From nobody Fri Dec 19 12:18:30 2025 Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) (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 BE7B11D4606 for ; Tue, 27 Aug 2024 23:11:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.202 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724800303; cv=none; b=qti8FzIgLPfa2Qn0X+b6T3oEHS65+NlD+4CunfWzIF9xLfRjA4gaWr8THM76XO+J0SSV3brzLRWUR5ZY2oPfh8mk8gGH2EXDtD/xNeNIZwpGClfB3tapSNMciOCAPNQYmHWouPXhdzx+UBX+T0N77MO1ykWXkmW9LCCTs59xEsk= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724800303; c=relaxed/simple; bh=GqTjpb1jAON2eyIaaYCsAsTBYc3LGYMO9CEcIpbdfhI=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=f64PnqtB26AkWyD5B4GrNJriPL5kzE795UepVYwZjLocj71qrjYdVStJJy4m3Lro7Ord5njAqJE058hjGSHOg/9S0awzUwHcIhd3Kxat+IHlbvha07mpZTGU8LMWOJOlZhHLyUf4ryfwO3MkI7wN4uFvFCugiRdvFuNAgyibiBI= 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=luWYdgww; arc=none smtp.client-ip=209.85.128.202 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="luWYdgww" Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-6b46a237b48so127399927b3.1 for ; Tue, 27 Aug 2024 16:11:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1724800301; x=1725405101; 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=QXZSgIWrSh7o4TMYH5HXuFFT6CvWnPSrTikMMv9Xu6c=; b=luWYdgwwCtalmPcjYLmG12A7J1JyH9hd5BjKy6ix1RIRFsvB8fmqyTBGV/hbtf5GN0 bpqruMsMwtW7Y+dvdLdKXC4R5A5LcSOoUOgvYFgTwnF/n8hesNz79ZREzZ1a1GH7ogBM XKt9/LyLJjg4UuvU9VXdYExxnv6SDg4lzx+Nw3sDb3tZzdIRyyrJYnNfgez1n4Pl3v/i rgQ0k6reNmGgwPYW2y4ZcV6zZ2TTjmaIkoU8D+T2WMTiNeWIlD3UI+vvhE57htyghBYg aUKwkJgEZMBB9YnFQKpEYpKEnIuJ3yFnHmtE1tpDLwLba71NSosomK4EnlCdKwYSl2o3 Uetw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1724800301; x=1725405101; 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=QXZSgIWrSh7o4TMYH5HXuFFT6CvWnPSrTikMMv9Xu6c=; b=AWN1/HpCCtjTcvv8LIZpFYN1mn+JFPQPYtcf8JTTYgCmZeasdAIkg2swvXBx1xfqIV EjAzE0n3JxE3yR6po/C5Yiq9Jj9PBfJmnVWuxP99NlonyPP3nbN6CWI5MtZcDPdZl6Tx Bmg21skeq/uroyqiNXwjk7VY0nnh5IVGLB/C1kzAcy6UsRgp+xhNzcKfwvDeq/VQ9OZu fwWN/JrG87VBtKi87+WiNFR8zb54jGkBJoKYfKT4SVPrmaH4EgRBrJBdauL+TcBjB6wF mVtfiTfiDPAKDVdRUa+AZzgtvhVWmYh3FsDme1mwZDz27z1BTkJHG17VUvFbuWoEMWnd Cjbw== X-Forwarded-Encrypted: i=1; AJvYcCUE8hJdpPV7WbSmEWOnQ7k9YEGJUUEIn9FybyDbW2YHmToODSmQUDTM3S3zkrttuQ7sw0g/74W5QzMm9mM=@vger.kernel.org X-Gm-Message-State: AOJu0YxqX1FcLagolorAaVJ2ltERVSsa6q0v3SJ12frKHNihnAjF7MQD JC/rsZsTKtA+smbZr7tvAuhSXbgFJEiMNi6RDttP5U3WrucTHtk1xNV1xrmXSfVxQbLimxovCRi 1GPU6ZtYRmA== X-Google-Smtp-Source: AGHT+IH/OEMHPdWupnXyjW/u5NZJ+Iwna53tKUi6hE4QApyk+GLsJI4xuXktR0o4bdU85PWSweUUV8sgsDuT4w== X-Received: from kinseyct.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:46b]) (user=kinseyho job=sendgmr) by 2002:a25:abad:0:b0:e11:69f2:e39 with SMTP id 3f1490d57ef6-e1a4580c518mr380276.9.1724800300879; Tue, 27 Aug 2024 16:11:40 -0700 (PDT) Date: Tue, 27 Aug 2024 23:07:39 +0000 In-Reply-To: <20240827230753.2073580-1-kinseyho@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240827230753.2073580-1-kinseyho@google.com> X-Mailer: git-send-email 2.46.0.295.g3b9ea8a38a-goog Message-ID: <20240827230753.2073580-3-kinseyho@google.com> Subject: [PATCH mm-unstable v3 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 Reviewed-by: T.J. Mercier --- mm/memcontrol.c | 18 +----------------- 1 file changed, 1 insertion(+), 17 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 35431035e782..67b1994377b7 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -1013,20 +1013,7 @@ 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; } @@ -1067,9 +1054,6 @@ 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 2.46.0.295.g3b9ea8a38a-goog