mpage.c

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/*
 * fs/mpage.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains functions related to preparing and submitting BIOs which contain
 * multiple pagecache pages.
 *
 * 15May2002    Andrew Morton
 *      Initial version
 * 27Jun2002    [email protected]
 *      use bio_add_page() to build bio's just the right size
 */

#include <linux/kernel.h>
#include <linux/export.h>
#include <linux/mm.h>
#include <linux/kdev_t.h>
#include <linux/gfp.h>
#include <linux/bio.h>
#include <linux/fs.h>
#include <linux/buffer_head.h>
#include <linux/blkdev.h>
#include <linux/highmem.h>
#include <linux/prefetch.h>
#include <linux/mpage.h>
#include <linux/writeback.h>
#include <linux/backing-dev.h>
#include <linux/pagevec.h>
#include <linux/cleancache.h>

/*
 * I/O completion handler for multipage BIOs.
 *
 * The mpage code never puts partial pages into a BIO (except for end-of-file).
 * If a page does not map to a contiguous run of blocks then it simply falls
 * back to block_read_full_page().
 *
 * Why is this?  If a page's completion depends on a number of different BIOs
 * which can complete in any order (or at the same time) then determining the
 * status of that page is hard.  See end_buffer_async_read() for the details.
 * There is no point in duplicating all that complexity.
 */
static void mpage_end_io(struct bio *bio, int err)
{
    const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
    struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;

    do {
        struct page *page = bvec->bv_page;

        if (--bvec >= bio->bi_io_vec)
            prefetchw(&bvec->bv_page->flags);
        if (bio_data_dir(bio) == READ) {
            if (uptodate) {
                SetPageUptodate(page);
            } else {
                ClearPageUptodate(page);
                SetPageError(page);
            }
            unlock_page(page);
        } else { /* bio_data_dir(bio) == WRITE */
            if (!uptodate) {
                SetPageError(page);
                if (page->mapping)
                    set_bit(AS_EIO, &page->mapping->flags);
            }
            end_page_writeback(page);
        }
    } while (bvec >= bio->bi_io_vec);
    bio_put(bio);
}

static struct bio *mpage_bio_submit(int rw, struct bio *bio)
{
    bio->bi_end_io = mpage_end_io;
    submit_bio(rw, bio);
    return NULL;
}

static struct bio *
mpage_alloc(struct block_device *bdev,
        sector_t first_sector, int nr_vecs,
        gfp_t gfp_flags)
{
    struct bio *bio;

    bio = bio_alloc(gfp_flags, nr_vecs);

    if (bio == NULL && (current->flags & PF_MEMALLOC)) {
        while (!bio && (nr_vecs /= 2))
            bio = bio_alloc(gfp_flags, nr_vecs);
    }

    if (bio) {
        bio->bi_bdev = bdev;
        bio->bi_sector = first_sector;
    }
    return bio;
}

/*
 * support function for mpage_readpages.  The fs supplied get_block might
 * return an up to date buffer.  This is used to map that buffer into
 * the page, which allows readpage to avoid triggering a duplicate call
 * to get_block.
 *
 * The idea is to avoid adding buffers to pages that don't already have
 * them.  So when the buffer is up to date and the page size == block size,
 * this marks the page up to date instead of adding new buffers.
 */
static void 
map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block) 
{
    struct inode *inode = page->mapping->host;
    struct buffer_head *page_bh, *head;
    int block = 0;

    if (!page_has_buffers(page)) {
        /*
         * don't make any buffers if there is only one buffer on
         * the page and the page just needs to be set up to date
         */
        if (inode->i_blkbits == PAGE_CACHE_SHIFT && 
            buffer_uptodate(bh)) {
            SetPageUptodate(page);    
            return;
        }
        create_empty_buffers(page, 1 << inode->i_blkbits, 0);
    }
    head = page_buffers(page);
    page_bh = head;
    do {
        if (block == page_block) {
            page_bh->b_state = bh->b_state;
            page_bh->b_bdev = bh->b_bdev;
            page_bh->b_blocknr = bh->b_blocknr;
            break;
        }
        page_bh = page_bh->b_this_page;
        block++;
    } while (page_bh != head);
}

