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    SubjectRe: [PATCH 1/6] arm64: pgtable: Fix pte_accessible()
    On Fri, Nov 20, 2020 at 02:35:52PM +0000, Will Deacon wrote:
    > pte_accessible() is used by ptep_clear_flush() to figure out whether TLB
    > invalidation is necessary when unmapping pages for reclaim. Although our
    > implementation is correct according to the architecture, returning true
    > only for valid, young ptes in the absence of racing page-table
    > modifications, this is in fact flawed due to lazy invalidation of old
    > ptes in ptep_clear_flush_young() where we elide the expensive DSB
    > instruction for completing the TLB invalidation.
    >
    > Rather than penalise the aging path, adjust pte_accessible() to return
    > true for any valid pte, even if the access flag is cleared.

    The chance of a system hitting reclaim is proportional to how long
    it's been running, and that of having mapped but yet to be accessed
    PTEs is reciprocal, so to speak. I don't reboot my devices everyday,
    and therefore:

    Acked-by: Yu Zhao <yuzhao@google.com>

    > Cc: <stable@vger.kernel.org>
    > Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()")
    > Reported-by: Yu Zhao <yuzhao@google.com>
    > Signed-off-by: Will Deacon <will@kernel.org>
    > ---
    > arch/arm64/include/asm/pgtable.h | 4 +---
    > 1 file changed, 1 insertion(+), 3 deletions(-)
    >
    > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
    > index 4ff12a7adcfd..1bdf51f01e73 100644
    > --- a/arch/arm64/include/asm/pgtable.h
    > +++ b/arch/arm64/include/asm/pgtable.h
    > @@ -115,8 +115,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
    > #define pte_valid(pte) (!!(pte_val(pte) & PTE_VALID))
    > #define pte_valid_not_user(pte) \
    > ((pte_val(pte) & (PTE_VALID | PTE_USER)) == PTE_VALID)
    > -#define pte_valid_young(pte) \
    > - ((pte_val(pte) & (PTE_VALID | PTE_AF)) == (PTE_VALID | PTE_AF))
    > #define pte_valid_user(pte) \
    > ((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER))
    >
    > @@ -126,7 +124,7 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
    > * remapped as PROT_NONE but are yet to be flushed from the TLB.
    > */
    > #define pte_accessible(mm, pte) \
    > - (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte))
    > + (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
    >
    > /*
    > * p??_access_permitted() is true for valid user mappings (subject to the
    > --
    > 2.29.2.454.gaff20da3a2-goog
    >

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