linux-ext4 - Re: [BUG] aborted ext4 leads to inifinity loop in balance_dirty

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Date:	Mon, 7 Nov 2011 18:29:39 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Kazuya Mio <k-mio@...jp.nec.com>, Jan Kara <jack@...e.cz>,
	ext4 <linux-ext4@...r.kernel.org>, Theodore Tso <tytso@....edu>,
	Andreas Dilger <adilger@...ger.ca>
Subject: Re: [BUG] aborted ext4 leads to inifinity loop in
 balance_dirty_pages

On Mon 07-11-11 12:00:41, Dmitry Monakhov wrote:
> On Fri, 28 Oct 2011 14:34:31 +0900, Kazuya Mio <k-mio@...jp.nec.com> wrote:
> > 2011/10/25 22:40, Jan Kara wrote:
> > >   Please no. Generally this boils down to what do we do with dirty data
> > > when there's error in writing them out. Currently we just throw them away
> > > (e.g. in media error case) but I don't think that's a generally good thing
> > > because e.g. admin may want to copy the data to other working storage or
> > > so. So I think we should rather keep the data and provide a mechanism for
> > > userspace to ask kernel to get rid of the data (so that we don't eventually
> > > run OOM).
> > 
> > I see. I agree with you.
> > 
> > >> Do you have any ideas?
> > >   So the question is what would you like to achieve. If you just want to
> > > unblock a thread then a solution would be to make a thread at
> > > balance_dirty_pages() killable. If generally you want to get rid of dirty
> > > memory, then I don't have a really good answer but throwing dirty data away
> > > seems like a bad answer to me.
> > 
> > The problem is that we cannot unmount the corrupted filesystem due to
> > un-killable dd process. We must bring down the system to resume the service
> > with no dirty pages. I think it is important for the service continuity
> > to be able to kill the thread handling in balance_dirty_pages().
> In fact you are very lucky because dd is just deadlocked, in many cases
> journal abort result in BUG_ON triggering(if IO load is high enough).
  Can you provide the exact kernel message? I'd be interested...

> This is because transaction abort check is racy. Right now i've no good
> fix which has reasonable performance. My latest idea is to protect
> transaction abort check via SRCU.
  Yeah, the code does not seem to care about races too much but I don't see
which BUG_ON would be triggered...

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