linux/fs/mpage.c
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   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#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 */
  44static void mpage_end_io(struct bio *bio, int err)
  45{
  46        const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
  47        struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;
  48
  49        do {
  50                struct page *page = bvec->bv_page;
  51
  52                if (--bvec >= bio->bi_io_vec)
  53                        prefetchw(&bvec->bv_page->flags);
  54                if (bio_data_dir(bio) == READ) {
  55                        if (uptodate) {
  56                                SetPageUptodate(page);
  57                        } else {
  58                                ClearPageUptodate(page);
  59                                SetPageError(page);
  60                        }
  61                        unlock_page(page);
  62                } else { /* bio_data_dir(bio) == WRITE */
  63                        if (!uptodate) {
  64                                SetPageError(page);
  65                                if (page->mapping)
  66                                        set_bit(AS_EIO, &page->mapping->flags);
  67                        }
  68                        end_page_writeback(page);
  69                }
  70        } while (bvec >= bio->bi_io_vec);
  71        bio_put(bio);
  72}
  73
  74static struct bio *mpage_bio_submit(int rw, struct bio *bio)
  75{
  76        bio->bi_end_io = mpage_end_io;
  77        submit_bio(rw, bio);
  78        return NULL;
  79}
  80
  81static struct bio *
  82mpage_alloc(struct block_device *bdev,
  83                sector_t first_sector, int nr_vecs,
  84                gfp_t gfp_flags)
  85{
  86        struct bio *bio;
  87
  88        bio = bio_alloc(gfp_flags, nr_vecs);
  89
  90        if (bio == NULL && (current->flags & PF_MEMALLOC)) {
  91                while (!bio && (nr_vecs /= 2))
  92                        bio = bio_alloc(gfp_flags, nr_vecs);
  93        }
  94
  95        if (bio) {
  96                bio->bi_bdev = bdev;
  97                bio->bi_sector = first_sector;
  98        }
  99        return bio;
 100}
 101
 102/*
 103 * support function for mpage_readpages.  The fs supplied get_block might
 104 * return an up to date buffer.  This is used to map that buffer into
 105 * the page, which allows readpage to avoid triggering a duplicate call
 106 * to get_block.
 107 *
 108 * The idea is to avoid adding buffers to pages that don't already have
 109 * them.  So when the buffer is up to date and the page size == block size,
 110 * this marks the page up to date instead of adding new buffers.
 111 */
 112static void 
 113map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block) 
 114{
 115        struct inode *inode = page->mapping->host;
 116        struct buffer_head *page_bh, *head;
 117        int block = 0;
 118
 119        if (!page_has_buffers(page)) {
 120                /*
 121                 * don't make any buffers if there is only one buffer on
 122                 * the page and the page just needs to be set up to date
 123                 */
 124                if (inode->i_blkbits == PAGE_CACHE_SHIFT && 
 125                    buffer_uptodate(bh)) {
 126                        SetPageUptodate(page);    
 127                        return;
 128                }
 129                create_empty_buffers(page, 1 << inode->i_blkbits, 0);
 130        }
 131        head = page_buffers(page);
 132        page_bh = head;
 133        do {
 134                if (block == page_block) {
 135                        page_bh->b_state = bh->b_state;
 136                        page_bh->b_bdev = bh->b_bdev;
 137                        page_bh->b_blocknr = bh->b_blocknr;
 138                        break;
 139                }
 140                page_bh = page_bh->b_this_page;
 141                block++;
 142        } while (page_bh != head);
 143}
 144
 145/*
 146 * This is the worker routine which does all the work of mapping the disk
 147 * blocks and constructs largest possible bios, submits them for IO if the
 148 * blocks are not contiguous on the disk.
 149 *
 150 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
 151 * represent the validity of its disk mapping and to decide when to do the next
 152 * get_block() call.
