• Eryu Guan's avatar
    ext4: don't read blocks from disk after extents being swapped · ce6288cc
    Eryu Guan authored
    commit bcff2488 upstream.
    
    I notice ext4/307 fails occasionally on ppc64 host, reporting md5
    checksum mismatch after moving data from original file to donor file.
    
    The reason is that move_extent_per_page() calls __block_write_begin()
    and block_commit_write() to write saved data from original inode blocks
    to donor inode blocks, but __block_write_begin() not only maps buffer
    heads but also reads block content from disk if the size is not block
    size aligned.  At this time the physical block number in mapped buffer
    head is pointing to the donor file not the original file, and that
    results in reading wrong data to page, which get written to disk in
    following block_commit_write call.
    
    This also can be reproduced by the following script on 1k block size ext4
    on x86_64 host:
    
        mnt=/mnt/ext4
        donorfile=$mnt/donor
        testfile=$mnt/testfile
        e4compact=~/xfstests/src/e4compact
    
        rm -f $donorfile $testfile
    
        # reserve space for donor file, written by 0xaa and sync to disk to
        # avoid EBUSY on EXT4_IOC_MOVE_EXT
        xfs_io -fc "pwrite -S 0xaa 0 1m" -c "fsync" $donorfile
    
        # create test file written by 0xbb
        xfs_io -fc "pwrite -S 0xbb 0 1023" -c "fsync" $testfile
    
        # compute initial md5sum
        md5sum $testfile | tee md5sum.txt
        # drop cache, force e4compact to read data from disk
        echo 3 > /proc/sys/vm/drop_caches
    
        # test defrag
        echo "$testfile" | $e4compact -i -v -f $donorfile
        # check md5sum
        md5sum -c md5sum.txt
    
    Fix it by creating & mapping buffer heads only but not reading blocks
    from disk, because all the data in page is guaranteed to be up-to-date
    in mext_page_mkuptodate().
    Signed-off-by: default avatarEryu Guan <guaneryu@gmail.com>
    Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
    [ luis: backported to 3.16: adjusted context ]
    Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
    ce6288cc
move_extent.c 44.2 KB