  * them.  So when the buffer is up to date and the page size == block size,
 127  * this marks the page up to date instead of adding new buffers.
 128  */
 129 static void
 130 map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
 131 {
 132         struct inode *inode = page->mapping->host;
 133         struct buffer_head *page_bh, *head;
 134         int block = 0;
 135
 136         if (!page_has_buffers(page)) {
 137                 /*
 138                  * don't make any buffers if there is only one buffer on
 139                  * the page and the page just needs to be set up to date
 140                  */
 141                 if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
 142                     buffer_uptodate(bh)) {
 143                         SetPageUptodate(page);
 144                         return;
 145                 }
 146                 create_empty_buffers(page, 1 << inode->i_blkbits, 0);
 147         }
 148         head = page_buffers(page);
 149         page_bh = head;
 150         do {
 151                 if (block == page_block) {
 152                         page_bh->b_state = bh->b_state;
 153                         page_bh->b_bdev = bh->b_bdev;
 154                         page_bh->b_blocknr = bh->b_blocknr;
 155                         break;
 156                 }
 157                 page_bh = page_bh->b_this_page;
 158                 block++;
 159         } while (page_bh != head);
 160 }
 161
 162 /**
 163  * mpage_readpages - populate an address space with some pages, and
 164  *                       start reads against them.
 165  *
 166  * @mapping: the address_space
 167  * @pages: The address of a list_head which contains the target pages.  These
 168  *   pages have their ->index populated and are otherwise uninitialised.
 169  *
 170  *   The page at @pages->prev has the lowest file offset, and reads should be
 171  *   issued in @pages->prev to @pages->next order.
 172  *
 173  * @nr_pages: The number of pages at *@pages
 174  * @get_block: The filesystem's block mapper function.
 175  *
 176  * This function walks the pages and the blocks within each page, building and
 177  * emitting large BIOs.
 178  *
 179  * If anything unusual happens, such as:
 180  *
 181  * - encountering a page which has buffers
 182  * - encountering a page which has a non-hole after a hole
 183  * - encountering a page with non-contiguous blocks
 184  *
 185  * then this code just gives up and calls the buffer_head-based read function.
 186  * It does handle a page which has holes at the end - that is a common case:
 187  * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
 188  *
 189  * BH_Boundary explanation:
 190  *
 191  * There is a problem.  The mpage read code assembles several pages, gets all
 192  * their disk mappings, and then submits them all.  That's fine, but obtaining
 193  * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 194  *
 195  * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 196  * submitted in the following order:
 197  *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 198  * because the indirect block has to be read to get the mappings of blocks
 199  * 13,14,15,16.  Obviously, this impacts performance.
 200  *
 201  * So what we do it to allow the filesystem's get_block() function to set
 202  * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 203  * after this one will require I/O against a block which is probably close to
 204  * this one.  So you should push what I/O you have currently accumulated.
 205  *
 206  * This all causes the disk requests to be issued in the correct order.
 207  */
 208 static struct bio *
 209 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 210                         sector_t *last_block_in_bio, get_block_t get_block)
 211 {
 212         struct inode *inode = page->mapping->host;
 213         const unsigned blkbits = inode->i_blkbits;
 214         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 215         const unsigned blocksize = 1 << blkbits;
 216         sector_t block_in_file;
 217         sector_t last_block;
 218         sector_t blocks[MAX_BUF_PER_PAGE];
 219         unsigned page_block;
 220         unsigned first_hole = blocks_per_page;
 221         struct block_device *bdev = NULL;
 222         struct buffer_head bh;
 223         int length;
 224         int fully_mapped = 1;
 225
 226         if (page_has_buffers(page))
 227                 goto confused;
 228
 229         block_in_file = page->index << (PAGE_CACHE_SHIFT - blkbits);
 230         last_block = (i_size_read(inode) + blocksize - 1) >> blkbits;
 231
 232         bh.b_page = page;
 233         for (page_block = 0; page_block < blocks_per_page;
 234                                 page_block++, block_in_file++) {
 235                 bh.b_state = 0;
 236                 if (block_in_file < last_block) {
 237                         if (get_block(inode, block_in_file, &bh, 0))
 238                                 goto confused;
 239                 }
 240
 241                 if (!buffer_mapped(&bh)) {
 242                         fully_mapped = 0;