 153 */
 154static struct bio *
 155do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 156                sector_t *last_block_in_bio, struct buffer_head *map_bh,
 157                unsigned long *first_logical_block, get_block_t get_block)
 158{
 159        struct inode *inode = page->mapping->host;
 160        const unsigned blkbits = inode->i_blkbits;
 161        const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 162        const unsigned blocksize = 1 << blkbits;
 163        sector_t block_in_file;
 164        sector_t last_block;
 165        sector_t last_block_in_file;
 166        sector_t blocks[MAX_BUF_PER_PAGE];
 167        unsigned page_block;
 168        unsigned first_hole = blocks_per_page;
 169        struct block_device *bdev = NULL;
 170        int length;
 171        int fully_mapped = 1;
 172        unsigned nblocks;
 173        unsigned relative_block;
 174
 175        if (page_has_buffers(page))
 176                goto confused;
 177
 178        block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 179        last_block = block_in_file + nr_pages * blocks_per_page;
 180        last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
 181        if (last_block > last_block_in_file)
 182                last_block = last_block_in_file;
 183        page_block = 0;
 184
 185        /*
 186         * Map blocks using the result from the previous get_blocks call first.
 187         */
 188        nblocks = map_bh->b_size >> blkbits;
 189        if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
 190                        block_in_file < (*first_logical_block + nblocks)) {
 191                unsigned map_offset = block_in_file - *first_logical_block;
 192                unsigned last = nblocks - map_offset;
 193
 194                for (relative_block = 0; ; relative_block++) {
 195                        if (relative_block == last) {
 196                                clear_buffer_mapped(map_bh);
 197                                break;
 198                        }
 199                        if (page_block == blocks_per_page)
 200                                break;
 201                        blocks[page_block] = map_bh->b_blocknr + map_offset +
 202                                                relative_block;
 203                        page_block++;
 204                        block_in_file++;
 205                }
 206                bdev = map_bh->b_bdev;
 207        }
 208
 209        /*
 210         * Then do more get_blocks calls until we are done with this page.
 211         */
 212        map_bh->b_page = page;
 213        while (page_block < blocks_per_page) {
 214                map_bh->b_state = 0;
 215                map_bh->b_size = 0;
 216
 217                if (block_in_file < last_block) {
 218                        map_bh->b_size = (last_block-block_in_file) << blkbits;
 219                        if (get_block(inode, block_in_file, map_bh, 0))
 220                                goto confused;
 221                        *first_logical_block = block_in_file;
 222                }
 223
 224                if (!buffer_mapped(map_bh)) {
 225                        fully_mapped = 0;
 226                        if (first_hole == blocks_per_page)
 227                                first_hole = page_block;
 228                        page_block++;
 229                        block_in_file++;
 230                        continue;
 231                }
 232
 233                /* some filesystems will copy data into the page during
 234                 * the get_block call, in which case we don't want to
 235                 * read it again.  map_buffer_to_page copies the data
 236                 * we just collected from get_block into the page's buffers
 237                 * so readpage doesn't have to repeat the get_block call
 238                 */
 239                if (buffer_uptodate(map_bh)) {
 240                        map_buffer_to_page(page, map_bh, page_block);
 241                        goto confused;
 242                }
 243        
 244                if (first_hole != blocks_per_page)
 245                        goto confused;          /* hole -> non-hole */
 246
 247                /* Contiguous blocks? */
 248                if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
 249                        goto confused;
 250                nblocks = map_bh->b_size >> blkbits;
 251                for (relative_block = 0; ; relative_block++) {
 252                        if (relative_block == nblocks) {
 253                                clear_buffer_mapped(map_bh);
 254                                break;
 255                        } else if (page_block == blocks_per_page)
 256                                break;
 257                        blocks[page_block] = map_bh->b_blocknr+relative_block;
 258                        page_block++;
 259                        block_in_file++;
 260                }
 261                bdev = map_bh->b_bdev;
 262        }
 263
 264        if (first_hole != blocks_per_page) {
 265                zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
 266                if (first_hole == 0) {
 267                        SetPageUptodate(page);
 268                        unlock_page(page);
 269                        goto out;
 270                }
 271        } else if (fully_mapped) {
 272                SetPageMappedToDisk(page);
 273        }
 274
 275        if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
 276            cleancache_get_page(page) == 0) {
 277                SetPageUptodate(page);
 278                goto confused;
 279        }
 280
 281        /*
 282         * This page will go to BIO.  Do we need to send this BIO off first?
