linux-kernel - [PATCH -V2] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

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Date:   Mon, 24 Apr 2023 14:54:08 +0800
From:   Huang Ying <ying.huang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Nadav Amit <namit@...are.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>,
        kernel test robot <yujie.liu@...el.com>,
        haoxin <xhao@...ux.alibaba.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
Subject: [PATCH -V2] mm,unmap: avoid flushing TLB in batch if PTE is inaccessible

Hi, Andrew,

The version 1 of this patch was merged in mm-unstable branch.  If you
want to move that patch into mm-stable recently, it may be better to
update that patch with this new version firstly.  If you want to do
that after v6.4-rc1, I will rebase this patch and resend it after
v6.4-rc1 is released.

Hi, Amit,

The patch has been changed based on comments from Xin.  I keep your
"reviewed-by" because I think the change is trivial.  But if you think
it's inappropriate,  I will change that.

Best Regards,
Huang, Ying

------------------------------->8------------------------------------------
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/
Fixes: 7e12beb8ca2a ("migrate_pages: batch flushing TLB")
Reported-by: kernel test robot <yujie.liu@...el.com>
Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
Reviewed-by: Nadav Amit <namit@...are.com>
Cc: haoxin <xhao@...ux.alibaba.com>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Hugh Dickins <hughd@...gle.com>
Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
Cc: David Hildenbrand <david@...hat.com>
---
 mm/rmap.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 8632e02661ac..be19232e94f4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -641,10 +641,14 @@ 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, nbatch;
+	bool writable = pte_dirty(pteval);
+
+	if (!pte_accessible(mm, pteval))
+		return;
 
 	arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
 	tlb_ubc->flush_required = true;
@@ -731,7 +735,7 @@ 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)
 {
 }
 
@@ -1582,7 +1586,7 @@ 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);
 			}
@@ -1963,7 +1967,7 @@ 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);
 			}
-- 
2.39.2