 243                         if (first_hole == blocks_per_page)
 244                                 first_hole = page_block;
 245                         continue;
 246                 }
 247
 248                 /* some filesystems will copy data into the page during
 249                  * the get_block call, in which case we don't want to
 250                  * read it again.  map_buffer_to_page copies the data
 251                  * we just collected from get_block into the page's buffers
 252                  * so readpage doesn't have to repeat the get_block call
 253                  */
 254                 if (buffer_uptodate(&bh)) {
 255                         map_buffer_to_page(page, &bh, page_block);
 256                         goto confused;
 257                 }
 258
 259                 if (first_hole != blocks_per_page)
 260                         goto confused;          /* hole -> non-hole */
 261
 262                 /* Contiguous blocks? */
 263                 if (page_block && blocks[page_block-1] != bh.b_blocknr-1)
 264                         goto confused;
 265                 blocks[page_block] = bh.b_blocknr;
 266                 bdev = bh.b_bdev;
 267         }
 268
 269         if (first_hole != blocks_per_page) {
 270                 char *kaddr = kmap_atomic(page, KM_USER0);
 271                 memset(kaddr + (first_hole << blkbits), 0,
 272                                 PAGE_CACHE_SIZE - (first_hole << blkbits));
 273                 flush_dcache_page(page);
 274                 kunmap_atomic(kaddr, KM_USER0);
 275                 if (first_hole == 0) {
 276                         SetPageUptodate(page);
 277                         unlock_page(page);
 278                         goto out;
 279                 }
 280         } else if (fully_mapped) {
 281                 SetPageMappedToDisk(page);
 282         }
 283
 284         /*
 285          * This page will go to BIO.  Do we need to send this BIO off first?
 286          */
 287         if (bio && (*last_block_in_bio != blocks[0] - 1))
 288                 bio = mpage_bio_submit(READ, bio);
 289
 290 alloc_new:
 291         if (bio == NULL) {
 292                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 293                                 min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
 294                                 GFP_KERNEL);
 295                 if (bio == NULL)
 296                         goto confused;
 297         }
 298
 299         length = first_hole << blkbits;
 300         if (bio_add_page(bio, page, length, 0) < length) {
 301                 bio = mpage_bio_submit(READ, bio);
 302                 goto alloc_new;
 303         }
 304
 305         if (buffer_boundary(&bh) || (first_hole != blocks_per_page))
 306                 bio = mpage_bio_submit(READ, bio);
 307         else
 308                 *last_block_in_bio = blocks[blocks_per_page - 1];
 309 out:
 310         return bio;
 311
 312 confused:
 313         if (bio)
 314                 bio = mpage_bio_submit(READ, bio);
 315         if (!PageUptodate(page))
 316                 block_read_full_page(page, get_block);
 317         else
 318                 unlock_page(page);
 319         goto out;
 320 }
 321
 322 int
 323 mpage_readpages(struct address_space *mapping, struct list_head *pages,
 324                                 unsigned nr_pages, get_block_t get_block)
 325 {
 326         struct bio *bio = NULL;
 327         unsigned page_idx;
 328         sector_t last_block_in_bio = 0;
 329         struct pagevec lru_pvec;
 330
 331         pagevec_init(&lru_pvec, 0);
 332         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 333                 struct page *page = list_entry(pages->prev, struct page, lru);
 334
 335                 prefetchw(&page->flags);
 336                 list_del(&page->lru);
 337                 if (!add_to_page_cache(page, mapping,
 338                                         page->index, GFP_KERNEL)) {
 339                         bio = do_mpage_readpage(bio, page,
 340                                         nr_pages - page_idx,
 341                                         &last_block_in_bio, get_block);
 342                         if (!pagevec_add(&lru_pvec, page))
 343                                 __pagevec_lru_add(&lru_pvec);
 344                 } else {
 345                         page_cache_release(page);
 346                 }
 347         }
 348         pagevec_lru_add(&lru_pvec);
 349         BUG_ON(!list_empty(pages));
 350         if (bio)
 351                 mpage_bio_submit(READ, bio);
 352         return 0;
 353 }
 354 EXPORT_SYMBOL(mpage_readpages);
 355
 356 /*
 357  * This isn't called much at all
 358  */
 359 int mpage_readpage(struct page *page, get_block_t get_block)
 360 {
 361         struct bio *bio = NULL;
 362         sector_t last_block_in_bio = 0;
 363
 364         bio = do_mpage_readpage(bio, page, 1,
 365                         &last_block_in_bio, get_block);
 366         if (bio)
 367                 mpage_bio_submit(READ, bio);
 368         return 0;
 369 }
 370 EXPORT_SYMBOL(mpage_readpage);
 371
 372 /*
 373  * Writing is not so simple.
