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

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 11 Nov 2008 20:48:16 +0200
Avi Kivity <avi@redhat.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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