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    SubjectRe: [PATCH] mmu_notifier, kvm: Introduce dirty bit tracking in spte and mmu notifier to help KSM dirty bit tracking
    On Thu, Jun 23, 2011 at 07:13:54AM +0800, Nai Xia wrote:
    > I agree on this point. Dirty bit , young bit, is by no means accurate. Even
    > on 4kB pages, there is always a chance that the pte are dirty but the contents
    > are actually the same. Yeah, the whole optimization contains trade-offs and

    Just a side note: the fact the dirty bit would be set even when the
    data is the same is actually a pros, not a cons. If the content is the
    same but the page was written to, it'd trigger a copy on write short
    after merging the page rendering the whole exercise wasteful. The
    cksum plays a double role, it both "stabilizes" the unstable tree, so
    there's less chance of bad lookups, but it also avoids us to merge
    stuff that is written to frequently triggering copy on writes, and the
    dirty bit would also catch overwrites with the same data, something
    the cksum can't do.

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