linux-kernel - Re: [PATCH 4/4] mm: vmscan: Take page buffers dirty and locked state into account

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Date:	Wed, 29 May 2013 12:53:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Rik van Riel <riel@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>, Michal Hocko <mhocko@...e.cz>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm: vmscan: Take page buffers dirty and locked
 state into account

On Mon, 27 May 2013 14:02:58 +0100 Mel Gorman <mgorman@...e.de> wrote:

> Page reclaim keeps track of dirty and under writeback pages and uses it to
> determine if wait_iff_congested() should stall or if kswapd should begin
> writing back pages. This fails to account for buffer pages that can be under
> writeback but not PageWriteback which is the case for filesystems like ext3
> ordered mode. Furthermore, PageDirty buffer pages can have all the buffers
> clean and writepage does no IO so it should not be accounted as congested.

iirc, the PageDirty-all-buffers-clean state is pretty rare.  It might
not be worth bothering about?

> This patch adds an address_space operation that filesystems may
> optionally use to check if a page is really dirty or really under
> writeback.

address_space_operations methods are Documented in
Documentation/filesystems/vfs.txt ;)

> An implementation is provided for for buffer_heads is added
> and used for block operations and ext3 in ordered mode. By default the
> page flags are obeyed.
> 
> Credit goes to Jan Kara for identifying that the page flags alone are
> not sufficient for ext3 and sanity checking a number of ideas on how
> the problem could be addressed.

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