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    Subject[PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access_flags()
    From: Rik van Riel <riel@redhat.com>

    If ptep_set_access_flags() is invoked to upgrade access permissions
    on a PTE, there is no security or data integrity reason to do a
    remote TLB flush.

    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>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/mm/pgtable.c | 17 ++++++++++++++++-
    1 file changed, 16 insertions(+), 1 deletion(-)

    Index: tip/arch/x86/mm/pgtable.c
    ===================================================================
    --- tip.orig/arch/x86/mm/pgtable.c
    +++ tip/arch/x86/mm/pgtable.c
    @@ -306,11 +306,26 @@ int ptep_set_access_flags(struct vm_area
    pte_t entry, int dirty)
    {
    int changed = !pte_same(*ptep, entry);
    + /*
    + * If the page used to be inaccessible (_PAGE_PROTNONE), or
    + * this call upgrades the access permissions on the same page,
    + * it is safe to skip the remote TLB flush.
    + */
    + bool flush_remote = false;
    + if (!pte_accessible(*ptep))
    + flush_remote = false;
    + else if (pte_pfn(*ptep) != pte_pfn(entry) ||
    + (pte_write(*ptep) && !pte_write(entry)) ||
    + (pte_exec(*ptep) && !pte_exec(entry)))
    + flush_remote = true;

    if (changed && dirty) {
    *ptep = entry;
    pte_update_defer(vma->vm_mm, address, ptep);
    - flush_tlb_page(vma, address);
    + if (flush_remote)
    + flush_tlb_page(vma, address);
    + else
    + __flush_tlb_one(address);
    }

    return changed;



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