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/mm_inline.h>
  28 #include <linux/writeback.h>
  29 #include <linux/backing-dev.h>
  30 #include <linux/pagevec.h>
  31 #include <linux/cleancache.h>
  32 #include "internal.h"
  33
  34 /*
  35  * I/O completion handler for multipage BIOs.
  36  *
  37  * The mpage code never puts partial pages into a BIO (except for end-of-file).
  38  * If a page does not map to a contiguous run of blocks then it simply falls
  39  * back to block_read_full_page().
  40  *
  41  * Why is this?  If a page's completion depends on a number of different BIOs
  42  * which can complete in any order (or at the same time) then determining the
  43  * status of that page is hard.  See end_buffer_async_read() for the details.
  44  * There is no point in duplicating all that complexity.
  45  */
  46 static void mpage_end_io(struct bio *bio)
  47 {
  48         struct bio_vec *bv;
  49         int i;
  50
  51         bio_for_each_segment_all(bv, bio, i) {
  52                 struct page *page = bv->bv_page;
  53                 page_endio(page, bio_data_dir(bio), bio->bi_error);
  54         }
  55
  56         bio_put(bio);
  57 }
  58
  59 static struct bio *mpage_bio_submit(int rw, struct bio *bio)
  60 {
  61         bio->bi_end_io = mpage_end_io;
  62         guard_bio_eod(rw, bio);
  63         submit_bio(rw, bio);
  64         return NULL;
  65 }
  66
  67 static struct bio *
  68 mpage_alloc(struct block_device *bdev,
  69                 sector_t first_sector, int nr_vecs,
  70                 gfp_t gfp_flags)
  71 {
  72         struct bio *bio;
  73
  74         bio = bio_alloc(gfp_flags, nr_vecs);
  75
  76         if (bio == NULL && (current->flags & PF_MEMALLOC)) {
  77                 while (!bio && (nr_vecs /= 2))
  78                         bio = bio_alloc(gfp_flags, nr_vecs);
  79         }
  80
  81         if (bio) {
  82                 bio->bi_bdev = bdev;
  83                 bio->bi_iter.bi_sector = first_sector;
  84         }
  85         return bio;
  86 }
  87
  88 /*
  89  * support function for mpage_readpages.  The fs supplied get_block might
  90  * return an up to date buffer.  This is used to map that buffer into
  91  * the page, which allows readpage to avoid triggering a duplicate call
  92  * to get_block.
  93  *
  94  * The idea is to avoid adding buffers to pages that don't already have
  95  * them.  So when the buffer is up to date and the page size == block size,
  96  * this marks the page up to date instead of adding new buffers.
  97  */
  98 static void
  99 map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
 100 {
 101         struct inode *inode = page->mapping->host;
 102         struct buffer_head *page_bh, *head;
 103         int block = 0;
 104
 105         if (!page_has_buffers(page)) {
 106                 /*
 107                  * don't make any buffers if there is only one buffer on
 108                  * the page and the page just needs to be set up to date
 109                  */
 110                 if (inode->i_blkbits == PAGE_CACHE_SHIFT &&
 111                     buffer_uptodate(bh)) {
 112                         SetPageUptodate(page);
 113                         return;
 114                 }
 115                 create_empty_buffers(page, 1 << inode->i_blkbits, 0);
 116         }
 117         head = page_buffers(page);
 118         page_bh = head;
 119         do {
 120                 if (block == page_block) {
 121                         page_bh->b_state = bh->b_state;
 122                         page_bh->b_bdev = bh->b_bdev;
 123                         page_bh->b_blocknr = bh->b_blocknr;
 124                         break;
 125                 }
 126                 page_bh = page_bh->b_this_page;
 127                 block++;
 128         } while (page_bh != head);
 129 }
 130
 131 /*
 132  * This is the worker routine which does all the work of mapping the disk
 133  * blocks and constructs largest possible bios, submits them for IO if the
 134  * blocks are not contiguous on the disk.
 135  *
 136  * We pass a buffer_head back and forth and use its buffer_mapped() flag to
 137  * represent the validity of its disk mapping and to decide when to do the next
 138  * get_block() call.
