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

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Message-ID: <1315471875.2814.3.camel@menhir>
Date:	Thu, 08 Sep 2011 09:51:15 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Jan Kara <jack@...e.cz>, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Wu Fengguang <fengguang.wu@...el.com>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

Hi,

On Wed, 2011-09-07 at 18:46 +0200, Christoph Hellwig wrote:
> On Wed, Sep 07, 2011 at 06:42:16PM +0200, Jan Kara wrote:
> >   Well, it depends on what you call common - usually, ->writepage is called
> > from kswapd which shouldn't be common compared to writeback from a flusher
> > thread. But now I've realized that JBD2 also calls ->writepage to fulfill
> > data=ordered mode guarantees and that's what causes most of redirtying of
> > pages on ext4. That's going away eventually but it will take some time. So
> > for now writeback has to handle redirtying...
> 
> Under the "right" loads it may also happen for xfs because we can't
> take lock non-blockingly in the fluser thread for example.
> 

GFS2 uses this trick for journaled data pages - the lock ordering is
transaction lock before page lock, so we cannot handle pages which are
already locked before they are handed to the fs if a transaction is
required. So we have our own ->writepages which gets the locks in the
correct order, and ->writepage will simply redirty the page if it would
have required a transaction in order to write out the page,

Steve.


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