From nobody Sat Feb 7 19:45:25 2026 Received: from smtpout.efficios.com (smtpout.efficios.com [158.69.130.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DAB33221DB9 for ; Fri, 7 Mar 2025 19:30:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=158.69.130.18 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741375854; cv=none; b=VW7NKKXjiQ4K+lIgF+FvTNj82CmKA47jY5RszpceUdWfNWtV79TIDMUy38fGD+EvnWOUAUURi7iKqe1GQsQ7N3D3asxBJ6hZ3dvijO3PBX352yFa2GR8ipsQQGgSwhkPTftoYz1ZtkIlTXHPDNQ7UFcVAFGhDVk9D9EJj4cypKk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741375854; c=relaxed/simple; bh=I7qUEBsf9z9DvFVeTUwEL2QF/mwYSpxIB5OX23QFtcU=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=eb15sAPBV51/VG3imYL8kIXLRitXVrG3xdb10IOYZZKdoXJLEVEKFBm2aeUUV/m3SEwIYFPhRs+ljqNih/I1NZ9uuhILMkt090F5SooZShEVyVpRzEPc1OuEMuFkpthCAzHR3TXAxgPalXdv71UTam4Un3hZHGo0m3gDVUUS+hE= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b=SFFHlPiQ; arc=none smtp.client-ip=158.69.130.18 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="SFFHlPiQ" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1741375851; bh=I7qUEBsf9z9DvFVeTUwEL2QF/mwYSpxIB5OX23QFtcU=; h=From:To:Cc:Subject:Date:From; b=SFFHlPiQ1dVRlkA83oEwuD3d/a54xPfyT7af1B56LbjBkK/qQx8EPBooVir5Xs/PP YxGzct4ODrmkgHTHkZwnmo0ATuRRc/KHU8rW3KKL6C4Imra6Y4zOIDMFvysDE/jOfb a0oimfFSYWEs5uXP9CkxbuL5QXmK6LtMlYVpaoabLXi1NzV7LLLRm6g/mYyAFiivtO 0prauRBELPV8Pqk0Yohajy7xlCeMI2qQHPAEYV4TlMwTZkIaytCPrRj32k7Y4zLiAC CUxskCVt+Bm9uC4wlzCauOiIw2ew0yIQRtXv00qPOUJAcGlSjwslPr/VYrOprNy4r8 zbwl56EPgfkrA== Received: from localhost.localdomain (96-127-217-162.qc.cable.ebox.net [96.127.217.162]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4Z8bx71SLyz1gPP; Fri, 7 Mar 2025 14:30:51 -0500 (EST) From: Mathieu Desnoyers To: Andrew Morton Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , Lorenzo Stoakes , Matthew Wilcox , Alan Stern , Andrea Parri , Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , "Paul E. McKenney" , linux-mm@kvack.org Subject: [RFC PATCH] mm: Add missing release barrier on PGDAT_RECLAIM_LOCKED unlock Date: Fri, 7 Mar 2025 14:30:47 -0500 Message-Id: <20250307193047.66079-1-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.25.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 with following changes: A) Use 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). B) Use test_and_set_bit_lock rather than test_and_set_bit to test-and-set PGDAT_RECLAIM_LOCKED with an acquire memory ordering semantic. This changes the "lock" acquisition from a full barrier to an acquire memory ordering, which is weaker. The acquire semi-permeable barrier paired with the release on unlock is sufficient for this mutual exclusion use-case. Signed-off-by: Mathieu Desnoyers Cc: Andrew Morton Cc: Lorenzo Stoakes Cc: Matthew Wilcox Cc: Alan Stern Cc: Andrea Parri Cc: Will Deacon Cc: Peter Zijlstra Cc: Boqun Feng Cc: Nicholas Piggin Cc: David Howells Cc: Jade Alglave Cc: Luc Maranget Cc: "Paul E. McKenney" Cc: linux-mm@kvack.org --- mm/vmscan.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index c22175120f5d..021b25bdba91 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -7567,11 +7567,11 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t g= fp_mask, unsigned int order) if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id !=3D numa_node_id= ()) return NODE_RECLAIM_NOSCAN; =20 - if (test_and_set_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags)) + if (test_and_set_bit_lock(PGDAT_RECLAIM_LOCKED, &pgdat->flags)) return NODE_RECLAIM_NOSCAN; =20 ret =3D __node_reclaim(pgdat, gfp_mask, order); - clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags); + clear_bit_unlock(PGDAT_RECLAIM_LOCKED, &pgdat->flags); =20 if (ret) count_vm_event(PGSCAN_ZONE_RECLAIM_SUCCESS); --=20 2.25.1