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    Subject[PATCH 3.16 022/132] jbd2: check superblock mapped prior to committing
    3.16.74-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Jiufei Xue <jiufei.xue@linux.alibaba.com>

    commit 742b06b5628f2cd23cb51a034cb54dc33c6162c5 upstream.

    We hit a BUG at fs/buffer.c:3057 if we detached the nbd device
    before unmounting ext4 filesystem.

    The typical chain of events leading to the BUG:
    jbd2_write_superblock
    submit_bh
    submit_bh_wbc
    BUG_ON(!buffer_mapped(bh));

    The block device is removed and all the pages are invalidated. JBD2
    was trying to write journal superblock to the block device which is
    no longer present.

    Fix this by checking the journal superblock's buffer head prior to
    submitting.

    Reported-by: Eric Ren <renzhen@linux.alibaba.com>
    Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Reviewed-by: Jan Kara <jack@suse.cz>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/jbd2/journal.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/fs/jbd2/journal.c
    +++ b/fs/jbd2/journal.c
    @@ -1344,6 +1344,10 @@ static int jbd2_write_superblock(journal
    journal_superblock_t *sb = journal->j_superblock;
    int ret;

    + /* Buffer got discarded which means block device got invalidated */
    + if (!buffer_mapped(bh))
    + return -EIO;
    +
    trace_jbd2_write_superblock(journal, write_op);
    if (!(journal->j_flags & JBD2_BARRIER))
    write_op &= ~(REQ_FUA | REQ_FLUSH);
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