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 09:55:20 +0200
From:	Jan Kara <jack@...e.cz>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	jack@...e.cz
Subject: Re: [PATCH] jbd: clear b_modified before moving the jh to a
 different transaction

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.

								Honza


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

View attachment "0001-jbd-clear-b_modified-before-moving-the-jh-to-a-diffe.patch" of type "text/x-patch" (2474 bytes)