 374  *
 375  * If the page has buffers then they will be used for obtaining the disk
 376  * mapping.  We only support pages which are fully mapped-and-dirty, with a
 377  * special case for pages which are unmapped at the end: end-of-file.
 378  *
 379  * If the page has no buffers (preferred) then the page is mapped here.
 380  *
 381  * If all blocks are found to be contiguous then the page can go into the
 382  * BIO.  Otherwise fall back to the mapping's writepage().
 383  *
 384  * FIXME: This code wants an estimate of how many pages are still to be
 385  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 386  * just allocate full-size (16-page) BIOs.
 387  */
 388 static struct bio *
 389 mpage_writepage(struct bio *bio, struct page *page, get_block_t get_block,
 390         sector_t *last_block_in_bio, int *ret, struct writeback_control *wbc)
 391 {
 392         struct address_space *mapping = page->mapping;
 393         struct inode *inode = page->mapping->host;
 394         const unsigned blkbits = inode->i_blkbits;
 395         unsigned long end_index;
 396         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 397         sector_t last_block;
 398         sector_t block_in_file;
 399         sector_t blocks[MAX_BUF_PER_PAGE];
 400         unsigned page_block;
 401         unsigned first_unmapped = blocks_per_page;
 402         struct block_device *bdev = NULL;
 403         int boundary = 0;
 404         sector_t boundary_block = 0;
 405         struct block_device *boundary_bdev = NULL;
 406         int length;
 407         struct buffer_head map_bh;
 408         loff_t i_size = i_size_read(inode);
 409
 410         if (page_has_buffers(page)) {
 411                 struct buffer_head *head = page_buffers(page);
 412                 struct buffer_head *bh = head;
 413
 414                 /* If they're all mapped and dirty, do it */
 415                 page_block = 0;
 416                 do {
 417                         BUG_ON(buffer_locked(bh));
 418                         if (!buffer_mapped(bh)) {
 419                                 /*
 420                                  * unmapped dirty buffers are created by
 421                                  * __set_page_dirty_buffers -> mmapped data
 422                                  */
 423                                 if (buffer_dirty(bh))
 424                                         goto confused;
 425                                 if (first_unmapped == blocks_per_page)
 426                                         first_unmapped = page_block;
 427                                 continue;
 428                         }
 429
 430                         if (first_unmapped != blocks_per_page)
 431                                 goto confused;  /* hole -> non-hole */
 432
 433                         if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 434                                 goto confused;
 435                         if (page_block) {
 436                                 if (bh->b_blocknr != blocks[page_block-1] + 1)
 437                                         goto confused;
 438                         }
 439                         blocks[page_block++] = bh->b_blocknr;
 440                         boundary = buffer_boundary(bh);
 441                         if (boundary) {
 442                                 boundary_block = bh->b_blocknr;
 443                                 boundary_bdev = bh->b_bdev;
 444                         }
 445                         bdev = bh->b_bdev;
 446                 } while ((bh = bh->b_this_page) != head);
 447
 448                 if (first_unmapped)
 449                         goto page_is_mapped;
 450
 451                 /*
 452                  * Page has buffers, but they are all unmapped. The page was
 453                  * created by pagein or read over a hole which was handled by
 454                  * block_read_full_page().  If this address_space is also
 455                  * using mpage_readpages then this can rarely happen.
 456                  */
 457                 goto confused;
 458         }
 459
 460         /*
 461          * The page has no buffers: map it to disk
 462          */
 463         BUG_ON(!PageUptodate(page));
 464         block_in_file = page->index << (PAGE_CACHE_SHIFT - blkbits);
 465         last_block = (i_size - 1) >> blkbits;
 466         map_bh.b_page = page;
 467         for (page_block = 0; page_block < blocks_per_page; ) {
 468
 469                 map_bh.b_state = 0;
 470                 if (get_block(inode, block_in_file, &map_bh, 1))
 471                         goto confused;
 472                 if (buffer_new(&map_bh))
 473                         unmap_underlying_metadata(map_bh.b_bdev,
 474                                                 map_bh.b_blocknr);
 475                 if (buffer_boundary(&map_bh)) {
 476                         boundary_block = map_bh.b_blocknr;
 477                         boundary_bdev = map_bh.b_bdev;
 478                 }
 479                 if (page_block) {
 480                         if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 481                                 goto confused;
 482                 }
 483                 blocks[page_block++] = map_bh.b_blocknr;
 484                 boundary = buffer_boundary(&map_bh);
 485                 bdev = map_bh.b_bdev;
 486                 if (block_in_file == last_block)
 487                         break;
 488                 block_in_file++;
 489         }
 490         BUG_ON(page_block == 0);
 491
 492         first_unmapped = page_block;
 493
 494 page_is_mapped:
 495         end_index = i_size >> PAGE_CACHE_SHIFT;
 496         if (page->index >= end_index) {
 497                 /*
 498                  * The page straddles i_size.  It must be zeroed out on each
 499                  * and every writepage invokation because it may be mmapped.
