From nobody Thu Dec 18 22:16:22 2025 Received: from mail-pg1-f201.google.com (mail-pg1-f201.google.com [209.85.215.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B993A8F58 for ; Thu, 5 Sep 2024 00:31:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725496312; cv=none; b=LD00j1Pv2OWWmZUi61VNv1Q9BeueMyKgqPWLTPryf1gbyY+O0/sIIzhpYNnsXao3VDHtW9lRgFxF6nJJeYL+lDeHVWs8cFdn+KhuXP1HdjaUn1/QN2XHZl+tz/6c6B4D8Unx31MjSzapueWx2RHK/K7D2I1lMYMduoMxKcyEFj0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725496312; c=relaxed/simple; bh=fk2/lGT/4GsriM/5ZGMKLoZ8zzkgaZL8fCE8hrnHHzY=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=MwUa8ASrMrsZc4/AobLU2hSra3syEAJfLuZauxB0mne31R4j41sH6Aeeh2NKIzAIM29R2Id71USbtYYz8/ZFJ/wXcH2bXk8u5smBtJRaEBffdkgwA1QOTYfVabdT5smzB3WuKxlK6UjLxow6bLgjEmCg6Ph4naKhsHdxKMG/hXI= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--kinseyho.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=hGwvOgz2; arc=none smtp.client-ip=209.85.215.201 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--kinseyho.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="hGwvOgz2" Received: by mail-pg1-f201.google.com with SMTP id 41be03b00d2f7-7cd98f27becso285541a12.3 for ; Wed, 04 Sep 2024 17:31:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1725496310; x=1726101110; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:from:to:cc:subject:date:message-id :reply-to; bh=sqTDHTCRiwsvIU2qvmmvg5mPCxzlppgliuAtX0a739I=; b=hGwvOgz2Okmct2aS5wPnYk+3JMzJDdVKq9LpGVZbThH/5uzZVGOnjnNea4TDiU6css yufw96/rt124h8rgD8cbFQNAK8UjxF2fW7ImIE/iybSA6L+7aE6W0rxjotuTD/yvicGH 6XMgNdWGvWQQDL2hheChL1N4p72iHSkqCZw9Dc68+VgQEYQ+Z79eoPYFD49JaVyJPTZ+ kzrzmTuLLNh8/GyaqYigqpEWUJ3yI2GbcR/EEVk3vboZo9xRte+kIQuXelckWsm2EHRY yaBeWTLw0D6wYjXXOKe06uGpnWpLXJMYIQzxdGZkYqbjsowslDz4WB2ewOGzP2souVfh HYyg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1725496310; x=1726101110; h=content-transfer-encoding:cc:to:from:subject:message-id:references :mime-version:in-reply-to:date:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to; bh=sqTDHTCRiwsvIU2qvmmvg5mPCxzlppgliuAtX0a739I=; b=ZA3dGjwVdo9HDPiOohQbD6tSZ5+W//6RJXMG2iRXNZAl8hSUzuq8iCOYAu9WlcgxfL 1W7ldb2xGlOe4Jqe+/zMAoigh0j+UNSLZd4fokNMVS1pa68uB7y990XFVDWMjToQzl5t S3giMdqK0pQLi9KDLE/VYU3sBayDtjDfewkJkzUh+jdbpT/C573QE3wRaFDEQNEq7+X+ tDABj3GeFaTyJyEnkX4jy6dUGRJ1AnTa4/VyJUVnk7h3+uRi0c3ev+61RrpIk5m7k3Kl dz1llBNcxhUyn8BYM9hWKD1wub6kXFMm34zpqyGd4tVypH5DgGoeVH3+eyeDTNP+eEy5 xeOg== X-Forwarded-Encrypted: i=1; AJvYcCVLhQ4wXach1u4QV4eEkB2KX8PbP9Aff5o3pfjKqcds7S4kZTdZspMTvxrG1QhdcJ6R1xtWVP948Y0WLe8=@vger.kernel.org X-Gm-Message-State: AOJu0Yy275t70sM+8R30/QqOPCRUUD4OXfVYs93uoLbBSKMctrWejpEx nm6sOqUv6WZCvEmSYnRq8Zfd+cfOLMXxkdNRP3uTemX6r2P1hqeSoe5jeFLraSR47rnWGwMMcCG X7nCC8ZcYMQ== X-Google-Smtp-Source: AGHT+IEOy0bHsy4chODc/dIKYmpQwZgXe72gHtWSiqNJNS/vQMmLAYyUUqkUY8wRavMe2EY8pkrILzNoQwJIVw== X-Received: from kinseyct.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:46b]) (user=kinseyho job=sendgmr) by 2002:a63:e34e:0:b0:6ea:d240:23b0 with SMTP id 41be03b00d2f7-7d4c1019e5emr56861a12.4.1725496309928; Wed, 04 Sep 2024 17:31:49 -0700 (PDT) Date: Thu, 5 Sep 2024 00:30:51 +0000 In-Reply-To: <20240905003058.1859929-1-kinseyho@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240905003058.1859929-1-kinseyho@google.com> X-Mailer: git-send-email 2.46.0.469.g59c65b2a67-goog Message-ID: <20240905003058.1859929-3-kinseyho@google.com> Subject: [PATCH mm-unstable v4 2/5] mm: don't hold css->refcnt during traversal From: Kinsey Ho To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Yosry Ahmed , Roman Gushchin , Johannes Weiner , Michal Hocko , Shakeel Butt , Muchun Song , Tejun Heo , Zefan Li , mkoutny@suse.com, "T . J . Mercier" , Hugh Dickins , Kinsey Ho Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" To obtain the pointer to the next memcg position, mem_cgroup_iter() currently holds css->refcnt during memcg traversal only to put css->refcnt at the end of the routine. This isn't necessary as an rcu_read_lock is already held throughout the function. The use of the RCU read lock with css_next_descendant_pre() guarantees that sibling linkage is safe without holding a ref on the passed-in @css. Remove css->refcnt usage during traversal by leveraging RCU. Signed-off-by: Kinsey Ho Reviewed-by: T.J. Mercier Cc: Johannes Weiner Cc: Michal Hocko Cc: Michal Koutn=C3=BD Cc: Muchun Song Cc: Roman Gushchin Cc: Shakeel Butt Cc: Tejun Heo Cc: Yosry Ahmed Cc: Zefan Li Cc: Hugh Dickins --- mm/memcontrol.c | 18 +----------------- 1 file changed, 1 insertion(+), 17 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index dbc1d43a5c4c..cdd324812b55 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1015,20 +1015,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup= *root, else if (reclaim->generation !=3D iter->generation) goto out_unlock; =20 - while (1) { - pos =3D READ_ONCE(iter->position); - if (!pos || css_tryget(&pos->css)) - break; - /* - * css reference reached zero, so iter->position will - * be cleared by ->css_released. However, we should not - * rely on this happening soon, because ->css_released - * is called from a work queue, and by busy-waiting we - * might block it. So we clear iter->position right - * away. - */ - (void)cmpxchg(&iter->position, pos, NULL); - } + pos =3D READ_ONCE(iter->position); } else if (prev) { pos =3D prev; } @@ -1069,9 +1056,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup = *root, */ (void)cmpxchg(&iter->position, pos, memcg); =20 - if (pos) - css_put(&pos->css); - if (!memcg) iter->generation++; } --=20 2.46.0.469.g59c65b2a67-goog