linux-kernel - [PATCH V3] mm/swap: take folio refcount after testing the LRU flag

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Message-Id: <1723542743-32179-1-git-send-email-yangge1116@126.com>
Date: Tue, 13 Aug 2024 17:52:23 +0800
From: yangge1116@....com
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	21cnbao@...il.com,
	david@...hat.com,
	baolin.wang@...ux.alibaba.com,
	yuzhao@...gle.com,
	liuzixing@...on.cn,
	yangge <yangge1116@....com>
Subject: [PATCH V3] mm/swap: take folio refcount after testing the LRU flag

From: yangge <yangge1116@....com>

Whoever passes a folio to __folio_batch_add_and_move() must hold
a reference, otherwise something else would already be messed up.
If the folio is referenced, it will not be freed elsewhere, so we
can safely clear the folio's lru flag. As discussed with David
in [1], we should take the reference after testing the LRU flag,
not before.

Link: https://lore.kernel.org/lkml/d41865b4-d6fa-49ba-890a-921eefad27dd@redhat.com/ [1]
Signed-off-by: yangge <yangge1116@....com>
Acked-by: David Hildenbrand <david@...hat.com>
---
 mm/swap.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

V3:
   Remove sanity check suggested by Yu
V2:
   Add sanity check suggested by David

diff --git a/mm/swap.c b/mm/swap.c
index 67a2467..6b83898 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -226,12 +226,10 @@ static void __folio_batch_add_and_move(struct folio_batch __percpu *fbatch,
 {
 	unsigned long flags;
 
-	folio_get(folio);
-
-	if (on_lru && !folio_test_clear_lru(folio)) {
-		folio_put(folio);
+	if (on_lru && !folio_test_clear_lru(folio))
 		return;
-	}
+
+	folio_get(folio);
 
 	if (disable_irq)
 		local_lock_irqsave(&cpu_fbatches.lock_irq, flags);
-- 
2.7.4