jbd2: check superblock mapped prior to committing

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

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 382c030cc78b85516cf6b9869094dbc70bafb4f3..37e16d969925683bfc03d9a020396cbe2ec55756 100644 (file)
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1350,6 +1350,10 @@ static int jbd2_write_superblock(journal_t *journal, int write_flags)
        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);