[PATCH 5.0 081/123] jbd2: check superblock mapped prior to committing

[Greg Kroah-Hartman]

From: Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxxx>
Signed-off-by: Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx>
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: stable@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/jbd2/journal.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1366,6 +1366,10 @@ 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_flags);
 	if (!(journal->j_flags & JBD2_BARRIER))
 		write_flags &= ~(REQ_FUA | REQ_PREFLUSH);