 500                  * "A file is mapped in multiples of the page size.  For a file
 501                  * that is not a multiple of the page size, the remaining memory
 502                  * is zeroed when mapped, and writes to that region are not
 503                  * written out to the file."
 504                  */
 505                 unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
 506                 char *kaddr;
 507
 508                 if (page->index > end_index || !offset)
 509                         goto confused;
 510                 kaddr = kmap_atomic(page, KM_USER0);
 511                 memset(kaddr + offset, 0, PAGE_CACHE_SIZE - offset);
 512                 flush_dcache_page(page);
 513                 kunmap_atomic(kaddr, KM_USER0);
 514         }
 515
 516         /*
 517          * This page will go to BIO.  Do we need to send this BIO off first?
 518          */
 519         if (bio && *last_block_in_bio != blocks[0] - 1)
 520                 bio = mpage_bio_submit(WRITE, bio);
 521
 522 alloc_new:
 523         if (bio == NULL) {
 524                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 525                                 bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
 526                 if (bio == NULL)
 527                         goto confused;
 528         }
 529
 530         /*
 531          * Must try to add the page before marking the buffer clean or
 532          * the confused fail path above (OOM) will be very confused when
 533          * it finds all bh marked clean (i.e. it will not write anything)
 534          */
 535         length = first_unmapped << blkbits;
 536         if (bio_add_page(bio, page, length, 0) < length) {
 537                 bio = mpage_bio_submit(WRITE, bio);
 538                 goto alloc_new;
 539         }
 540
 541         /*
 542          * OK, we have our BIO, so we can now mark the buffers clean.  Make
 543          * sure to only clean buffers which we know we'll be writing.
 544          */
 545         if (page_has_buffers(page)) {
 546                 struct buffer_head *head = page_buffers(page);
 547                 struct buffer_head *bh = head;
 548                 unsigned buffer_counter = 0;
 549
 550                 do {
 551                         if (buffer_counter++ == first_unmapped)
 552                                 break;
 553                         clear_buffer_dirty(bh);
 554                         bh = bh->b_this_page;
 555                 } while (bh != head);
 556
 557                 /*
 558                  * we cannot drop the bh if the page is not uptodate
 559                  * or a concurrent readpage would fail to serialize with the bh
 560                  * and it would read from disk before we reach the platter.
 561                  */
 562                 if (buffer_heads_over_limit && PageUptodate(page))
 563                         try_to_free_buffers(page);
 564         }
 565
 566         BUG_ON(PageWriteback(page));
 567         set_page_writeback(page);
 568         unlock_page(page);
 569         if (boundary || (first_unmapped != blocks_per_page)) {
 570                 bio = mpage_bio_submit(WRITE, bio);
 571                 if (boundary_block) {
 572                         write_boundary_block(boundary_bdev,
 573                                         boundary_block, 1 << blkbits);
 574                 }
 575         } else {
 576                 *last_block_in_bio = blocks[blocks_per_page - 1];
 577         }
 578         goto out;
 579
 580 confused:
 581         if (bio)
 582                 bio = mpage_bio_submit(WRITE, bio);
 583         *ret = page->mapping->a_ops->writepage(page, wbc);
 584         /*
 585          * The caller has a ref on the inode, so *mapping is stable
 586          */
 587         if (*ret) {
 588                 if (*ret == -ENOSPC)
 589                         set_bit(AS_ENOSPC, &mapping->flags);
 590                 else
 591                         set_bit(AS_EIO, &mapping->flags);
 592         }
 593 out:
 594         return bio;
 595 }
 596
 597 /**
 598  * mpage_writepages - walk the list of dirty pages of the given
 599  * address space and writepage() all of them.
 600  *
 601  * @mapping: address space structure to write
 602  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 603  * @get_block: the filesystem's block mapper function.
 604  *             If this is NULL then use a_ops->writepage.  Otherwise, go
 605  *             direct-to-BIO.
 606  *
 607  * This is a library function, which implements the writepages()
 608  * address_space_operation.
