linux-kernel - [PATCH 1/2] mm: Add missing release barrier on PGDAT_RECLAIM

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Message-Id: <20250312141014.129725-1-mathieu.desnoyers@efficios.com>
Date: Wed, 12 Mar 2025 10:10:13 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Nicholas Piggin <npiggin@...il.com>,
	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,
	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	linux-mm@...ck.org
Subject: [PATCH 1/2] mm: Add missing release barrier on PGDAT_RECLAIM_LOCKED unlock

The PGDAT_RECLAIM_LOCKED bit is used to provide mutual exclusion of
node reclaim for struct pglist_data using a single bit.

It is "locked" with a test_and_set_bit (similarly to a try lock) which
provides full ordering with respect to loads and stores done within
__node_reclaim().

It is "unlocked" with clear_bit(), which does not provide any ordering
with respect to loads and stores done before clearing the bit.

The lack of clear_bit() memory ordering with respect to stores within
__node_reclaim() can cause a subsequent CPU to fail to observe stores
from a prior node reclaim. This is not an issue in practice on TSO (e.g.
x86), but it is an issue on weakly-ordered architectures (e.g. arm64).

Fix this by using clear_bit_unlock rather than clear_bit to clear
PGDAT_RECLAIM_LOCKED with a release memory ordering semantic.

This provides stronger memory ordering (release rather than relaxed).

Fixes: d773ed6b856a ("mm: test and set zone reclaim lock before starting reclaim")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Andrea Parri <parri.andrea@...il.com>
Cc: Will Deacon <will@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Jade Alglave <j.alglave@....ac.uk>
Cc: Luc Maranget <luc.maranget@...ia.fr>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-mm@...ck.org
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c22175120f5d..d7e27d5c24e7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -7571,7 +7571,7 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
 		return NODE_RECLAIM_NOSCAN;
 
 	ret = __node_reclaim(pgdat, gfp_mask, order);
-	clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
+	clear_bit_unlock(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
 
 	if (ret)
 		count_vm_event(PGSCAN_ZONE_RECLAIM_SUCCESS);
-- 
2.25.1