[PATCH] jbd2: check superblock mapped prior to committing

[Jiufei Xue <jiufei.xue@linux.alibaba.com>]

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.

Cc: stable@kernel.org
Reported-by: Eric Ren <renzhen@linux.alibaba.com>
Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
---
 fs/jbd2/journal.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 382c030cc78b..90fd3ed0a232 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1350,6 +1350,9 @@ static int jbd2_write_superblock(journal_t *journal, int write_flags)
 	journal_superblock_t *sb = journal->j_superblock;
 	int ret;
 
+	if (!buffer_mapped(bh))
+		return -EIO;
+
 	trace_jbd2_write_superblock(journal, write_flags);
 	if (!(journal->j_flags & JBD2_BARRIER))
 		write_flags &= ~(REQ_FUA | REQ_PREFLUSH);
-- 
2.19.1.856.g8858448bb