linux-kernel - Re: [PATCH 2/9] ksm: let shared pages be swappable

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Message-ID: <20091204144908.GJ28697@random.random>
Date:	Fri, 4 Dec 2009 15:49:08 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <ieidus@...hat.com>,
	Chris Wright <chrisw@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable

On Fri, Dec 04, 2009 at 02:16:17PM +0900, KAMEZAWA Hiroyuki wrote:
> Hmm, can't we use ZERO_PAGE we have now ?
> If do so,
>  - no mapcount check
>  - never on LRU
>  - don't have to maintain shared information because ZERO_PAGE itself has
>    copy-on-write nature.

The zero page could be added to the stable tree always to avoid a
memcmp and we could try to merge anon pages into it, instead of
merging it into ksmpages, but it's not a ksm page so it would require
special handling with branches. We considered doing a magic on
zeropage but we though it's not worth it. We need CPU to be efficient
on very shared pages not just zero page without magics, and the memory
saving is just 4k system-wide (all zero pages of all windows are
already shared).
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