From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 0419923F40D; Thu, 15 Oct 2026 13:49:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792072185; cv=none; b=SAlvOajwspNgBWr9kX0Qabl1vG0Y1vkS00iPNB0ml7r+wmKUg9LfmNyMJ2f1zHd+f5aUUjVrjEEm+kBqit6ZSeMTvYznspqmTXG1IerX5KJljS1xDbs4cU8az8LVNOjxmINhcsKuFldEMfzq1oMbsnnRnlv8MtCxnM2wOCHbPZA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792072185; c=relaxed/simple; bh=o6nVC085f3Vj+LPAGK4PW96/pbDYu9j2D8FLRR2JjBk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bknAZF8Rxou5hRyY/BpwyZrjEs7u0e8pQfgIIFLAwdaBtOH8hswHkUMZqTfptz6k1ZKUmdc5oEwGCZAYcwlrcVsnEkjDOcQPJ3p/vbG8CHved1LfTZn/4lcKYF4n9vNUJ20q7yO8eZLuZpdZI0wV5lMtxL+/G/+mdTwJFQGmvlY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=d861A3BS; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="d861A3BS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6444F1F000C4; Thu, 15 Oct 2026 13:49:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792072183; bh=VP0jF5bnf0g3p1HRHBbIMSTGCx+taYCBpLmJnYYBFYM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=d861A3BS0NNc1P1ZuaLP81CBT0yRUV/MSIHHcvdDZV+dT+6mcQz8xzHjOWGCjrKj/ +Su2EAXRXRa0OPgm4DdRtvGpP68bzQQeOo8SY23c/1GGvCjHgOVvQapCfnqyJjF5OQ Y9YuFfH+rFf1Krh1H8iZACaIiRQhkuFM1zfag8a7UHqYOw+H8JpOjtD/RRjjHlNxgE PwsuxctdlD0cIFn5F3Bt2VPCIW6kxs6mh/656KSn49syyDwLR3nmksNR3RlfxExqn8 4lEbI3hGXBW8JisgrkrlA/gajaSBDI/jmSMRTo9TBPj8vfkbY9eWlhYd2PEivpAckz 6A6+I8MG3zhNw== Message-ID: <10932a95-28c9-4b7a-9d57-95890bdaee51@kernel.org> Date: Thu, 15 Oct 2026 15:49:38 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] cgroup/cpuset: use mems_allowed_seq when checking a policy nodemask Content-Language: en-US To: Gregory Price Cc: linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, longman@redhat.com, ridong.chen@linux.dev, tj@kernel.org, hannes@cmpxchg.org, mkoutny@suse.com, akpm@linux-foundation.org, surenb@google.com, mhocko@suse.com, brendan.jackman@linux.dev, ziy@nvidia.com, david@kernel.org, mgorman@techsingularity.net References: <20261014222456.2318284-1-gourry@gourry.net> <20261014222456.2318284-3-gourry@gourry.net> <8d611a0a-4ada-4536-875d-061492773e05@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/15/26 15:38, Gregory Price wrote: > On Thu, Oct 15, 2026 at 09:59:31AM +0200, Vlastimil Babka (SUSE) wrote: >> On 10/15/26 00:24, Gregory Price (Meta) wrote: >> > A cpuset update changes the task's mems_allowed and rebinds its >> > mempolicy in separate steps under mems_allowed_seq, but the check >> > reads both without it. This can cause a spurious numa placement >> > miss under some cpuset/mempolicy combinations. >> > >> > A reader that loads one mask before the update and the other after >> > it can see a spurious empty intersection and drop the nodemask. >> > >> > For example, a task with cpuset.mems={0} and MPOL_BIND={0}. If the >> > cpuset moves to mems={1,2}, the policy is rebound to {1}. >> > >> > The allocator can race and compare the old mems_allowed {0} against >> > the new policy {1} - which will result in a dropped nodemask and a >> > misplaced allocation on node 2. >> >> > Simply retry the intersection until mems_allowed_seq is stable. >> > >> > This covers the task policy, which is rebound under mems_allowed_seq. >> > VMA policies are rebound afterwards by mpol_rebind_mm(), outside >> > mems_allowed_seq, and are not covered by this change. >> > >> > Fixes: cc9a6c877661 ("cpuset: mm: reduce large amounts of memory barrier related damage v3") >> > Signed-off-by: Gregory Price (Meta) >> >> Should the comment and/or code in check_retry_cpuset() be updated as well? >> > > No I don't think so. We're checking different things. > > The loop here makes sure nodemask and mems_allowed are stable when > compared so we don't erroneously drop the nodemask on a race. > > The cookie-check in check_retry_cpuset() determines whether the entire > allocation should restart. It runs after the nodemask would already > have been dropped, so it can't catch this race. Ah, right. Reviewed-by: Vlastimil Babka (SUSE) > ~Gregory