From nobody Wed Dec 17 05:46:50 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 66EAA16A95E for ; Wed, 24 Jul 2024 19:02:21 +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=1721847742; cv=none; b=QY/tQXFFF07zrwKlhGJ7IWrRL0XdTggbd23eb1wAmEMuENsdKz9l4e3s5Sp+HyD4zVr9EAFzy1gekRQ3c7zN290m7bIlqjeuJhA09A5hajrHZgbfFqGSrZJK3tUwN4nnMrT6964SNcasXMOqOPzH15CcSk/iipt7Ci4V/SL7udw= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721847742; c=relaxed/simple; bh=dnIpnYFNL7tcZIOMfeqJiwj7BfmA8twPI/chQqFF0z0=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=bkEzVhtyTgTq3ymwlQyemQAs0KDZGyj+jJpl+oZUj65OSrxEluZqkKa9eo1q5yk7AEYrLD7A2P1vnxhc21RjW9v/3bHKHQ14Rqppy4i0WCR9muL9L6Fni1hly0I7vYEQWu3dicX1hi02DJLiPhBZiHKs1e/BJkcATGPaQYUlGh8= 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=qIXZx/sC; 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="qIXZx/sC" Received: by mail-pg1-f201.google.com with SMTP id 41be03b00d2f7-7908626a4a6so1129814a12.1 for ; Wed, 24 Jul 2024 12:02:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1721847740; x=1722452540; 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=iLDZhQ0id4uDXJarhBRTOmxWLKOk3mwU62F8HwEfdow=; b=qIXZx/sCrPq2bQDUPrsl7BdTS28VE3ZfmFF1KDt3gnTIg8EVVTy41oCGqKV93s1gqK M2DM/EJnsK4kU4rlq7qnkiywnl7l3xwnZTK7PgSkygXfEJSTME5FE+kCmwjkYwr81WbK aChG5D+R/ti4ajsha7PwlOn5r0qlQ0+PnMbCuVLAt5FZVLwI62AHcZUq3poob/nd5rqU 31wpGct5x8aWXT6+tfqdZQKwARa3LxmdI4fSMRzkA1jEEg3fouW3bBmYRZtP1dfpQ9Ue Kl9YnNf/RvN37rar8jFVBLuUNLZI+YzYP+8GEko9wlmGZnsQrTkHkif6aufEAubvEXq7 k7mA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1721847740; x=1722452540; 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=iLDZhQ0id4uDXJarhBRTOmxWLKOk3mwU62F8HwEfdow=; b=nYxaQpdkkEBueROQfSLiNPAPs1fH0zvChnZyfk7Y5TDPpp3UtPYQVXupy8iFQ1ZFQM 5gVimbDxUJ2RpI+ZeqQpCv7ghn8opM63l59MBc01dnzG/1a0hMGPfyjYfAoePkg2q21w 91eLslnHCkaLNoasC8jexxk+5ss/AM3lnXBwGhGY1XTLCj91IrEkIm01IXeVn+me7/uI /j7KKvU9TlkM92vs6lwX5UcNoIZB0E65bEvUNdjRifYHTSkcD3PmE5WEhTB4Q1TvRC21 Wq4f4eLq7y+5NkvZNxisLIRpvuhFWFxk39ZGSkYJY7qaH/Ij5jLXbMLtflzZ1M38qJWE /g8Q== X-Forwarded-Encrypted: i=1; AJvYcCW1bCMKw61MSY52fUFye3bq6wyKnTK2L82ATuzmNmWTQYFQh2p/VGWBebv6oXYmIYe/m/mdl1fQRwUbl+/vHbG6Sp+VC7ba5RNGrRoU X-Gm-Message-State: AOJu0YzEwyoztnfsbOZ0nSJrZaOIj+qlKrk3p8KRbtpEMmM3VP5FncdF qZ3CRZT8X/dcLA3MXiFMt6m7z/fDMsQcCCtIUNNHI32u2ktZyPPG3QbOkim1N+l8CIJy8We6Gma vK7KVpBNNnQ== X-Google-Smtp-Source: AGHT+IE2wliyUgg36eRbrdQG0eW2t+LBhnewXuB2szeGvK9iDIHirGHs2LzRSu2WcnyoQhiliA2iavEfWfM/Kg== X-Received: from kinseyct.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:46b]) (user=kinseyho job=sendgmr) by 2002:a17:90a:6b0a:b0:2c8:632:7efe with SMTP id 98e67ed59e1d1-2cdb9661b65mr32367a91.4.1721847739514; Wed, 24 Jul 2024 12:02:19 -0700 (PDT) Date: Wed, 24 Jul 2024 19:02:11 +0000 In-Reply-To: <20240724190214.1108049-1-kinseyho@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240724190214.1108049-1-kinseyho@google.com> X-Mailer: git-send-email 2.45.2.1089.g2a221341d9-goog Message-ID: <20240724190214.1108049-2-kinseyho@google.com> Subject: [PATCH mm-unstable v1 1/4] mm: don't hold css->refcnt during traversal From: Kinsey Ho To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Yosry Ahmed , Roman Gushchin , Kinsey Ho Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" To obtain the pointer to the saved 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. 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 7e2eb091049a..4cbab85e2e56 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 960371788687..062bfeee799c 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1019,20 +1019,7 @@ 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; } @@ -1073,9 +1060,6 @@ 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 2.45.2.1089.g2a221341d9-goog