 139  */
 140 static struct bio *
 141 do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 142                 sector_t *last_block_in_bio, struct buffer_head *map_bh,
 143                 unsigned long *first_logical_block, get_block_t get_block,
 144                 gfp_t gfp)
 145 {
 146         struct inode *inode = page->mapping->host;
 147         const unsigned blkbits = inode->i_blkbits;
 148         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 149         const unsigned blocksize = 1 << blkbits;
 150         sector_t block_in_file;
 151         sector_t last_block;
 152         sector_t last_block_in_file;
 153         sector_t blocks[MAX_BUF_PER_PAGE];
 154         unsigned page_block;
 155         unsigned first_hole = blocks_per_page;
 156         struct block_device *bdev = NULL;
 157         int length;
 158         int fully_mapped = 1;
 159         unsigned nblocks;
 160         unsigned relative_block;
 161
 162         if (page_has_buffers(page))
 163                 goto confused;
 164
 165         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 166         last_block = block_in_file + nr_pages * blocks_per_page;
 167         last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
 168         if (last_block > last_block_in_file)
 169                 last_block = last_block_in_file;
 170         page_block = 0;
 171
 172         /*
 173          * Map blocks using the result from the previous get_blocks call first.
 174          */
 175         nblocks = map_bh->b_size >> blkbits;
 176         if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
 177                         block_in_file < (*first_logical_block + nblocks)) {
 178                 unsigned map_offset = block_in_file - *first_logical_block;
 179                 unsigned last = nblocks - map_offset;
 180
 181                 for (relative_block = 0; ; relative_block++) {
 182                         if (relative_block == last) {
 183                                 clear_buffer_mapped(map_bh);
 184                                 break;
 185                         }
 186                         if (page_block == blocks_per_page)
 187                                 break;
 188                         blocks[page_block] = map_bh->b_blocknr + map_offset +
 189                                                 relative_block;
 190                         page_block++;
 191                         block_in_file++;
 192                 }
 193                 bdev = map_bh->b_bdev;
 194         }
 195
 196         /*
 197          * Then do more get_blocks calls until we are done with this page.
 198          */
 199         map_bh->b_page = page;
 200         while (page_block < blocks_per_page) {
 201                 map_bh->b_state = 0;
 202                 map_bh->b_size = 0;
 203
 204                 if (block_in_file < last_block) {
 205                         map_bh->b_size = (last_block-block_in_file) << blkbits;
 206                         if (get_block(inode, block_in_file, map_bh, 0))
 207                                 goto confused;
 208                         *first_logical_block = block_in_file;
 209                 }
 210
 211                 if (!buffer_mapped(map_bh)) {
 212                         fully_mapped = 0;
 213                         if (first_hole == blocks_per_page)
 214                                 first_hole = page_block;
 215                         page_block++;
 216                         block_in_file++;
 217                         continue;
 218                 }
 219
 220                 /* some filesystems will copy data into the page during
 221                  * the get_block call, in which case we don't want to
 222                  * read it again.  map_buffer_to_page copies the data
 223                  * we just collected from get_block into the page's buffers
 224                  * so readpage doesn't have to repeat the get_block call
 225                  */
 226                 if (buffer_uptodate(map_bh)) {
 227                         map_buffer_to_page(page, map_bh, page_block);
 228                         goto confused;
 229                 }
 230
 231                 if (first_hole != blocks_per_page)
 232                         goto confused;          /* hole -> non-hole */
 233
 234                 /* Contiguous blocks? */
 235                 if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
 236                         goto confused;
 237                 nblocks = map_bh->b_size >> blkbits;
 238                 for (relative_block = 0; ; relative_block++) {
 239                         if (relative_block == nblocks) {
 240                                 clear_buffer_mapped(map_bh);
 241                                 break;
 242                         } else if (page_block == blocks_per_page)
 243                                 break;
 244                         blocks[page_block] = map_bh->b_blocknr+relative_block;
 245                         page_block++;
 246                         block_in_file++;
 247                 }
 248                 bdev = map_bh->b_bdev;
 249         }
 250
 251         if (first_hole != blocks_per_page) {
 252                 zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
 253                 if (first_hole == 0) {
 254                         SetPageUptodate(page);
 255                         unlock_page(page);
 256                         goto out;
 257                 }
 258         } else if (fully_mapped) {
 259                 SetPageMappedToDisk(page);
 260         }
 261
 262         if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
 263             cleancache_get_page(page) == 0) {
 264                 SetPageUptodate(page);
 265                 goto confused;
 266         }
 267
 268         /*
 269          * This page will go to BIO.  Do we need to send this BIO off first?
