linux-kernel - [PATCH] mm/rmap: optimize folio_move_anon

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Date: Sun, 12 May 2024 20:35:55 +0800
From: Chen Taotao <chentt10@...natelecom.cn>
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] mm/rmap: optimize folio_move_anon_rmap()

When a folio belongs exclusively to one process after a COW event,
folio_move_anon_rmap() always moves the folio into the anon_vma
belongs only to this process.

However, if the folio already belongs to the anon_vma of the this
process, we don't need to move it again. In this case, we first
check if the folio already belongs to the anna_vma of the this
process, and only move it if it does not.

The above changes may improve the performance of vm faults in some
scenarios, because the performance loss caused by WRITE_ONCE() is
much more than the performance loss caused by add a judgment.

Signed-off-by: Chen Taotao <chentt10@...natelecom.cn>
---
 mm/rmap.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 3746a5531018..5720cb78162d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1216,12 +1216,15 @@ void folio_move_anon_rmap(struct folio *folio, struct vm_area_struct *vma)
 	VM_BUG_ON_VMA(!anon_vma, vma);

 	anon_vma += PAGE_MAPPING_ANON;
+
 	/*
-	 * Ensure that anon_vma and the PAGE_MAPPING_ANON bit are written
-	 * simultaneously, so a concurrent reader (eg folio_referenced()'s
-	 * folio_test_anon()) will not see one without the other.
+	 * If anon_vma != folio->mapping ensure that anon_vma and the
+	 * PAGE_MAPPING_ANON bit are writtensimultaneously, so a concurrent
+	 * reader (eg folio_referenced()'s folio_test_anon()) will not see
+	 * one without the other.
 	 */
-	WRITE_ONCE(folio->mapping, anon_vma);
+	if (anon_vma != folio->mapping)
+		WRITE_ONCE(folio->mapping, anon_vma);
 }

 /**
-- 
2.27.0