/*
 * This is the worker routine which does all the work of mapping the disk
 * blocks and constructs largest possible bios, submits them for IO if the
 * blocks are not contiguous on the disk.
 *
 * We pass a buffer_head back and forth and use its buffer_mapped() flag to
 * represent the validity of its disk mapping and to decide when to do the next
 * get_block() call.
 */
static struct bio *
do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
        sector_t *last_block_in_bio, struct buffer_head *map_bh,
        unsigned long *first_logical_block, get_block_t get_block)
{
    struct inode *inode = page->mapping->host;
    const unsigned blkbits = inode->i_blkbits;
    const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
    const unsigned blocksize = 1 << blkbits;
    sector_t block_in_file;
    sector_t last_block;
    sector_t last_block_in_file;
    sector_t blocks[MAX_BUF_PER_PAGE];
    unsigned page_block;
    unsigned first_hole = blocks_per_page;
    struct block_device *bdev = NULL;
    int length;
    int fully_mapped = 1;
    unsigned nblocks;
    unsigned relative_block;

    if (page_has_buffers(page))
        goto confused;

    block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
    last_block = block_in_file + nr_pages * blocks_per_page;
    last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
    if (last_block > last_block_in_file)
        last_block = last_block_in_file;
    page_block = 0;

    /*
     * Map blocks using the result from the previous get_blocks call first.
     */
    nblocks = map_bh->b_size >> blkbits;
    if (buffer_mapped(map_bh) && block_in_file > *first_logical_block &&
            block_in_file < (*first_logical_block + nblocks)) {
        unsigned map_offset = block_in_file - *first_logical_block;
        unsigned last = nblocks - map_offset;

        for (relative_block = 0; ; relative_block++) {
            if (relative_block == last) {
                clear_buffer_mapped(map_bh);
                break;
            }
            if (page_block == blocks_per_page)
                break;
            blocks[page_block] = map_bh->b_blocknr + map_offset +
                        relative_block;
            page_block++;
            block_in_file++;
        }
        bdev = map_bh->b_bdev;
    }

    /*
     * Then do more get_blocks calls until we are done with this page.
     */
    map_bh->b_page = page;
    while (page_block < blocks_per_page) {
        map_bh->b_state = 0;
        map_bh->b_size = 0;

        if (block_in_file < last_block) {
            map_bh->b_size = (last_block-block_in_file) << blkbits;
            if (get_block(inode, block_in_file, map_bh, 0))
                goto confused;
            *first_logical_block = block_in_file;
        }

        if (!buffer_mapped(map_bh)) {
            fully_mapped = 0;
            if (first_hole == blocks_per_page)
                first_hole = page_block;
            page_block++;
            block_in_file++;
            continue;
        }

        /* some filesystems will copy data into the page during
         * the get_block call, in which case we don't want to
         * read it again.  map_buffer_to_page copies the data
         * we just collected from get_block into the page's buffers
         * so readpage doesn't have to repeat the get_block call
         */
        if (buffer_uptodate(map_bh)) {
            map_buffer_to_page(page, map_bh, page_block);
            goto confused;
        }
    
        if (first_hole != blocks_per_page)
            goto confused;      /* hole -> non-hole */

        /* Contiguous blocks? */
        if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
            goto confused;
        nblocks = map_bh->b_size >> blkbits;
        for (relative_block = 0; ; relative_block++) {
            if (relative_block == nblocks) {
                clear_buffer_mapped(map_bh);
                break;
            } else if (page_block == blocks_per_page)
                break;
            blocks[page_block] = map_bh->b_blocknr+relative_block;
            page_block++;
            block_in_file++;
        }
        bdev = map_bh->b_bdev;
    }

    if (first_hole != blocks_per_page) {
        zero_user_segment(page, first_hole << blkbits, PAGE_CACHE_SIZE);
        if (first_hole == 0) {
            SetPageUptodate(page);
            unlock_page(page);
            goto out;
        }
    } else if (fully_mapped) {
        SetPageMappedToDisk(page);
    }

    if (fully_mapped && blocks_per_page == 1 && !PageUptodate(page) &&
        cleancache_get_page(page) == 0) {
        SetPageUptodate(page);
        goto confused;
    }

