linux-kernel - Re: [PATCH 17/18] writeback: fix dirtied pages accounting on redirty

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Message-ID: <20110907002222.GF31945@quack.suse.cz>
Date:	Wed, 7 Sep 2011 02:22:22 +0200
From:	Jan Kara <jack@...e.cz>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 17/18] writeback: fix dirtied pages accounting on
 redirty

On Tue 06-09-11 18:18:56, Peter Zijlstra wrote:
> On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
> > De-account the accumulative dirty counters on page redirty.
> > 
> > Page redirties (very common in ext4) will introduce mismatch between
> > counters (a) and (b)
> > 
> > a) NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied
> > b) NR_WRITTEN, BDI_WRITTEN
> > 
> > This will introduce systematic errors in balanced_rate and result in
> > dirty page position errors (ie. the dirty pages are no longer balanced
> > around the global/bdi setpoints).
> > 
> 
> So wtf is ext4 doing? Shouldn't a page stay dirty until its written out?
> 
> That is, should we really frob around this behaviour or fix ext4 because
> its on crack?
  Fengguang, could you please verify your findings with recent kernel? I
believe ext4 got fixed in this regard some time ago already (and yes, old
delalloc writeback code in ext4 was terrible).

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