linux-ext4 - Re: [PATCH] jbd: clear b_modified before moving the jh to a different transaction

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Date:	Wed, 4 Apr 2012 23:14:44 +0200
From:	Jan Kara <jack@...e.cz>
To:	Josef Bacik <josef@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] jbd: clear b_modified before moving the jh to a
 different transaction

On Wed 04-04-12 12:46:57, Josef Bacik wrote:
> On Wed, Apr 04, 2012 at 09:55:20AM +0200, Jan Kara wrote:
> > 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 <josef@...hat.com>
> >   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.
> > 
> 
> Ok now it's my turn to be unsure ;).  I thought invalidatepage could only be
> called via truncate?  You say it happens when someone flushes pagecache, do you
> mean like echo 3 > /proc/sys/vm/drop_caches?
  Yup, or things like BLKFLSBUF ioctl. But yes, you are right they don't
end up calling ext3_invalidatepage() I often get confused by the name of
invalidate_mapping_pages()... Anyway ext3_invalidatepage() definitely gets
called (I see that in my traces) and now I tend to thing it's from
ext3_evict_inode(). The guy was using 2.6.37 kernel which doesn't have
b22570d9abb3d844e65c15c8bc0d57a78129e3b4 so truncate_inode_pages() gets
called from ext3_evict_inode() before the buffer is checkpointed and that
causes the described scenario. But the guy claims he's seen the problem
with 3.2 as well. So I guess I'll forward-port the buffer tracking patches
and ask him to reproduce with 3.2.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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