 270          */
 271         if (bio && (*last_block_in_bio != blocks[0] - 1))
 272                 bio = mpage_bio_submit(READ, bio);
 273
 274 alloc_new:
 275         if (bio == NULL) {
 276                 if (first_hole == blocks_per_page) {
 277                         if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
 278                                                                 page))
 279                                 goto out;
 280                 }
 281                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 282                                 min_t(int, nr_pages, BIO_MAX_PAGES), gfp);
 283                 if (bio == NULL)
 284                         goto confused;
 285         }
 286
 287         length = first_hole << blkbits;
 288         if (bio_add_page(bio, page, length, 0) < length) {
 289                 bio = mpage_bio_submit(READ, bio);
 290                 goto alloc_new;
 291         }
 292
 293         relative_block = block_in_file - *first_logical_block;
 294         nblocks = map_bh->b_size >> blkbits;
 295         if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
 296             (first_hole != blocks_per_page))
 297                 bio = mpage_bio_submit(READ, bio);
 298         else
 299                 *last_block_in_bio = blocks[blocks_per_page - 1];
 300 out:
 301         return bio;
 302
 303 confused:
 304         if (bio)
 305                 bio = mpage_bio_submit(READ, bio);
 306         if (!PageUptodate(page))
 307                 block_read_full_page(page, get_block);
 308         else
 309                 unlock_page(page);
 310         goto out;
 311 }
 312
 313 /**
 314  * mpage_readpages - populate an address space with some pages & start reads against them
 315  * @mapping: the address_space
 316  * @pages: The address of a list_head which contains the target pages.  These
 317  *   pages have their ->index populated and are otherwise uninitialised.
 318  *   The page at @pages->prev has the lowest file offset, and reads should be
 319  *   issued in @pages->prev to @pages->next order.
 320  * @nr_pages: The number of pages at *@pages
 321  * @get_block: The filesystem's block mapper function.
 322  *
 323  * This function walks the pages and the blocks within each page, building and
 324  * emitting large BIOs.
 325  *
 326  * If anything unusual happens, such as:
 327  *
 328  * - encountering a page which has buffers
 329  * - encountering a page which has a non-hole after a hole
 330  * - encountering a page with non-contiguous blocks
 331  *
 332  * then this code just gives up and calls the buffer_head-based read function.
 333  * It does handle a page which has holes at the end - that is a common case:
 334  * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
 335  *
 336  * BH_Boundary explanation:
 337  *
 338  * There is a problem.  The mpage read code assembles several pages, gets all
 339  * their disk mappings, and then submits them all.  That's fine, but obtaining
 340  * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 341  *
 342  * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 343  * submitted in the following order:
 344  *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 345  *
 346  * because the indirect block has to be read to get the mappings of blocks
 347  * 13,14,15,16.  Obviously, this impacts performance.
 348  *
 349  * So what we do it to allow the filesystem's get_block() function to set
 350  * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 351  * after this one will require I/O against a block which is probably close to
 352  * this one.  So you should push what I/O you have currently accumulated.
 353  *
 354  * This all causes the disk requests to be issued in the correct order.
