arm64: pgtable: Fix pte_accessible()

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.

Cc: <stable@vger.kernel.org>
Fixes: 76c714be ("arm64: pgtable: implement pte_accessible()")
Reported-by: Yu Zhao <yuzhao@google.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20201120143557.6715-2-will@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

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))
 
@@ -124,9 +122,12 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
  * Could the pte be present in the TLB? We must check mm_tlb_flush_pending
  * so that we don't erroneously return false for pages that have been
  * remapped as PROT_NONE but are yet to be flushed from the TLB.
+ * Note that we can't make any assumptions based on the state of the access
+ * flag, since ptep_clear_flush_young() elides a DSB when invalidating 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