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    SubjectRe: question on NUMA page migration
    On 10/19/2012 11:53 PM, Rik van Riel wrote:
    > Hi Andrea, Peter,
    >
    > I have a question on page refcounting in your NUMA
    > page migration code.
    >
    > In Peter's case, I wonder why you introduce a new
    > MIGRATE_FAULT migration mode. If the normal page
    > migration / compaction logic can do without taking
    > an extra reference count, why does your code need it?

    Hi Rik van Riel,

    This is which part of codes? Why I can't find MIGRATE_FAULT in latest
    v3.7-rc2?

    Regards,
    Chen

    >
    > In Andrea's case, we have a comment suggesting an
    > extra refcount is needed, immediately followed by
    > a put_page:
    >
    > /*
    > * Pin the head subpage at least until the first
    > * __isolate_lru_page succeeds (__isolate_lru_page pins it
    > * again when it succeeds). If we unpin before
    > * __isolate_lru_page successd, the page could be freed and
    > * reallocated out from under us. Thus our previous checks on
    > * the page, and the split_huge_page, would be worthless.
    > *
    > * We really only need to do this if "ret > 0" but it doesn't
    > * hurt to do it unconditionally as nobody can reference
    > * "page" anymore after this and so we can avoid an "if (ret >
    > * 0)" branch here.
    > */
    > put_page(page);
    >
    > This also confuses me.
    >
    > If we do not need the extra refcount (and I do not
    > understand why NUMA migrate-on-fault needs one more
    > refcount than normal page migration), we can get
    > rid of the MIGRATE_FAULT mode.
    >
    > If we do need the extra refcount, why is normal
    > page migration safe? :)
    >



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