linux-kernel - [PATCH] mm/vmscan: drop checking if _deferred_list is empty before using TTU

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Date: Sun, 30 Jun 2024 11:41:55 +1200
From: Barry Song <21cnbao@...il.com>
To: akpm@...ux-foundation.org,
	linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	Barry Song <v-songbaohua@...o.com>,
	David Hildenbrand <david@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>
Subject: [PATCH] mm/vmscan: drop checking if _deferred_list is empty before using TTU_SYNC

From: Barry Song <v-songbaohua@...o.com>

The optimization of list_empty(&folio->_deferred_list) aimed to prevent
increasing the PTL duration when a large folio is partially unmapped,
for example, from subpage 0 to subpage (nr - 2).

But Ryan's commit 5ed890ce5147 ("mm: vmscan: avoid split during
shrink_folio_list()") actually splits this kind of large folios. This
makes the "optimization" useless.

Signed-off-by: Barry Song <v-songbaohua@...o.com>
Cc: David Hildenbrand <david@...hat.com>
Cc: Ryan Roberts <ryan.roberts@....com>
---
 -v1
 * I remember David and Ryan once suggested that this check could be dropped
   while the patch was being pulled into mm-unstable. However, for some reason,
   I forgot to request squashing this change.

 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3d4c681c6d40..0761f91b407f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1291,7 +1291,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 			 * try_to_unmap acquire PTL from the first PTE,
 			 * eliminating the influence of temporary PTE values.
 			 */
-			if (folio_test_large(folio) && list_empty(&folio->_deferred_list))
+			if (folio_test_large(folio))
 				flags |= TTU_SYNC;
 
 			try_to_unmap(folio, flags);
-- 
2.34.1