 283         */
 284        if (bio && (*last_block_in_bio != blocks[0] - 1))
 285                bio = mpage_bio_submit(READ, bio);
 286
 287alloc_new:
 288        if (bio == NULL) {
 289                bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 290                                min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
 291                                GFP_KERNEL);
 292                if (bio == NULL)
 293                        goto confused;
 294        }
 295
 296        length = first_hole << blkbits;
 297        if (bio_add_page(bio, page, length, 0) < length) {
 298                bio = mpage_bio_submit(READ, bio);
 299                goto alloc_new;
 300        }
 301
 302        relative_block = block_in_file - *first_logical_block;
 303        nblocks = map_bh->b_size >> blkbits;
 304        if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
 305            (first_hole != blocks_per_page))
 306                bio = mpage_bio_submit(READ, bio);
 307        else
 308                *last_block_in_bio = blocks[blocks_per_page - 1];
 309out:
 310        return bio;
 311
 312confused:
 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/**
 323 * mpage_readpages - populate an address space with some pages & start reads against them
 324 * @mapping: the address_space
 325 * @pages: The address of a list_head which contains the target pages.  These
 326 *   pages have their ->index populated and are otherwise uninitialised.
 327 *   The page at @pages->prev has the lowest file offset, and reads should be
 328 *   issued in @pages->prev to @pages->next order.
 329 * @nr_pages: The number of pages at *@pages
 330 * @get_block: The filesystem's block mapper function.
 331 *
 332 * This function walks the pages and the blocks within each page, building and
 333 * emitting large BIOs.
 334 *
 335 * If anything unusual happens, such as:
 336 *
 337 * - encountering a page which has buffers
 338 * - encountering a page which has a non-hole after a hole
 339 * - encountering a page with non-contiguous blocks
 340 *
 341 * then this code just gives up and calls the buffer_head-based read function.
 342 * It does handle a page which has holes at the end - that is a common case:
 343 * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
 344 *
 345 * BH_Boundary explanation:
 346 *
 347 * There is a problem.  The mpage read code assembles several pages, gets all
 348 * their disk mappings, and then submits them all.  That's fine, but obtaining
 349 * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 350 *
 351 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 352 * submitted in the following order:
 353 *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 354 *
 355 * because the indirect block has to be read to get the mappings of blocks
 356 * 13,14,15,16.  Obviously, this impacts performance.
 357 *
 358 * So what we do it to allow the filesystem's get_block() function to set
 359 * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 360 * after this one will require I/O against a block which is probably close to
 361 * this one.  So you should push what I/O you have currently accumulated.
 362 *
 363 * This all causes the disk requests to be issued in the correct order.
