From: Josef Bacik <josef@redhat.com>
Subject: Re: [PATCH] jbd: clear b_modified before moving the jh to a
 different transaction
Date: Wed, 4 Apr 2012 12:46:57 -0400
Message-ID: <20120404164656.GA2097@localhost.localdomain>
References: <1326219175-4529-1-git-send-email-josef@redhat.com>
 <20120404075520.GA5725@quack.suse.cz>
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Cc: Josef Bacik <josef@redhat.com>, linux-fsdevel@vger.kernel.org,
	linux-ext4@vger.kernel.org
To: Jan Kara <jack@suse.cz>
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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@redhat.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?  I've followed invalidatepage and
can't see what you are talking about, so as usual I need it explained to me
because I'm stupid.  Thanks,

Josef