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    Subject[CHECKER] crash after fsync causing serious FS corruptions (ext2, 2.6.11)

    Hi,

    FiSC (our FS checker) issues a warning on ext2, complaining that crash
    after fsync causes file system to corrupt. FS corrupts in two different
    ways: 1. file contains illegal blocks (such as block # -2) 2. one block
    owned by two different files.

    I diagnosed the warning a little bit and it appears that this warning can
    be triggered by the following steps:

    1. a file is truncated, so several blocks are freed
    2. a new file is created, and the blocks freed in step 1 are reused
    3. fsync on the new file
    4. crash and run fsck to recover.

    fsync should guarantee that a specific file is persistent on disk.
    Presumably, operations on other files should not mess up with the file we
    just fsync (true ?) However, I also understand that ext2 by default
    relies on e2fsck to provide file system consistency. Do you guys consider
    the above warning as a bug or not? Any clarification on this will be very
    helpful.

    To reproduce the warning, please download the test case at
    http://fisc.stanford.edu/bug3/crash.tar.bz2, untar, compile and run the
    executable ./crash <disk partition> <mount point> This test case is
    semi-automatically generated. It may contain more than enough FS
    operations to trigger the warning. **NOTE**: it'll run mke2fs on <disk
    partition> and reboot your machine!

    e2fsck output:
    e2fsck 1.36 (05-Feb-2005)
    /dev/ide/host0/bus0/target0/lun0/part9 was not cleanly unmounted, check
    forced.
    Pass 1: Checking inodes, blocks, and sizes
    Inode 12 has illegal block(s). Clear? yes

    Illegal block #-2 (2305145833) in inode 12. CLEARED.
    Inode 12, i_blocks is 24, should be 16. Fix? yes

    Duplicate blocks found... invoking duplicate block passes.
    Pass 1B: Rescan for duplicate/bad blocks
    Duplicate/bad block(s) in inode 12: 24
    Duplicate/bad block(s) in inode 13: 24
    Pass 1C: Scan directories for inodes with dup blocks.
    Pass 1D: Reconciling duplicate blocks
    (There are 2 inodes containing duplicate/bad blocks.)

    File ... (inode #12, mod time Mon Mar 7 01:27:12 2005)
    has 1 duplicate block(s), shared with 1 file(s):
    ... (inode #13, mod time Mon Mar 7 01:27:14 2005)
    Clone duplicate/bad blocks? yes

    File ... (inode #13, mod time Mon Mar 7 01:27:14 2005)
    has 1 duplicate block(s), shared with 1 file(s):
    ... (inode #12, mod time Mon Mar 7 01:27:12 2005)
    Duplicated blocks already reassigned or cloned.

    Pass 2: Checking directory structure
    Pass 3: Checking directory connectivity
    Pass 4: Checking reference counts
    Unattached inode 12
    Connect to /lost+found? yes

    Inode 12 ref count is 2, should be 1. Fix? yes

    Unattached inode 13
    Connect to /lost+found? yes

    Inode 13 ref count is 2, should be 1. Fix? yes

    Pass 5: Checking group summary information
    Block bitmap differences: +(21--22) +(29--31)
    Fix? yes

    Free blocks count wrong for group #0 (37, counted=31).
    Fix? yes

    Free blocks count wrong (37, counted=31).
    Fix? yes


    /dev/ide/host0/bus0/target0/lun0/part9: ***** FILE SYSTEM WAS MODIFIED
    *****
    /dev/ide/host0/bus0/target0/lun0/part9: 13/16 files (7.7% non-contiguous),
    29/60 blocks
    running cmd "sudo mount -t ext2 /dev/ide/host0/bus0/target0/lun0/part9
    /mnt/sbd1

    -Junfeng

    -
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