 609  *
 610  * If a page is already under I/O, generic_writepages() skips it, even
 611  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 612  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 613  * and msync() need to guarantee that all the data which was dirty at the time
 614  * the call was made get new I/O started against them.  If wbc->sync_mode is
 615  * WB_SYNC_ALL then we were called for data integrity and we must wait for
 616  * existing IO to complete.
 617  */
 618 int
 619 mpage_writepages(struct address_space *mapping,
 620                 struct writeback_control *wbc, get_block_t get_block)
 621 {
 622         struct backing_dev_info *bdi = mapping->backing_dev_info;
 623         struct bio *bio = NULL;
 624         sector_t last_block_in_bio = 0;
 625         int ret = 0;
 626         int done = 0;
 627         int (*writepage)(struct page *page, struct writeback_control *wbc);
 628         struct pagevec pvec;
 629         int nr_pages;
 630         pgoff_t index;
 631         pgoff_t end = -1;               /* Inclusive */
 632         int scanned = 0;
 633         int is_range = 0;
 634
 635         if (wbc->nonblocking && bdi_write_congested(bdi)) {
 636                 wbc->encountered_congestion = 1;
 637                 return 0;
 638         }
 639
 640         writepage = NULL;
 641         if (get_block == NULL)
 642                 writepage = mapping->a_ops->writepage;
 643
 644         pagevec_init(&pvec, 0);
 645         if (wbc->sync_mode == WB_SYNC_NONE) {
 646                 index = mapping->writeback_index; /* Start from prev offset */
 647         } else {
 648                 index = 0;                        /* whole-file sweep */
 649                 scanned = 1;
 650         }
 651         if (wbc->start || wbc->end) {
 652                 index = wbc->start >> PAGE_CACHE_SHIFT;
 653                 end = wbc->end >> PAGE_CACHE_SHIFT;
 654                 is_range = 1;
 655                 scanned = 1;
 656         }
 657 retry:
 658         while (!done && (index <= end) &&
 659                         (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
 660                         PAGECACHE_TAG_DIRTY,
 661                         min(end - index, (pgoff_t)PAGEVEC_SIZE-1) + 1))) {
 662                 unsigned i;
 663
 664                 scanned = 1;
 665                 for (i = 0; i < nr_pages; i++) {
 666                         struct page *page = pvec.pages[i];
 667
 668                         /*
 669                          * At this point we hold neither mapping->tree_lock nor
 670                          * lock on the page itself: the page may be truncated or
 671                          * invalidated (changing page->mapping to NULL), or even
 672                          * swizzled back from swapper_space to tmpfs file
 673                          * mapping
 674                          */
 675
 676                         lock_page(page);
 677
 678                         if (unlikely(page->mapping != mapping)) {
 679                                 unlock_page(page);
 680                                 continue;
 681                         }
 682
 683                         if (unlikely(is_range) && page->index > end) {
 684                                 done = 1;
 685                                 unlock_page(page);
 686                                 continue;
 687                         }
 688
 689                         if (wbc->sync_mode != WB_SYNC_NONE)
 690                                 wait_on_page_writeback(page);
 691
 692                         if (PageWriteback(page) ||
 693                                         !clear_page_dirty_for_io(page)) {
 694                                 unlock_page(page);
 695                                 continue;
 696                         }
 697
 698                         if (writepage) {
 699                                 ret = (*writepage)(page, wbc);
 700                                 if (ret) {
 701                                         if (ret == -ENOSPC)
 702                                                 set_bit(AS_ENOSPC,
 703                                                         &mapping->flags);
 704                                         else
 705                                                 set_bit(AS_EIO,
 706                                                         &mapping->flags);
 707                                 }
 708                         } else {
 709                                 bio = mpage_writepage(bio, page, get_block,
 710                                                 &last_block_in_bio, &ret, wbc);
 711                         }
 712                         if (ret || (--(wbc->nr_to_write) <= 0))
 713                                 done = 1;
 714                         if (wbc->nonblocking && bdi_write_congested(bdi)) {
 715                                 wbc->encountered_congestion = 1;
 716                                 done = 1;
 717                         }
 718                 }
 719                 pagevec_release(&pvec);
 720                 cond_resched();
 721         }
 722         if (!scanned && !done) {
 723                 /*
 724                  * We hit the last page and there is more work to be done: wrap
 725                  * back to the start of the file
 726                  */
 727                 scanned = 1;
 728                 index = 0;
 729                 goto retry;
 730         }
 731         if (!is_range)
 732                 mapping->writeback_index = index;
 733         if (bio)
 734                 mpage_bio_submit(WRITE, bio);
 735         return ret;
 736 }
 737 EXPORT_SYMBOL(mpage_writepages);