 364 */
 365int
 366mpage_readpages(struct address_space *mapping, struct list_head *pages,
 367                                unsigned nr_pages, get_block_t get_block)
 368{
 369        struct bio *bio = NULL;
 370        unsigned page_idx;
 371        sector_t last_block_in_bio = 0;
 372        struct buffer_head map_bh;
 373        unsigned long first_logical_block = 0;
 374        struct blk_plug plug;
 375
 376        blk_start_plug(&plug);
 377
 378        map_bh.b_state = 0;
 379        map_bh.b_size = 0;
 380        for (page_idx = 0; page_idx < nr_pages; page_idx++) {
 381                struct page *page = list_entry(pages->prev, struct page, lru);
 382
 383                prefetchw(&page->flags);
 384                list_del(&page->lru);
 385                if (!add_to_page_cache_lru(page, mapping,
 386                                        page->index, GFP_KERNEL)) {
 387                        bio = do_mpage_readpage(bio, page,
 388                                        nr_pages - page_idx,
 389                                        &last_block_in_bio, &map_bh,
 390                                        &first_logical_block,
 391                                        get_block);
 392                }
 393                page_cache_release(page);
 394        }
 395        BUG_ON(!list_empty(pages));
 396        if (bio)
 397                mpage_bio_submit(READ, bio);
 398        blk_finish_plug(&plug);
 399        return 0;
 400}
 401EXPORT_SYMBOL(mpage_readpages);
 402
 403/*
 404 * This isn't called much at all
 405 */
 406int mpage_readpage(struct page *page, get_block_t get_block)
 407{
 408        struct bio *bio = NULL;
 409        sector_t last_block_in_bio = 0;
 410        struct buffer_head map_bh;
 411        unsigned long first_logical_block = 0;
 412
 413        map_bh.b_state = 0;
 414        map_bh.b_size = 0;
 415        bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
 416                        &map_bh, &first_logical_block, get_block);
 417        if (bio)
 418                mpage_bio_submit(READ, bio);
 419        return 0;
 420}
 421EXPORT_SYMBOL(mpage_readpage);
 422
 423/*
 424 * Writing is not so simple.
 425 *
 426 * If the page has buffers then they will be used for obtaining the disk
 427 * mapping.  We only support pages which are fully mapped-and-dirty, with a
 428 * special case for pages which are unmapped at the end: end-of-file.
 429 *
 430 * If the page has no buffers (preferred) then the page is mapped here.
 431 *
 432 * If all blocks are found to be contiguous then the page can go into the
 433 * BIO.  Otherwise fall back to the mapping's writepage().
 434 * 
 435 * FIXME: This code wants an estimate of how many pages are still to be
 436 * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 437 * just allocate full-size (16-page) BIOs.
 438 */
 439
 440struct mpage_data {
 441        struct bio *bio;
 442        sector_t last_block_in_bio;
 443        get_block_t *get_block;
 444        unsigned use_writepage;
 445};
 446
 447static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 448                      void *data)
 449{
 450        struct mpage_data *mpd = data;
 451        struct bio *bio = mpd->bio;
 452        struct address_space *mapping = page->mapping;
 453        struct inode *inode = page->mapping->host;
 454        const unsigned blkbits = inode->i_blkbits;
 455        unsigned long end_index;
 456        const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
 457        sector_t last_block;
 458        sector_t block_in_file;
 459        sector_t blocks[MAX_BUF_PER_PAGE];
 460        unsigned page_block;
 461        unsigned first_unmapped = blocks_per_page;
 462        struct block_device *bdev = NULL;
 463        int boundary = 0;
 464        sector_t boundary_block = 0;
 465        struct block_device *boundary_bdev = NULL;
 466        int length;
 467        struct buffer_head map_bh;
 468        loff_t i_size = i_size_read(inode);
 469        int ret = 0;
 470
 471        if (page_has_buffers(page)) {
 472                struct buffer_head *head = page_buffers(page);
 473                struct buffer_head *bh = head;
 474
 475                /* If they're all mapped and dirty, do it */
 476                page_block = 0;
 477                do {
 478                        BUG_ON(buffer_locked(bh));
 479                        if (!buffer_mapped(bh)) {
 480                                /*
 481                                 * unmapped dirty buffers are created by
 482                                 * __set_page_dirty_buffers -> mmapped data
 483                                 */
 484                                if (buffer_dirty(bh))
 485                                        goto confused;
 486                                if (first_unmapped == blocks_per_page)
 487                                        first_unmapped = page_block;
 488                                continue;
 489                        }
 490
 491                        if (first_unmapped != blocks_per_page)
 492                                goto confused;  /* hole -> non-hole */
 493
 494                        if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 495                                goto confused;
 496                        if (page_block) {
 497                                if (bh->b_blocknr != blocks[page_block-1] + 1)
 498                                        goto confused;
 499                        }
 500                        blocks[page_block++] = bh->b_blocknr;
 501                        boundary = buffer_boundary(bh);
 502                        if (boundary) {
 503                                boundary_block = bh->b_blocknr;
 504                                boundary_bdev = bh->b_bdev;
 505                        }
 506                        bdev = bh->b_bdev;
 507                } while ((bh = bh->b_this_page) != head);
 508
 509                if (first_unmapped)
 510                        goto page_is_mapped;
 511
 512                /*
 513                 * Page has buffers, but they are all unmapped. The page was
 514                 * created by pagein or read over a hole which was handled by
 515                 * block_read_full_page().  If this address_space is also
 516                 * using mpage_readpages then this can rarely happen.
