linux-kernel - Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking

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Message-ID: <BANLkTi=oNXAmYaC44EYnNRU2_4KFoJjubw@mail.gmail.com>
Date:	Thu, 23 Jun 2011 08:42:24 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Avi Kivity <avi@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-mm <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte
 and mmu notifier to help KSM dirty bit tracking

On Thu, Jun 23, 2011 at 8:00 AM, Rik van Riel <riel@...hat.com> wrote:
> On 06/22/2011 07:37 PM, Nai Xia wrote:
>
>> On 2MB pages, I'd like to remind you and Rik that ksmd currently splits
>> huge pages before their sub pages gets really merged to stable tree.
>
> Your proposal appears to add a condition that causes ksmd to skip
> doing that, which can cause the system to start using swap instead
> of sharing memory.

Hmm, yes, no swapping. So we should make the checksum default
for huge pages, right?

>
>> So when there are many 2MB pages each having a 4kB subpage
>> changed for all time, this is already a concern for ksmd to judge
>> if it's worthwhile to split 2MB page and get its sub-pages merged.
>> I think the policy for ksmd in a system should be "If you cannot do sth
>> good,
>> at least do nothing evil". So I really don't think we can satisfy _all_
>> people.
>> Get a general method and give users one or two knobs to tune it when they
>> are the corner cases. How do  you think of my proposal ?
>
> I think your proposal makes sense for 4kB pages, but the ksmd
> policy for 2MB pages probably needs to be much more aggressive.

I now agree with you on the whole point. Let's fall back to checksum
Thanks for make my mind clear! :)

And shall we provide a interface to users if he _really_ what to judge the dirty
bit from the pmd level? I think we should agree to one point before I
misunderstand
you and spam you with my next submission :P


And thanks for your time viewing!

-Nai


>
> --
> All rights reversed
>
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