linux-kernel - Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux

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Date:	Tue, 11 Nov 2008 21:18:23 +0200
From:	Izik Eidus <ieidus@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kvm@...r.kernel.org, aarcange@...hat.com,
	chrisw@...hat.com
Subject: Re: [PATCH 0/4] ksm - dynamic page sharing driver for linux

Andrew Morton wrote:
> On Tue, 11 Nov 2008 20:48:16 +0200
> Avi Kivity <avi@...hat.com> wrote:
>
>   
>> Andrew Morton wrote:
>>     
>>> The whole approach seems wrong to me.  The kernel lost track of these
>>> pages and then we run around post-facto trying to fix that up again. 
>>> Please explain (for the changelog) why the kernel cannot get this right
>>> via the usual sharing, refcounting and COWing approaches.
>>>   
>>>       
>> For kvm, the kernel never knew those pages were shared.  They are loaded 
>> from independent (possibly compressed and encrypted) disk images.  These 
>> images are different; but some pages happen to be the same because they 
>> came from the same installation media.
>>     
>
> What userspace-only changes could fix this?  Identify the common data,
> write it to a flat file and mmap it, something like that?
>
>   
>> For OpenVZ the situation is less clear, but if you allow users to 
>> independently upgrade their chroots you will eventually arrive at the 
>> same scenario (unless of course you apply the same merging strategy at 
>> the filesystem level).
>>     
>
> hm.
>
> There has been the occasional discussion about idenfifying all-zeroes
> pages and scavenging them, repointing them at the zero page.  Could
> this infrastructure be used for that?  (And how much would we gain from
> it?)
>
> [I'm looking for reasons why this is more than a muck-up-the-vm-for-kvm
> thing here ;) ]
KSM is separate driver , it doesn't change anything in the VM but adding 
two helper functions.
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