 355  */
 356 int
 357 mpage_readpages(struct address_space *mapping, struct list_head *pages,
 358                                 unsigned nr_pages, get_block_t get_block)
 359 {
 360         struct bio *bio = NULL;
 361         unsigned page_idx;
 362         sector_t last_block_in_bio = 0;
 363         struct buffer_head map_bh;
 364         unsigned long first_logical_block = 0;
 365         gfp_t gfp = mapping_gfp_constraint(mapping, GFP_KERNEL);
 366
 367         map_bh.b_state = 0;
 368         map_bh.b_size = 0;
 369         for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 370                 struct page *page = lru_to_page(pages);
 371
 372                 prefetchw(&page->flags);
 373                 list_del(&page->lru);
 374                 if (!add_to_page_cache_lru(page, mapping,
 375                                         page->index,
 376                                         gfp)) {
 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, gfp);
 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         gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);
 402
 403         map_bh.b_state = 0;
 404         map_bh.b_size = 0;
 405         bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
 406                         &map_bh, &first_logical_block, get_block, gfp);
 407         if (bio)
 408                 mpage_bio_submit(READ, bio);
 409         return 0;
 410 }
 411 EXPORT_SYMBOL(mpage_readpage);
 412
 413 /*
 414  * Writing is not so simple.
 415  *
 416  * If the page has buffers then they will be used for obtaining the disk
 417  * mapping.  We only support pages which are fully mapped-and-dirty, with a
 418  * special case for pages which are unmapped at the end: end-of-file.
 419  *
 420  * If the page has no buffers (preferred) then the page is mapped here.
 421  *
 422  * If all blocks are found to be contiguous then the page can go into the
 423  * BIO.  Otherwise fall back to the mapping's writepage().
 424  *
 425  * FIXME: This code wants an estimate of how many pages are still to be
 426  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 427  * just allocate full-size (16-page) BIOs.
 428  */
 429
 430 struct mpage_data {
 431         struct bio *bio;
 432         sector_t last_block_in_bio;
 433         get_block_t *get_block;
 434         unsigned use_writepage;
 435 };
 436
 437 /*
 438  * We have our BIO, so we can now mark the buffers clean.  Make
 439  * sure to only clean buffers which we know we'll be writing.
 440  */
 441 static void clean_buffers(struct page *page, unsigned first_unmapped)
 442 {
 443         unsigned buffer_counter = 0;
 444         struct buffer_head *bh, *head;
 445         if (!page_has_buffers(page))
 446                 return;
 447         head = page_buffers(page);
 448         bh = head;
 449
 450         do {
 451                 if (buffer_counter++ == first_unmapped)
 452                         break;
 453                 clear_buffer_dirty(bh);
 454                 bh = bh->b_this_page;
 455         } while (bh != head);
 456
 457         /*
 458          * we cannot drop the bh if the page is not uptodate or a concurrent
 459          * readpage would fail to serialize with the bh and it would read from
 460          * disk before we reach the platter.
 461          */
 462         if (buffer_heads_over_limit && PageUptodate(page))
 463                 try_to_free_buffers(page);
 464 }
 465
 466 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 467                       void *data)
 468 {
 469         struct mpage_data *mpd = data;
 470         struct bio *bio = mpd->bio;
 471         struct address_space *mapping = page->mapping;
 472         struct inode *inode = page->mapping->host;
 473         const unsigned blkbits = inode->i_blkbits;
 474         unsigned long end_index;
 475         const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 476         sector_t last_block;
 477         sector_t block_in_file;
 478         sector_t blocks[MAX_BUF_PER_PAGE];
 479         unsigned page_block;
 480         unsigned first_unmapped = blocks_per_page;
 481         struct block_device *bdev = NULL;
 482         int boundary = 0;
 483         sector_t boundary_block = 0;
 484         struct block_device *boundary_bdev = NULL;
 485         int length;
 486         struct buffer_head map_bh;
 487         loff_t i_size = i_size_read(inode);
 488         int ret = 0;
 489         int wr = (wbc->sync_mode == WB_SYNC_ALL ?  WRITE_SYNC : WRITE);
 490
 491         if (page_has_buffers(page)) {
 492                 struct buffer_head *head = page_buffers(page);
 493                 struct buffer_head *bh = head;
 494
 495                 /* If they're all mapped and dirty, do it */
 496                 page_block = 0;
