root/fs/mpage.c

/*
 * fs/mpage.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * Contains functions related to preparing and submitting BIOs which contain
 * multiple pagecache pages.
 *
 * 15May2002    akpm@zip.com.au
 *              Initial version
 * 27Jun2002    axboe@suse.de
 *              use bio_add_page() to build bio's just the right size
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/kdev_t.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>

/*
 * 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 int mpage_end_io_read(struct bio *bio, unsigned int bytes_done, int err)
{
        const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
        struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;

        if (bio->bi_size)
                return 1;

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

                if (--bvec >= bio->bi_io_vec)
                        prefetchw(&bvec->bv_page->flags);

                if (uptodate) {
                        SetPageUptodate(page);
                } else {
                        ClearPageUptodate(page);
                        SetPageError(page);
                }
                unlock_page(page);
        } while (bvec >= bio->bi_io_vec);
        bio_put(bio);
        return 0;
}

static int mpage_end_io_write(struct bio *bio, unsigned int bytes_done, int err)
{
        const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
        struct bio_vec *bvec = bio->bi_io_vec + bio->bi_vcnt - 1;

        if (bio->bi_size)
                return 1;

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

                if (--bvec >= bio->bi_io_vec)
                        prefetchw(&bvec->bv_page->flags);

                if (!uptodate)
                        SetPageError(page);
                end_page_writeback(page);
        } while (bvec >= bio->bi_io_vec);
        bio_put(bio);
        return 0;
}

struct bio *mpage_bio_submit(int rw, struct bio *bio)
{
        bio->bi_end_io = mpage_end_io_read;
        if (rw == WRITE)
                bio->bi_end_io = mpage_end_io_write;
        submit_bio(rw, bio);
        return NULL;
}

static struct bio *
mpage_alloc(struct block_device *bdev,
                sector_t first_sector, int nr_vecs, int 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);
}

/**
 * mpage_readpages - populate an address space with some pages, and
 *                       start reads against them.
 *
 * @mapping: the address_space
 * @pages: The address of a list_head which contains the target pages.  These
 *   pages have their ->index populated and are otherwise uninitialised.
 *
 *   The page at @pages->prev has the lowest file offset, and reads should be
 *   issued in @pages->prev to @pages->next order.
 *
 * @nr_pages: The number of pages at *@pages
 * @get_block: The filesystem's block mapper function.
 *
 * This function walks the pages and the blocks within each page, building and
 * emitting large BIOs.
 *
 * If anything unusual happens, such as:
 *
 * - encountering a page which has buffers
 * - encountering a page which has a non-hole after a hole
 * - encountering a page with non-contiguous blocks
 *
 * then this code just gives up and calls the buffer_head-based read function.
 * It does handle a page which has holes at the end - that is a common case:
 * the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
 *
 * BH_Boundary explanation:
 *
 * There is a problem.  The mpage read code assembles several pages, gets all
 * their disk mappings, and then submits them all.  That's fine, but obtaining
 * the disk mappings may require I/O.  Reads of indirect blocks, for example.
 *
 * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
 * submitted in the following order:
 *      12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
 * because the indirect block has to be read to get the mappings of blocks
 * 13,14,15,16.  Obviously, this impacts performance.
 * 
 * So what we do it to allow the filesystem's get_block() function to set
 * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
 * after this one will require I/O against a block which is probably close to
 * this one.  So you should push what I/O you have currently accumulated.
 *
 * This all causes the disk requests to be issued in the correct order.
 */
static struct bio *
do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
                        sector_t *last_block_in_bio, 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 blocks[MAX_BUF_PER_PAGE];
        unsigned page_block;
        unsigned first_hole = blocks_per_page;
        struct block_device *bdev = NULL;
        struct buffer_head bh;
        int length;
        int fully_mapped = 1;

        if (page_has_buffers(page))
                goto confused;

        block_in_file = page->index << (PAGE_CACHE_SHIFT - blkbits);
        last_block = (i_size_read(inode) + blocksize - 1) >> blkbits;

        bh.b_page = page;
        for (page_block = 0; page_block < blocks_per_page;
                                page_block++, block_in_file++) {
                bh.b_state = 0;
                if (block_in_file < last_block) {
                        if (get_block(inode, block_in_file, &bh, 0))
                                goto confused;
                }

                if (!buffer_mapped(&bh)) {
                        fully_mapped = 0;
                        if (first_hole == blocks_per_page)
                                first_hole = page_block;
                        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(&bh)) {
                        map_buffer_to_page(page, &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] != bh.b_blocknr-1)
                        goto confused;
                blocks[page_block] = bh.b_blocknr;
                bdev = bh.b_bdev;
        }

        if (first_hole != blocks_per_page) {
                char *kaddr = kmap_atomic(page, KM_USER0);
                memset(kaddr + (first_hole << blkbits), 0,
                                PAGE_CACHE_SIZE - (first_hole << blkbits));
                flush_dcache_page(page);
                kunmap_atomic(kaddr, KM_USER0);
                if (first_hole == 0) {
                        SetPageUptodate(page);
                        unlock_page(page);
                        goto out;
                }
        } else if (fully_mapped) {
                SetPageMappedToDisk(page);
        }

        /*
         * 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),
                                        nr_pages, 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;
        }

        if (buffer_boundary(&bh) || (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 pagevec lru_pvec;

        pagevec_init(&lru_pvec, 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(page, mapping,
                                        page->index, GFP_KERNEL)) {
                        bio = do_mpage_readpage(bio, page,
                                        nr_pages - page_idx,
                                        &last_block_in_bio, get_block);
                        if (!pagevec_add(&lru_pvec, page))
                                __pagevec_lru_add(&lru_pvec);
                } else {
                        page_cache_release(page);
                }
        }
        pagevec_lru_add(&lru_pvec);
        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;

        bio = do_mpage_readpage(bio, page, 1,
                        &last_block_in_bio, 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.
 */
static struct bio *
mpage_writepage(struct bio *bio, struct page *page, get_block_t get_block,
        sector_t *last_block_in_bio, int *ret, struct writeback_control *wbc)
{
        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;

