tree abe30c09b0d0ea26491a78ba2ed22d5bdc8b051b
parent 948c9cec0457f27e7810a88493e6292178c331a8
author Jiufei Xue <jiufei.xue@linux.alibaba.com> 1554591460 -0400
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1558457339 +0200

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>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>