    /*
     * This page will go to BIO.  Do we need to send this BIO off first?
     */
    if (bio && (*last_block_in_bio != blocks[0] - 1))
        bio = mpage_bio_submit(READ, bio);

alloc_new:
    if (bio == NULL) {
        bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
                min_t(int, nr_pages, bio_get_nr_vecs(bdev)),
                GFP_KERNEL);
        if (bio == NULL)
            goto confused;
    }

    length = first_hole << blkbits;
    if (bio_add_page(bio, page, length, 0) < length) {
        bio = mpage_bio_submit(READ, bio);
        goto alloc_new;
    }

    relative_block = block_in_file - *first_logical_block;
    nblocks = map_bh->b_size >> blkbits;
    if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
        (first_hole != blocks_per_page))
        bio = mpage_bio_submit(READ, bio);
    else
        *last_block_in_bio = blocks[blocks_per_page - 1];
out:
    return bio;

confused:
    if (bio)
        bio = mpage_bio_submit(READ, bio);
    if (!PageUptodate(page))
            block_read_full_page(page, get_block);
    else
        unlock_page(page);
    goto out;
}

int
mpage_readpages(struct address_space *mapping, struct list_head *pages,
                unsigned nr_pages, get_block_t get_block)
{
    struct bio *bio = NULL;
    unsigned page_idx;
    sector_t last_block_in_bio = 0;
    struct buffer_head map_bh;
    unsigned long first_logical_block = 0;

    map_bh.b_state = 0;
    map_bh.b_size = 0;
    for (page_idx = 0; page_idx < nr_pages; page_idx++) {
        struct page *page = list_entry(pages->prev, struct page, lru);

        prefetchw(&page->flags);
        list_del(&page->lru);
        if (!add_to_page_cache_lru(page, mapping,
                    page->index, GFP_KERNEL)) {
            bio = do_mpage_readpage(bio, page,
                    nr_pages - page_idx,
                    &last_block_in_bio, &map_bh,
                    &first_logical_block,
                    get_block);
        }
        page_cache_release(page);
    }
    BUG_ON(!list_empty(pages));
    if (bio)
        mpage_bio_submit(READ, bio);
    return 0;
}
EXPORT_SYMBOL(mpage_readpages);

/*
 * This isn't called much at all
 */
int mpage_readpage(struct page *page, get_block_t get_block)
{
    struct bio *bio = NULL;
    sector_t last_block_in_bio = 0;
    struct buffer_head map_bh;
    unsigned long first_logical_block = 0;

    map_bh.b_state = 0;
    map_bh.b_size = 0;
    bio = do_mpage_readpage(bio, page, 1, &last_block_in_bio,
            &map_bh, &first_logical_block, get_block);
    if (bio)
        mpage_bio_submit(READ, bio);
    return 0;
}
EXPORT_SYMBOL(mpage_readpage);

/*
 * Writing is not so simple.
 *
 * If the page has buffers then they will be used for obtaining the disk
 * mapping.  We only support pages which are fully mapped-and-dirty, with a
 * special case for pages which are unmapped at the end: end-of-file.
 *
 * If the page has no buffers (preferred) then the page is mapped here.
 *
 * If all blocks are found to be contiguous then the page can go into the
 * BIO.  Otherwise fall back to the mapping's writepage().
 * 
 * FIXME: This code wants an estimate of how many pages are still to be
 * written, so it can intelligently allocate a suitably-sized BIO.  For now,
 * just allocate full-size (16-page) BIOs.
 */