 517                 */
 518                goto confused;
 519        }
 520
 521        /*
 522         * The page has no buffers: map it to disk
 523         */
 524        BUG_ON(!PageUptodate(page));
 525        block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
 526        last_block = (i_size - 1) >> blkbits;
 527        map_bh.b_page = page;
 528        for (page_block = 0; page_block < blocks_per_page; ) {
 529
 530                map_bh.b_state = 0;
 531                map_bh.b_size = 1 << blkbits;
 532                if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 533                        goto confused;
 534                if (buffer_new(&map_bh))
 535                        unmap_underlying_metadata(map_bh.b_bdev,
 536                                                map_bh.b_blocknr);
 537                if (buffer_boundary(&map_bh)) {
 538                        boundary_block = map_bh.b_blocknr;
 539                        boundary_bdev = map_bh.b_bdev;
 540                }
 541                if (page_block) {
 542                        if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 543                                goto confused;
 544                }
 545                blocks[page_block++] = map_bh.b_blocknr;
 546                boundary = buffer_boundary(&map_bh);
 547                bdev = map_bh.b_bdev;
 548                if (block_in_file == last_block)
 549                        break;
 550                block_in_file++;
 551        }
 552        BUG_ON(page_block == 0);
 553
 554        first_unmapped = page_block;
 555
 556page_is_mapped:
 557        end_index = i_size >> PAGE_CACHE_SHIFT;
 558        if (page->index >= end_index) {
 559                /*
 560                 * The page straddles i_size.  It must be zeroed out on each
 561                 * and every writepage invocation because it may be mmapped.
 562                 * "A file is mapped in multiples of the page size.  For a file
 563                 * that is not a multiple of the page size, the remaining memory
 564                 * is zeroed when mapped, and writes to that region are not
 565                 * written out to the file."
 566                 */
 567                unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);
 568
 569                if (page->index > end_index || !offset)
 570                        goto confused;
 571                zero_user_segment(page, offset, PAGE_CACHE_SIZE);
 572        }
 573
 574        /*
 575         * This page will go to BIO.  Do we need to send this BIO off first?
 576         */
 577        if (bio && mpd->last_block_in_bio != blocks[0] - 1)
 578                bio = mpage_bio_submit(WRITE, bio);
 579
 580alloc_new:
 581        if (bio == NULL) {
 582                bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
 583                                bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
 584                if (bio == NULL)
 585                        goto confused;
 586        }
 587
 588        /*
 589         * Must try to add the page before marking the buffer clean or
 590         * the confused fail path above (OOM) will be very confused when
 591         * it finds all bh marked clean (i.e. it will not write anything)
 592         */
 593        length = first_unmapped << blkbits;
 594        if (bio_add_page(bio, page, length, 0) < length) {
 595                bio = mpage_bio_submit(WRITE, bio);
 596                goto alloc_new;
 597        }
 598
 599        /*
 600         * OK, we have our BIO, so we can now mark the buffers clean.  Make
 601         * sure to only clean buffers which we know we'll be writing.
