+ mmunmap-avoid-flushing-tlb-in-batch-if-pte-is-inaccessible.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm,unmap: avoid flushing TLB in batch if PTE is inaccessible
has been added to the -mm mm-unstable branch.  Its filename is
     mmunmap-avoid-flushing-tlb-in-batch-if-pte-is-inaccessible.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mmunmap-avoid-flushing-tlb-in-batch-if-pte-is-inaccessible.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Huang Ying <ying.huang@intel.com>
Subject: mm,unmap: avoid flushing TLB in batch if PTE is inaccessible
Date: Mon, 24 Apr 2023 14:54:08 +0800

0Day/LKP reported a performance regression for commit 7e12beb8ca2a
("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/202303192325.ecbaf968-yujie.liu@intel.com
Link: https://lore.kernel.org/oe-lkp/ab92aaddf1b52ede15e2c608696c36765a2602c1.camel@intel.com/
Link: https://lkml.kernel.org/r/20230424065408.188498-1-ying.huang@intel.com
Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reported-by: kernel test robot <yujie.liu@intel.com>
Reviewed-by: Nadav Amit <namit@vmware.com>
Reviewed-by: Xin Hao <xhao@linux.alibaba.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/rmap.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

--- a/mm/rmap.c~mmunmap-avoid-flushing-tlb-in-batch-if-pte-is-inaccessible
+++ a/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
 	}
 }
 #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 foli
 				 */
 				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 fo
 				 */
 				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);
 			}
_

Patches currently in -mm which might be from ying.huang@intel.com are

mmunmap-avoid-flushing-tlb-in-batch-if-pte-is-inaccessible.patch