struct mpage_data {
    struct bio *bio;
    sector_t last_block_in_bio;
    get_block_t *get_block;
    unsigned use_writepage;
};

static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
              void *data)
{
    struct mpage_data *mpd = data;
    struct bio *bio = mpd->bio;
    struct address_space *mapping = page->mapping;
    struct inode *inode = page->mapping->host;
    const unsigned blkbits = inode->i_blkbits;
    unsigned long end_index;
    const unsigned blocks_per_page = PAGE_CACHE_SIZE >> blkbits;
    sector_t last_block;
    sector_t block_in_file;
    sector_t blocks[MAX_BUF_PER_PAGE];
    unsigned page_block;
    unsigned first_unmapped = blocks_per_page;
    struct block_device *bdev = NULL;
    int boundary = 0;
    sector_t boundary_block = 0;
    struct block_device *boundary_bdev = NULL;
    int length;
    struct buffer_head map_bh;
    loff_t i_size = i_size_read(inode);
    int ret = 0;

    if (page_has_buffers(page)) {
        struct buffer_head *head = page_buffers(page);
        struct buffer_head *bh = head;

        /* If they're all mapped and dirty, do it */
        page_block = 0;
        do {
            BUG_ON(buffer_locked(bh));
            if (!buffer_mapped(bh)) {
                /*
                 * unmapped dirty buffers are created by
                 * __set_page_dirty_buffers -> mmapped data
                 */
                if (buffer_dirty(bh))
                    goto confused;
                if (first_unmapped == blocks_per_page)
                    first_unmapped = page_block;
                continue;
            }

            if (first_unmapped != blocks_per_page)
                goto confused;  /* hole -> non-hole */

            if (!buffer_dirty(bh) || !buffer_uptodate(bh))
                goto confused;
            if (page_block) {
                if (bh->b_blocknr != blocks[page_block-1] + 1)
                    goto confused;
            }
            blocks[page_block++] = bh->b_blocknr;
            boundary = buffer_boundary(bh);
            if (boundary) {
                boundary_block = bh->b_blocknr;
                boundary_bdev = bh->b_bdev;
            }
            bdev = bh->b_bdev;
        } while ((bh = bh->b_this_page) != head);

        if (first_unmapped)
            goto page_is_mapped;

        /*
         * Page has buffers, but they are all unmapped. The page was
         * created by pagein or read over a hole which was handled by
         * block_read_full_page().  If this address_space is also
         * using mpage_readpages then this can rarely happen.
         */
        goto confused;
    }

    /*
     * The page has no buffers: map it to disk
     */
    BUG_ON(!PageUptodate(page));
    block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);
    last_block = (i_size - 1) >> blkbits;
    map_bh.b_page = page;
    for (page_block = 0; page_block < blocks_per_page; ) {

        map_bh.b_state = 0;
        map_bh.b_size = 1 << blkbits;
        if (mpd->get_block(inode, block_in_file, &map_bh, 1))
            goto confused;
        if (buffer_new(&map_bh))
            unmap_underlying_metadata(map_bh.b_bdev,
                        map_bh.b_blocknr);
        if (buffer_boundary(&map_bh)) {
            boundary_block = map_bh.b_blocknr;
            boundary_bdev = map_bh.b_bdev;
        }
        if (page_block) {
            if (map_bh.b_blocknr != blocks[page_block-1] + 1)
                goto confused;
        }
        blocks[page_block++] = map_bh.b_blocknr;
        boundary = buffer_boundary(&map_bh);
        bdev = map_bh.b_bdev;
        if (block_in_file == last_block)
            break;
        block_in_file++;
    }
    BUG_ON(page_block == 0);

    first_unmapped = page_block;

page_is_mapped:
    end_index = i_size >> PAGE_CACHE_SHIFT;
    if (page->index >= end_index) {
        /*
         * The page straddles i_size.  It must be zeroed out on each
         * and every writepage invocation because it may be mmapped.
         * "A file is mapped in multiples of the page size.  For a file
         * that is not a multiple of the page size, the remaining memory
         * is zeroed when mapped, and writes to that region are not
         * written out to the file."
         */
        unsigned offset = i_size & (PAGE_CACHE_SIZE - 1);

        if (page->index > end_index || !offset)
            goto confused;
        zero_user_segment(page, offset, PAGE_CACHE_SIZE);
    }

