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

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Message-ID: <20110907065635.GA12619@lst.de>
Date:	Wed, 7 Sep 2011 08:56:35 +0200
From:	Christoph Hellwig <hch@....de>
To:	Jan Kara <jack@...e.cz>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Wu Fengguang <fengguang.wu@...el.com>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Wed, Sep 07, 2011 at 02:22:22AM +0200, Jan Kara wrote:
> > 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).

The pattern we do in writeback is:

in pageout / write_cache_pages:
	lock_page();
	clear_page_dirty_for_io();

in ->writepage:
	set_page_writeback();
	unlock_page();
	end_page_writeback();

So whenever ->writepage decides it doesn't want to write things back
we have to redirty pages.  We have this happen quite a bit in every
filesystem, but ext4 hits it a lot more than usual because it refuses
to write out delalloc pages from plain ->writepage and only allows
->writepages to do it.

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