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

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Date:	Tue, 11 Nov 2008 11:11:10 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	ieidus@...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

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 ;) ]
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