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    SubjectRe: [PATCH 2/9] ksm: let shared pages be swappable
    * KAMEZAWA Hiroyuki (kamezawa.hiroyu@jp.fujitsu.com) wrote:
    > On Fri, 4 Dec 2009 09:16:40 -0800
    > Chris Wright <chrisw@redhat.com> wrote:
    > > * KAMEZAWA Hiroyuki (kamezawa.hiroyu@jp.fujitsu.com) wrote:
    > > > Hmm, can't we use ZERO_PAGE we have now ?
    > > > If do so,
    > > > - no mapcount check
    > > > - never on LRU
    > > > - don't have to maintain shared information because ZERO_PAGE itself has
    > > > copy-on-write nature.
    > >
    > > It's a somewhat special case, but wouldn't it be useful to have a generic
    > > method to recognize this kind of sharing since it's a generic issue?
    >
    > I just remembered that why ZERO_PAGE was removed (in past). It was becasue
    > cache-line ping-pong at fork beacause of page->mapcount. And KSM introduces
    > zero-pages which have mapcount again. If no problems in realitsitc usage of
    > KVM, ignore me.

    KVM is not exactly fork heavy (although it's not the only possible user
    of KSM). And the CoW path has fault + copy already.

    Semi-related...it can make good sense to make the KSM trees per NUMA
    node. Would mean things like page of zeroes would collapse to number
    of NUMA nodes pages rather than a single page, but has the benefit of
    not adding remote access (although, probably more useful for text pages
    than zero pages).

    thanks,
    -chris


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