linux-ext4 - Re: [PATCH] jbd: Fix assertion failure in commit code due to lacking transaction credits

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Date:	Fri, 17 Aug 2012 08:48:14 -0400
From:	Josef Bacik <jbacik@...ionio.com>
To:	Jan Kara <jack@...e.cz>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Josef Bacik <JBacik@...ionio.com>
Subject: Re: [PATCH] jbd: Fix assertion failure in commit code due to lacking
 transaction credits

On Thu, Aug 16, 2012 at 04:04:32PM -0600, Jan Kara wrote:
> ext3 users of data=journal mode with blocksize < pagesize were occasionally
> hitting assertion failure in journal_commit_transaction() checking whether the
> transaction has at least as many credits reserved as buffers attached.  The
> core of the problem is that when a file gets truncated, buffers that still need
> checkpointing or that are attached to the committing transaction are left with
> buffer_mapped set. When this happens to buffers beyond i_size attached to a
> page stradding i_size, subsequent write extending the file will see these
> buffers and as they are mapped (but underlying blocks were freed) things go
> awry from here.
> 
> The assertion failure just coincidentally (and in this case luckily as we would
> start corrupting filesystem) triggers due to journal_head not being properly
> cleaned up as well.
> 
> Under some rare circumstances this bug could even hit data=ordered mode users.
> There the assertion won't trigger and we would end up corrupting the
> filesystem.
> 
> We fix the problem by unmapping buffers if possible (in lots of cases we just
> need a buffer attached to a transaction as a place holder but it must not be
> written out anyway). And in one case, we just have to bite the bullet and wait
> for transaction commit to finish.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

You are my hero Jan

Reviewed-by: Josef Bacik <jbacik@...ionio.com>

Thanks,

Josef
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