jbd2: check superblock mapped prior to committing

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>

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 7cc112108df6edf06acbdcbf45c78f3d37ca6864..59c56e6c41bbf339723318846379e40758f96467 100644 (file)
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1344,6 +1344,10 @@ static int jbd2_write_superblock(journal_t *journal, int write_op)
        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);