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

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Message-ID: <20091204171640.GE19624@x200.localdomain>
Date:	Fri, 4 Dec 2009 09:16:40 -0800
From:	Chris Wright <chrisw@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.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

* KAMEZAWA Hiroyuki (kamezawa.hiroyu@...fujitsu.com) wrote:
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> > Windows kernel have zero page thread and it clear the pages in free list
> > periodically. because many windows subsystem prerefer zero filled page.
> > hen, if we use windows guest, zero filled page have plenty mapcount rather
> > than other typical sharing pages, I guess.
> > 
> > So, can we mark as unevictable to zero filled ksm page? 

That's why I mentioned the page of zeroes as the prime example of
something with a high mapcount that shouldn't really ever be evicted.

> 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.

It's a somewhat special case, but wouldn't it be useful to have a generic
method to recognize this kind of sharing since it's a generic issue?

thanks,
-chris
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