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

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Message-Id: <20091210085413.0fe4369e.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 10 Dec 2009 08:54:13 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Chris Wright <chrisw@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.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 Wed, 9 Dec 2009 17:12:19 +0100
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Wed, Dec 09, 2009 at 09:43:31AM +0900, KAMEZAWA Hiroyuki wrote:
> > 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.
> 
> The whole memory marked MADV_MERGEABLE by KVM is also marked
> MADV_DONTFORK, so if KVM was to fork (and if it did, if it wasn't for
> MADV_DONTFORK, it would also trigger all O_DIRECT vs fork race
> conditions too, as KVM is one of the many apps that uses threads and
> O_DIRECT - we try not to fork though but we sure did in the past), no
> slowdown could ever happen in mapcount because of KSM, all KSM pages
> aren't visibile by child.
> 
> It's still something to keep in mind for other KSM users, but I don't
> think mapcount is big deal if compared to the risk of triggering COWs
> later on those pages, in general KSM is all about saving tons of
> memory at the expense of some CPU cycle (kksmd, cows, mapcount with
> parallel forks etc...).
> 
Okay, thank you for kindlt explanation.

and sorry for noise.

Thanks,
-Kame

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