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    SubjectRe: [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@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 ;) ]
    KSM is separate driver , it doesn't change anything in the VM but adding
    two helper functions.


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