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

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Message-ID: <20091209010419.GG28655@x200.localdomain>
Date:	Tue, 8 Dec 2009 17:04:19 -0800
From:	Chris Wright <chrisw@...hat.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Chris Wright <chrisw@...hat.com>,
	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>, 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:
> On Fri, 4 Dec 2009 09:16:40 -0800
> Chris Wright <chrisw@...hat.com> wrote:
> > * KAMEZAWA Hiroyuki (kamezawa.hiroyu@...fujitsu.com) 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.
> > 
> > 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?
> 
> I just remembered that why ZERO_PAGE was removed (in past). It was becasue
> cache-line ping-pong at fork beacause of page->mapcount. And KSM introduces
> zero-pages which have mapcount again. If no problems in realitsitc usage of
> KVM, ignore me.

KVM is not exactly fork heavy (although it's not the only possible user
of KSM).  And the CoW path has fault + copy already.

Semi-related...it can make good sense to make the KSM trees per NUMA
node.  Would mean things like page of zeroes would collapse to number
of NUMA nodes pages rather than a single page, but has the benefit of
not adding remote access (although, probably more useful for text pages
than zero pages).

thanks,
-chris
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