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

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Message-ID: <20120110202119.GA3523@localhost.localdomain>
Date:	Tue, 10 Jan 2012 15:21:20 -0500
From:	Josef Bacik <josef@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Josef Bacik <josef@...hat.com>, 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 Tue, Jan 10, 2012 at 09:17:06PM +0100, 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,
>   Ho hum, I'm inclined to apply this just because it makes sense. But I
> still don't see how a transaction can reuse a buffer from BJ_Forget list.
> We attach there only truncated buffers and their underlying block can be
> reallocated only after the transaction freeing them is committed. So have
> you some incentive that this patch indeed fixes the t_outstanding_credits
> assertion you were hunting?
> 

So more the problem is where we set b_next_transaction, since it could be
reallocated in the next transaction after the current transaction commits and
then we're really screwed.  I have no real evidence to prove that this is
causing my problem yet, but it's definitely wrong and I want to get it fixed
before I forget it :).  Thanks,

Josef
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