On Tue 10-01-12 13:12:55, Josef Bacik wrote:

If we are journalling data (ie journal=data or big symlinks) we can discard
buffers and move them to different transactions to make sure they get cleaned up
properly.  The problem is b_modified could still be set from the last
transaction that touched it, so putting it on the currently running transaction
or setting it up to be put on the next transaction will run into problems if the
buffer gets reused in that transaction as the space accounting logic won't be
done, which will result in panics at commit time because t_nr_buffers will end
up being more than t_outstanding_credits.  Thanks to Jan Kara for pointing out
the other part of this problem a few months ago.  Thanks,

Signed-off-by: Josef Bacik <redacted>

  So I think I've nailed this down. Your feeling that the problem is with
refiling buffer to BJ_Forget list of the running transaction was right. The
missing piece to the puzzle was that journal_invalidatepage() can get
called not only when underlying block is freed but also when someone
flushes page cache. The traces I have suggest that someone has flushed page
cache (likely of the block device), that moved buffer from the checkpoint
list to BJ_Forget list of the running transaction and then the same running
transaction tried to modify the buffer which triggered the accounting
problem you spotted.

I have updated the changelog and pushed the patch to my tree (for JBD
only). I'll duplicate the patch for JBD2 tomorrow.

								Honza


-- 
Jan Kara [off-list ref]
SUSE Labs, CR