 602         */
 603        if (page_has_buffers(page)) {
 604                struct buffer_head *head = page_buffers(page);
 605                struct buffer_head *bh = head;
 606                unsigned buffer_counter = 0;
 607
 608                do {
 609                        if (buffer_counter++ == first_unmapped)
 610                                break;
 611                        clear_buffer_dirty(bh);
 612                        bh = bh->b_this_page;
 613                } while (bh != head);
 614
 615                /*
 616                 * we cannot drop the bh if the page is not uptodate
 617                 * or a concurrent readpage would fail to serialize with the bh
 618                 * and it would read from disk before we reach the platter.
 619                 */
 620                if (buffer_heads_over_limit && PageUptodate(page))
 621                        try_to_free_buffers(page);
 622        }
 623
 624        BUG_ON(PageWriteback(page));
 625        set_page_writeback(page);
 626        unlock_page(page);
 627        if (boundary || (first_unmapped != blocks_per_page)) {
 628                bio = mpage_bio_submit(WRITE, bio);
 629                if (boundary_block) {
 630                        write_boundary_block(boundary_bdev,
 631                                        boundary_block, 1 << blkbits);
 632                }
 633        } else {
 634                mpd->last_block_in_bio = blocks[blocks_per_page - 1];
 635        }
 636        goto out;
 637
 638confused:
 639        if (bio)
 640                bio = mpage_bio_submit(WRITE, bio);
 641
 642        if (mpd->use_writepage) {
 643                ret = mapping->a_ops->writepage(page, wbc);
 644        } else {
 645                ret = -EAGAIN;
 646                goto out;
 647        }
 648        /*
 649         * The caller has a ref on the inode, so *mapping is stable
 650         */
 651        mapping_set_error(mapping, ret);
 652out:
 653        mpd->bio = bio;
 654        return ret;
 655}
 656
 657/**
 658 * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
 659 * @mapping: address space structure to write
 660 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 661 * @get_block: the filesystem's block mapper function.
 662 *             If this is NULL then use a_ops->writepage.  Otherwise, go
 663 *             direct-to-BIO.
 664 *
 665 * This is a library function, which implements the writepages()
 666 * address_space_operation.
 667 *
 668 * If a page is already under I/O, generic_writepages() skips it, even
 669 * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 670 * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 671 * and msync() need to guarantee that all the data which was dirty at the time
 672 * the call was made get new I/O started against them.  If wbc->sync_mode is
 673 * WB_SYNC_ALL then we were called for data integrity and we must wait for
 674 * existing IO to complete.
 675 */
 676int
 677mpage_writepages(struct address_space *mapping,
 678                struct writeback_control *wbc, get_block_t get_block)
 679{
 680        struct blk_plug plug;
 681        int ret;
 682
 683        blk_start_plug(&plug);
 684
 685        if (!get_block)
 686                ret = generic_writepages(mapping, wbc);
 687        else {
 688                struct mpage_data mpd = {
 689                        .bio = NULL,
 690                        .last_block_in_bio = 0,
 691                        .get_block = get_block,
 692                        .use_writepage = 1,
 693                };
 694
 695                ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
 696                if (mpd.bio)
 697                        mpage_bio_submit(WRITE, mpd.bio);
 698        }
 699        blk_finish_plug(&plug);
 700        return ret;
 701}
 702EXPORT_SYMBOL(mpage_writepages);
 703
 704int mpage_writepage(struct page *page, get_block_t get_block,
 705        struct writeback_control *wbc)
 706{
 707        struct mpage_data mpd = {
 708                .bio = NULL,
 709                .last_block_in_bio = 0,
 710                .get_block = get_block,
 711                .use_writepage = 0,
 712        };
 713        int ret = __mpage_writepage(page, wbc, &mpd);
 714        if (mpd.bio)
 715                mpage_bio_submit(WRITE, mpd.bio);
 716        return ret;
 717}
 718EXPORT_SYMBOL(mpage_writepage);
 719
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