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    SubjectRe: [PATCH 0/2] x86/mm: remove arch-specific PTE/PMD get-and-clear functions
    On Wed, Jun 13, 2012 at 11:20:43AM +0100, David Vrabel wrote:
    > This series removes the x86-specific implementation of
    > ptep_get_and_clear() and pmdp_get_and_clear().
    >
    > The principal reason for this is it allows Xen paravitualized guests
    > to batch the PTE clears which is a significant performance
    > optimization of munmap() and mremap() -- the number of entries into
    > the hypervisor is reduced by about a factor of about 30 (60 in 32-bit
    > guests) for munmap().
    >
    > There may be minimal gains on native and KVM guests due to the removal
    > of the locked xchg.

    What about lguest?
    >
    > Removal of arch-specific functions where generic ones are suitable
    > seems to be a generally useful thing to me.
    >
    > The full reasoning for why this is safe is included in the commit
    > message of patch 1 but to summarize. The atomic get-and-clear does
    > not guarantee that the latest dirty/accessed bits are returned as TLB
    > as there is a still a window after the get-and-clear and before the
    > TLB flush that the bits may be updated on other processors. So, user
    > space applications accessing pages that are being unmapped or remapped
    > already have unpredictable behaviour.
    >
    > David


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