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