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

Andrew Morton
2008-11-11T19:12:19Z

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 ;) ]