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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 9 Dec 2009 17:12:19 +0100
Andrea Arcangeli <aarcange@redhat.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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