 497                 do {
 498                         BUG_ON(buffer_locked(bh));
 499                         if (!buffer_mapped(bh)) {
 500                                 /*
 501                                  * unmapped dirty buffers are created by
 502                                  * __set_page_dirty_buffers -> mmapped data
 503                                  */
 504                                 if (buffer_dirty(bh))
 505                                         goto confused;
 506                                 if (first_unmapped == blocks_per_page)
 507                                         first_unmapped = page_block;
 508                                 continue;
 509                         }
 510
 511                         if (first_unmapped != blocks_per_page)
 512                                 goto confused;  /* hole -> non-hole */
 513
 514                         if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 515                                 goto confused;
 516                         if (page_block) {
 517                                 if (bh->b_blocknr != blocks[page_block-1] + 1)
 518                                         goto confused;
 519                         }
 520                         blocks[page_block++] = bh->b_blocknr;
 521                         boundary = buffer_boundary(bh);
 522                         if (boundary) {
 523                                 boundary_block = bh->b_blocknr;
 524                                 boundary_bdev = bh->b_bdev;
 525                         }
 526                         bdev = bh->b_bdev;
 527                 } while ((bh = bh->b_this_page) != head);
 528
 529                 if (first_unmapped)
 530                         goto page_is_mapped;
 531
 532                 /*
 533                  * Page has buffers, but they are all unmapped. The page was
 534                  * created by pagein or read over a hole which was handled by
 535                  * block_read_full_page().  If this address_space is also
 536                  * using mpage_readpages then this can rarely happen.
 537                  */
 538                 goto confused;
 539         }
 540
 541         /*
 542          * The page has no buffers: map it to disk
 543          */
 544         BUG_ON(!PageUptodate(page));
 545         block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 546         last_block = (i_size - 1) >> blkbits;
 547         map_bh.b_page = page;
 548         for (page_block = 0; page_block < blocks_per_page; ) {
 549
 550                 map_bh.b_state = 0;
 551                 map_bh.b_size = 1 << blkbits;
 552                 if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 553                         goto confused;
 554                 if (buffer_new(&map_bh))
 555                         unmap_underlying_metadata(map_bh.b_bdev,
 556                                                 map_bh.b_blocknr);
 557                 if (buffer_boundary(&map_bh)) {
 558                         boundary_block = map_bh.b_blocknr;
 559                         boundary_bdev = map_bh.b_bdev;
 560                 }
 561                 if (page_block) {
 562                         if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 563                                 goto confused;
 564                 }
 565                 blocks[page_block++] = map_bh.b_blocknr;
 566                 boundary = buffer_boundary(&map_bh);
 567                 bdev = map_bh.b_bdev;
 568                 if (block_in_file == last_block)
 569                         break;
 570                 block_in_file++;
 571         }
 572         BUG_ON(page_block == 0);
 573
 574         first_unmapped = page_block;
 575
 576 page_is_mapped:
 577         end_index = i_size >> PAGE_CACHE_SHIFT;
 578         if (page->index >= end_index) {
 579                 /*
 580                  * The page straddles i_size.  It must be zeroed out on each
 581                  * and every writepage invocation because it may be mmapped.
 582                  * "A file is mapped in multiples of the page size.  For a file
 583                  * that is not a multiple of the page size, the remaining memory
 584                  * is zeroed when mapped, and writes to that region are not
 585                  * written out to the file."
 586                  */
 587                 unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
 588
 589                 if (page->index > end_index || !offset)
 590                         goto confused;
 591                 zero_user_segment(page, offset, PAGE_CACHE_SIZE);
 592         }
 593
 594         /*
 595          * This page will go to BIO.  Do we need to send this BIO off first?
