Date: Wed, 9 Dec 2009 09:43:31 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Chris Wright <chrisw@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Andrea Arcangeli <aarcange@redhat.com>, Rik van Riel <riel@redhat.com>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Izik Eidus <ieidus@redhat.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable
Message-Id: <20091209094331.a1f53e6d.kamezawa.hiroyu@jp.fujitsu.com>
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On Fri, 4 Dec 2009 09:16:40 -0800
Chris Wright <chrisw@redhat.com> wrote:

> * KAMEZAWA Hiroyuki (kamezawa.hiroyu@jp.fujitsu.com) wrote:
> > KOSAKI Motohiro <kosaki.motohiro@jp.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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