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

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Date:	Wed, 9 Dec 2009 09:43:31 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Chris Wright <chrisw@...hat.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>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable

On Fri, 4 Dec 2009 09:16:40 -0800
Chris Wright <chrisw@...hat.com> wrote:

> * 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?
> 

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.

Thanks,
-Kame

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