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    Subject[PATCH 33/43] x86: mm: only do a local tlb flush in ptep_set_access_flags()
    Date
    From: Rik van Riel <riel@redhat.com>

    The function ptep_set_access_flags() is only ever invoked to set access
    flags or add write permission on a PTE. The write bit is only ever set
    together with the dirty bit.

    Because we only ever upgrade a PTE, it is safe to skip flushing entries on
    remote TLBs. The worst that can happen is a spurious page fault on other
    CPUs, which would flush that TLB entry.

    Lazily letting another CPU incur a spurious page fault occasionally is
    (much!) cheaper than aggressively flushing everybody else's TLB.

    Signed-off-by: Rik van Riel <riel@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/mm/pgtable.c | 9 ++++++++-
    1 file changed, 8 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
    index 8573b83..be3bb46 100644
    --- a/arch/x86/mm/pgtable.c
    +++ b/arch/x86/mm/pgtable.c
    @@ -301,6 +301,13 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd)
    free_page((unsigned long)pgd);
    }

    +/*
    + * Used to set accessed or dirty bits in the page table entries
    + * on other architectures. On x86, the accessed and dirty bits
    + * are tracked by hardware. However, do_wp_page calls this function
    + * to also make the pte writeable at the same time the dirty bit is
    + * set. In that case we do actually need to write the PTE.
    + */
    int ptep_set_access_flags(struct vm_area_struct *vma,
    unsigned long address, pte_t *ptep,
    pte_t entry, int dirty)
    @@ -310,7 +317,7 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
    if (changed && dirty) {
    *ptep = entry;
    pte_update_defer(vma->vm_mm, address, ptep);
    - flush_tlb_page(vma, address);
    + __flush_tlb_one(address);
    }

    return changed;
    --
    1.7.9.2


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