From nobody Mon Feb 9 15:27:12 2026 Received: from mail-yb1-f201.google.com (mail-yb1-f201.google.com [209.85.219.201]) (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 7ADA71A76A5 for ; Tue, 13 Aug 2024 20:47:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.219.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723582078; cv=none; b=ABQOH6xHTJxLpV85/b6eiHKeksBk0VUZPcNEL6FSbdEYHo+zD9TPQBErobntAf6lGm8VU2esc+Sma2CYMhf0LYkTbZvGcGaw4U77KwX904YB1OKmhpCXU5l9m2F3GnOqvh97BQYefOkr2DJxom0i5uiDX4JJJPSfuxCwIk7x8Z4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723582078; c=relaxed/simple; bh=KPTZtUl0BijCL5/vrgLCA/Byfa3nBjCHG6N9mdvcBjM=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=bhYRwMjXk/d12v4Oo540Y8NNDZ4xugiVDB94cMXg215237bgm9dEhjzuGHfIG0wojB5mCPd+cdly9M/XYBJ0nMm4CtljVpxurtPoE1iRGzgXSE0+OzxqRa1EjwSd1rIm1zbeq36gyjLxBTRxuXCpQaKJPr9w0WofKFByXhLhGIw= 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=TosVy3aD; arc=none smtp.client-ip=209.85.219.201 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="TosVy3aD" Received: by mail-yb1-f201.google.com with SMTP id 3f1490d57ef6-e0be7c74d79so8776303276.0 for ; Tue, 13 Aug 2024 13:47:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1723582076; x=1724186876; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=hmvEk3aWaELcFNHaifCCcbwnogY+IyR2g5JVb5or6kw=; b=TosVy3aDbg3ag/3xrFjPACUDxbfsynvJqjyHB1MSN1nTpA7bXECTlrAwFkdmBkAGUz Xql1idM8Vrtxwwd5O+ZP5QxCyhYZ2pDfjSX1YuYyQjO96lS8LetxGZ/xBPt8cPXUa/qB pjS40boryFdK5D5uq9spvw0fUIXWzpg7rBKbI9k83236C7KRBThnNg4M6QrUocdnzCSQ Lfpdovh5GA8ifXDNs3WE/A/iRnUu75rLD71ssoC/MqgcgDNaqHIGXlWdlVAVR9/FZ3Cv VQggOBRZUfZ57rmsFrH0uu+tC/ucAhZsFFDF4ETmXf+lyAJ3b+qbfw+zVBoc/7hh4H0R WQYg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1723582076; x=1724186876; h=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=hmvEk3aWaELcFNHaifCCcbwnogY+IyR2g5JVb5or6kw=; b=wQwyqCVTwXzKSzDKoyNU5MuVFBKJXnnv6RCVf2A7NuyLhByp+rcaE/pF17tSml5iD0 ygVNc41Cptbr7x7D9vo2hDx7FQZIx4Ggw03kstD4GozopvYDQ4vraaZAG9/MYFeW1cCb fcfQK/AL6128R97+WH7xNjW1GuBwdY5MJ2weGUjSqEsfCBWViMPWcuL9yfBWbvqIH6FW riMRiW/4UwCKxw1Dt9tefEYzjhR4e2Dats80RmtixOuj2Qs8VS6ANG6CZrASiaBbN0LB TVfAQPURMcEcqAo0OqRCkq2wytmoELV4sOFUdaKIjDIjhDpc7GeY1neLnc1iDigEkK7O 7Syw== X-Forwarded-Encrypted: i=1; AJvYcCVN61SW+TAufpWdcCPkjqlO2q3tdgyqX6sV8+mEJILoG1VpWAuFAjV7N2SmiGPSQnKCtENenkgXSZhnVWOy2lOVNMkpUQVM2Ms8lmq6 X-Gm-Message-State: AOJu0Yybb19U0UJanTDcQHZ+WUzhIyv6wCViNTC+2luFaH2SyYBSMMZh 3m/tUEdkZ6tAwShmMSaPUkA3SORNiCjy6xNi6xu7NEKHpBbhuBPGTTJfuGWc7Bhx54blIjKG7fk rJV8rWIgfsA== X-Google-Smtp-Source: AGHT+IHiDHCjFRbPR5+mkcQhIrf9lDLp4M5EcHeImCSP6xD1TY/GSvpgSst1fxEwRAEkFihKnMepRjSKepm4bA== X-Received: from kinseyct.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:46b]) (user=kinseyho job=sendgmr) by 2002:a25:c788:0:b0:e03:62dc:63de with SMTP id 3f1490d57ef6-e1155ada202mr39205276.6.1723582076373; Tue, 13 Aug 2024 13:47:56 -0700 (PDT) Date: Tue, 13 Aug 2024 20:47:12 +0000 In-Reply-To: <20240813204716.842811-1-kinseyho@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240813204716.842811-1-kinseyho@google.com> X-Mailer: git-send-email 2.46.0.76.ge559c4bf1a-goog Message-ID: <20240813204716.842811-3-kinseyho@google.com> Subject: [PATCH mm-unstable v2 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, 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 --- include/linux/memcontrol.h | 2 +- mm/memcontrol.c | 18 +----------------- 2 files changed, 2 insertions(+), 18 deletions(-) diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index 90ecd2dbca06..1aaed2f1f6ae 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -75,7 +75,7 @@ struct lruvec_stats_percpu; struct lruvec_stats; =20 struct mem_cgroup_reclaim_iter { - struct mem_cgroup *position; + struct mem_cgroup __rcu *position; /* scan generation, increased every round-trip */ unsigned int generation; }; diff --git a/mm/memcontrol.c b/mm/memcontrol.c index dacf4fec4541..1688aae3b1b4 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1052,20 +1052,7 @@ 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 rcu_dereference(iter->position); } else if (prev) { pos =3D prev; } @@ -1106,9 +1093,6 @@ 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 2.46.0.76.ge559c4bf1a-goog