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    SubjectRe: [patches] [PATCH] [28/34] i386: pte xchg optimization
    >>> Andi Kleen <ak@suse.de> 30.04.07 17:50 >>>
    >
    >From: Zachary Amsden <zach@vmware.com>
    >
    >In situations where page table updates need only be made locally, and there is
    >no cross-processor A/D bit races involved, we need not use the heavyweight
    >xchg instruction to atomically fetch and clear page table entries. Instead,
    >we can just read and clear them directly.

    I always wondered why the xchg is necessary here at all. If the process of
    tearing down a page table entry has started, other users of the mapped
    linear address are broken anyway - why is it necessary to still monitor the
    effect they may have on the A/D bits, unless this is a transient tear down?

    Checking for the uses of ptep_get_and_clear, I would judge that the use in
    change_pte_range() may in fact need the xchg, but the uses in
    vunmap_pte_range() and zap_pte_range() shouldn't.

    Jan

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