 596          */
 597         if (bio && mpd->last_block_in_bio != blocks[0] - 1)
 598                 bio = mpage_bio_submit(wr, bio);
 599
 600 alloc_new:
 601         if (bio == NULL) {
 602                 if (first_unmapped == blocks_per_page) {
 603                         if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
 604                                                                 page, wbc)) {
 605                                 clean_buffers(page, first_unmapped);
 606                                 goto out;
 607                         }
 608                 }
 609                 bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 610                                 BIO_MAX_PAGES, GFP_NOFS|__GFP_HIGH);
 611                 if (bio == NULL)
 612                         goto confused;
 613
 614                 wbc_init_bio(wbc, bio);
 615         }
 616
 617         /*
 618          * Must try to add the page before marking the buffer clean or
 619          * the confused fail path above (OOM) will be very confused when
 620          * it finds all bh marked clean (i.e. it will not write anything)
 621          */
 622         wbc_account_io(wbc, page, PAGE_SIZE);
 623         length = first_unmapped << blkbits;
 624         if (bio_add_page(bio, page, length, 0) < length) {
 625                 bio = mpage_bio_submit(wr, bio);
 626                 goto alloc_new;
 627         }
 628
 629         clean_buffers(page, first_unmapped);
 630
 631         BUG_ON(PageWriteback(page));
 632         set_page_writeback(page);
 633         unlock_page(page);
 634         if (boundary || (first_unmapped != blocks_per_page)) {
 635                 bio = mpage_bio_submit(wr, bio);
 636                 if (boundary_block) {
 637                         write_boundary_block(boundary_bdev,
 638                                         boundary_block, 1 << blkbits);
 639                 }
 640         } else {
 641                 mpd->last_block_in_bio = blocks[blocks_per_page - 1];
 642         }
 643         goto out;
 644
 645 confused:
 646         if (bio)
 647                 bio = mpage_bio_submit(wr, bio);
 648
 649         if (mpd->use_writepage) {
 650                 ret = mapping->a_ops->writepage(page, wbc);
 651         } else {
 652                 ret = -EAGAIN;
 653                 goto out;
 654         }
 655         /*
 656          * The caller has a ref on the inode, so *mapping is stable
 657          */
 658         mapping_set_error(mapping, ret);
 659 out:
 660         mpd->bio = bio;
 661         return ret;
 662 }
 663
 664 /**
 665  * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
 666  * @mapping: address space structure to write
 667  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 668  * @get_block: the filesystem's block mapper function.
 669  *             If this is NULL then use a_ops->writepage.  Otherwise, go
 670  *             direct-to-BIO.
 671  *
 672  * This is a library function, which implements the writepages()
 673  * address_space_operation.
 674  *
 675  * If a page is already under I/O, generic_writepages() skips it, even
 676  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 677  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 678  * and msync() need to guarantee that all the data which was dirty at the time
 679  * the call was made get new I/O started against them.  If wbc->sync_mode is
 680  * WB_SYNC_ALL then we were called for data integrity and we must wait for
 681  * existing IO to complete.
 682  */
 683 int
 684 mpage_writepages(struct address_space *mapping,
 685                 struct writeback_control *wbc, get_block_t get_block)
 686 {
 687         struct blk_plug plug;
 688         int ret;
 689
 690         blk_start_plug(&plug);
 691
 692         if (!get_block)
 693                 ret = generic_writepages(mapping, wbc);
 694         else {
 695                 struct mpage_data mpd = {
 696                         .bio = NULL,
 697                         .last_block_in_bio = 0,
 698                         .get_block = get_block,
 699                         .use_writepage = 1,
 700                 };
 701
 702                 ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
 703                 if (mpd.bio) {
 704                         int wr = (wbc->sync_mode == WB_SYNC_ALL ?
 705                                   WRITE_SYNC : WRITE);
 706                         mpage_bio_submit(wr, mpd.bio);
 707                 }
 708         }
 709         blk_finish_plug(&plug);
 710         return ret;
 711 }
 712 EXPORT_SYMBOL(mpage_writepages);
 713
 714 int mpage_writepage(struct page *page, get_block_t get_block,
 715         struct writeback_control *wbc)
 716 {
 717         struct mpage_data mpd = {
 718                 .bio = NULL,
 719                 .last_block_in_bio = 0,
 720                 .get_block = get_block,
 721                 .use_writepage = 0,
 722         };
 723         int ret = __mpage_writepage(page, wbc, &mpd);
 724         if (mpd.bio) {
 725                 int wr = (wbc->sync_mode == WB_SYNC_ALL ?
 726                           WRITE_SYNC : WRITE);
 727                 mpage_bio_submit(wr, mpd.bio);
 728         }
 729         return ret;
 730 }
 731 EXPORT_SYMBOL(mpage_writepage);