linux-kernel - Re: [PATCH 0/3] explicitly document overloaded page flags

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Date:	Thu, 15 May 2008 10:26:25 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andy Whitcroft <apw@...dowen.org>
cc:	linux-mm@...ck.org, Andrew Morton <akpm@...l.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] explicitly document overloaded page flags

On Thu, 15 May 2008, Andy Whitcroft wrote:

> With the recent page flag reorganisation we have a single enum which
> defines the valid page flags and their values, nice and clear.  However
> there are a number of bits which are overloaded by different subsystems.
> Firstly there is PG_owner_priv_1 which is used by filesystems and by XEN.
> Secondly both SLOB and SLUB use a couple of extra page bits to manage
> internal state for pages they own; both overlay other bits.  All of these
> "aliases" are scattered about the source making it very hard for a reader
> to know if the bits are safe to rely on in all contexts; confusion here
> is bad.
> 
> As we now have a single place where the bits are clearly assigned it makes
> sense to clarify the reuse of bits by making the aliases explicit and
> visible with the original bit assignments.  This patch creates explicit
> aliases within the enum itself for the overloaded bits and uses those
> aliases throughout.

Ahh. Great! I considered doing that work too but never got around to it.
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