        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 = page->index << (PAGE_CACHE_SHIFT - blkbits);
        last_block = (i_size_read(inode) - 1) >> blkbits;
        map_bh.b_page = page;
        for (page_block = 0; page_block < blocks_per_page; ) {

                map_bh.b_state = 0;
                if (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;

        end_index = i_size_read(inode) >> PAGE_CACHE_SHIFT;
        if (page->index >= end_index) {
                unsigned offset = i_size_read(inode) & (PAGE_CACHE_SIZE - 1);
                char *kaddr;

                if (page->index > end_index || !offset)
                        goto confused;
                kaddr = kmap_atomic(page, KM_USER0);
                memset(kaddr + offset, 0, PAGE_CACHE_SIZE - offset);
                flush_dcache_page(page);
                kunmap_atomic(kaddr, KM_USER0);
        }

page_is_mapped:

        /*
         * 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(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;
        }

        /*
         * 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);

                if (buffer_heads_over_limit)
                        try_to_free_buffers(page);
        }

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

        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 {
                *last_block_in_bio = blocks[blocks_per_page - 1];
        }
        goto out;

confused:
        if (bio)
                bio = mpage_bio_submit(WRITE, bio);
        *ret = page->mapping->a_ops->writepage(page, wbc);
        /*
         * The caller has a ref on the inode, so *mapping is stable
         */
        if (*ret) {
                if (*ret == -ENOSPC)
                        set_bit(AS_ENOSPC, &mapping->flags);
                else
                        set_bit(AS_EIO, &mapping->flags);
        }
out:
        return bio;
}

/**
 * mpage_writepages - walk the list of dirty pages of the given
 * address space and writepage() all of them.
 * 
 * @mapping: address space structure to write
 * @wbc: subtract the number of written pages from *@wbc->nr_to_write
 * @get_block: the filesystem's block mapper function.
 *             If this is NULL then use a_ops->writepage.  Otherwise, go
 *             direct-to-BIO.
 *
 * This is a library function, which implements the writepages()
 * address_space_operation.
 *
 * If a page is already under I/O, generic_writepages() skips it, even
 * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
 * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
 * and msync() need to guarantee that all the data which was dirty at the time
 * the call was made get new I/O started against them.  If wbc->sync_mode is
 * WB_SYNC_ALL then we were called for data integrity and we must wait for
 * existing IO to complete.
 */
int
mpage_writepages(struct address_space *mapping,
                struct writeback_control *wbc, get_block_t get_block)
{
        struct backing_dev_info *bdi = mapping->backing_dev_info;
        struct bio *bio = NULL;
        sector_t last_block_in_bio = 0;
        int ret = 0;
        int done = 0;
        int (*writepage)(struct page *page, struct writeback_control *wbc);
        struct pagevec pvec;
        int nr_pages;
        pgoff_t index;
        int scanned = 0;

        if (wbc->nonblocking && bdi_write_congested(bdi)) {
                wbc->encountered_congestion = 1;
                return 0;
        }

        writepage = NULL;
        if (get_block == NULL)
                writepage = mapping->a_ops->writepage;

        pagevec_init(&pvec, 0);
        if (wbc->sync_mode == WB_SYNC_NONE) {
                index = mapping->writeback_index; /* Start from prev offset */
        } else {
                index = 0;                        /* whole-file sweep */
                scanned = 1;
        }
retry:
        while (!done && (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
                                        PAGECACHE_TAG_DIRTY, PAGEVEC_SIZE))) {
                unsigned i;

                scanned = 1;
                for (i = 0; i < nr_pages; i++) {
                        struct page *page = pvec.pages[i];

                        /*
                         * At this point we hold neither mapping->tree_lock nor
                         * lock on the page itself: the page may be truncated or
                         * invalidated (changing page->mapping to NULL), or even
                         * swizzled back from swapper_space to tmpfs file
                         * mapping
                         */

                        lock_page(page);

                        if (wbc->sync_mode != WB_SYNC_NONE)
                                wait_on_page_writeback(page);

                        if (page->mapping != mapping || PageWriteback(page) ||
                                        !clear_page_dirty_for_io(page)) {
                                unlock_page(page);
                                continue;
                        }

                        if (writepage) {
                                ret = (*writepage)(page, wbc);
                                if (ret) {
                                        if (ret == -ENOSPC)
                                                set_bit(AS_ENOSPC,
                                                        &mapping->flags);
                                        else
                                                set_bit(AS_EIO,
                                                        &mapping->flags);
                                }
                        } else {
                                bio = mpage_writepage(bio, page, get_block,
                                                &last_block_in_bio, &ret, wbc);
                        }
                        if (ret || (--(wbc->nr_to_write) <= 0))
                                done = 1;
                        if (wbc->nonblocking && bdi_write_congested(bdi)) {
                                wbc->encountered_congestion = 1;
                                done = 1;
                        }
                }
                pagevec_release(&pvec);
                cond_resched();
        }
        if (!scanned && !done) {
                /*
                 * We hit the last page and there is more work to be done: wrap
                 * back to the start of the file
                 */
                scanned = 1;
                index = 0;
                goto retry;
        }
        mapping->writeback_index = index;
        if (bio)
                mpage_bio_submit(WRITE, bio);
        return ret;
}
EXPORT_SYMBOL(mpage_writepages);