Date: Fri, 4 Dec 2009 09:16:40 -0800
From: Chris Wright <chrisw@redhat.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.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>, Chris Wright <chrisw@redhat.com>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 2/9] ksm: let shared pages be swappable
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* 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?

thanks,
-chris
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