    /*
     * This page will go to BIO.  Do we need to send this BIO off first?
     */
    if (bio && mpd->last_block_in_bio != blocks[0] - 1)
        bio = mpage_bio_submit(WRITE, bio);

alloc_new:
    if (bio == NULL) {
        bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
                bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
        if (bio == NULL)
            goto confused;
    }

    /*
     * Must try to add the page before marking the buffer clean or
     * the confused fail path above (OOM) will be very confused when
     * it finds all bh marked clean (i.e. it will not write anything)
     */
    length = first_unmapped << blkbits;
    if (bio_add_page(bio, page, length, 0) < length) {
        bio = mpage_bio_submit(WRITE, bio);
        goto alloc_new;
    }

    /*
     * OK, we have our BIO, so we can now mark the buffers clean.  Make
     * sure to only clean buffers which we know we'll be writing.
     */
    if (page_has_buffers(page)) {
        struct buffer_head *head = page_buffers(page);
        struct buffer_head *bh = head;
        unsigned buffer_counter = 0;

        do {
            if (buffer_counter++ == first_unmapped)
                break;
            clear_buffer_dirty(bh);
            bh = bh->b_this_page;
        } while (bh != head);

        /*
         * we cannot drop the bh if the page is not uptodate
         * or a concurrent readpage would fail to serialize with the bh
         * and it would read from disk before we reach the platter.
         */
        if (buffer_heads_over_limit && PageUptodate(page))
            try_to_free_buffers(page);
    }

    BUG_ON(PageWriteback(page));
    set_page_writeback(page);
    unlock_page(page);
    if (boundary || (first_unmapped != blocks_per_page)) {
        bio = mpage_bio_submit(WRITE, bio);
        if (boundary_block) {
            write_boundary_block(boundary_bdev,
                    boundary_block, 1 << blkbits);
        }
    } else {
        mpd->last_block_in_bio = blocks[blocks_per_page - 1];
    }
    goto out;

confused:
    if (bio)
        bio = mpage_bio_submit(WRITE, bio);

    if (mpd->use_writepage) {
        ret = mapping->a_ops->writepage(page, wbc);
    } else {
        ret = -EAGAIN;
        goto out;
    }
    /*
     * The caller has a ref on the inode, so *mapping is stable
     */
    mapping_set_error(mapping, ret);
out:
    mpd->bio = bio;
    return ret;
}

int
mpage_writepages(struct address_space *mapping,
        struct writeback_control *wbc, get_block_t get_block)
{
    struct blk_plug plug;
    int ret;

    blk_start_plug(&plug);

    if (!get_block)
        ret = generic_writepages(mapping, wbc);
    else {
        struct mpage_data mpd = {
            .bio = NULL,
            .last_block_in_bio = 0,
            .get_block = get_block,
            .use_writepage = 1,
        };

        ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
        if (mpd.bio)
            mpage_bio_submit(WRITE, mpd.bio);
    }
    blk_finish_plug(&plug);
    return ret;
}
EXPORT_SYMBOL(mpage_writepages);

int mpage_writepage(struct page *page, get_block_t get_block,
    struct writeback_control *wbc)
{
    struct mpage_data mpd = {
        .bio = NULL,
        .last_block_in_bio = 0,
        .get_block = get_block,
        .use_writepage = 0,
    };
    int ret = __mpage_writepage(page, wbc, &mpd);
    if (mpd.bio)
        mpage_bio_submit(WRITE, mpd.bio);
    return ret;
}
EXPORT_SYMBOL(mpage_writepage);