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    SubjectRe: [PATCH 0/2] x86/mm: remove arch-specific PTE/PMD get-and-clear functions
    On 13/06/12 15:04, Konrad Rzeszutek Wilk wrote:
    > 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?

    As I note in the description of patch 1:

    "There may be a performance regression with lguest guests as
    an optimization for avoiding calling pte_update() when doing a full
    teardown of an mm is removed."

    I don't know how much this performance regression would be or if the
    performance of lguest guests is something people care about.

    We could have an x86-specific ptep_get_and_clear_full() which looks like:

    pte_t ptep_get_and_clear_full(
    struct mm_struct *mm, unsigned long addr, pte_t *ptep,
    int full)
    pte_t pte = *ptep;

    pte_clear(mm, address, ptep);
    if (!full)
    pte_update(mm, addr, ptep);

    return pte;

    Which would have all the performance benefits of the proposed patch
    without the performance regression with 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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