mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

0Day/LKP reported a performance regression for commit 7e12beb
("migrate_pages: batch flushing TLB").  In the commit, the TLB flushing
during page migration is batched.  So, in try_to_migrate_one(),
ptep_clear_flush() is replaced with set_tlb_ubc_flush_pending().  In
further investigation, it is found that the TLB flushing can be avoided in
ptep_clear_flush() if the PTE is inaccessible.  In fact, we can optimize
in similar way for the batched TLB flushing too to improve the
performance.

So in this patch, we check pte_accessible() before
set_tlb_ubc_flush_pending() in try_to_unmap/migrate_one().  Tests show
that the benchmark score of the anon-cow-rand-mt test case of
vm-scalability test suite can improve up to 2.1% with the patch on a Intel
server machine.  The TLB flushing IPI can reduce up to 44.3%.

Link: https://lore.kernel.org/oe-lkp/[email protected]
Link: https://lore.kernel.org/oe-lkp/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7e12beb ("migrate_pages: batch flushing TLB")
Signed-off-by: "Huang, Ying" <[email protected]>
Reported-by: kernel test robot <[email protected]>
Reviewed-by: Nadav Amit <[email protected]>
Reviewed-by: Xin Hao <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/rmap.c

@@ -642,10 +642,14 @@ void try_to_unmap_flush_dirty(void)
 #define TLB_FLUSH_BATCH_PENDING_LARGE			\
 	(TLB_FLUSH_BATCH_PENDING_MASK / 2)
 
-static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
+static void set_tlb_ubc_flush_pending(struct mm_struct *mm, pte_t pteval)
 {
 	struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
 	int batch;
+	bool writable = pte_dirty(pteval);
+
+	if (!pte_accessible(mm, pteval))
+		return;
 
 	arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
 	tlb_ubc->flush_required = true;
@@ -729,7 +733,7 @@ void flush_tlb_batched_pending(struct mm_struct *mm)
 	}
 }
 #else
-static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
+static void set_tlb_ubc_flush_pending(struct mm_struct *mm, pte_t pteval)
 {
 }
 
@@ -1580,7 +1584,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 */
 				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
 
-				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
+				set_tlb_ubc_flush_pending(mm, pteval);
 			} else {
 				pteval = ptep_clear_flush(vma, address, pvmw.pte);
 			}
@@ -1961,7 +1965,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				 */
 				pteval = ptep_get_and_clear(mm, address, pvmw.pte);
 
-				set_tlb_ubc_flush_pending(mm, pte_dirty(pteval));
+				set_tlb_ubc_flush_pending(mm, pteval);
 			} else {
 				pteval = ptep_clear_flush(vma, address, pvmw.pte);
 			}