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