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 971A63DD86F; Thu, 15 Oct 2026 07:59:38 +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=1792051179; cv=none; b=FZU1nYiiLL2f6LAbUeNwyIeVcUPXqpiAiXExY/1/nTLd0Y8Lvtr55f4eZ/2u116j2wVupgL7NoABgLlwYR0D5VU3Mt+BNh4lNZW0Oev0LlxsfW13/FJwF/e8aR134RHreCmP0thGG2yczgOp6mySMdquK5WbIGkJKRaRaIwsJXo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792051179; c=relaxed/simple; bh=Bzh4jsN+sNKRR5/mXutSsDbZ4OrUdZP76LPRV3A1S/A=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=SPQ1k94hkbC5L875OgxwZDqt/KIi/3tdV4MUD8H4rTVRjrHEGK03QBjysILeghYsZpi7PkQy2SY+a8Dh1FKmbKgzqgHOhHZVafXBpO9x3qP4vmwTMyZDeE1uvrzhHPJvrKjQ4fBqSOs8iqX0ZltOxqpSIUDrCdWyipTLINDN0F4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Vit6hou7; 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="Vit6hou7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 935271F000C4; Thu, 15 Oct 2026 07:59:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792051178; bh=Um2S9IpesnEJKKZWLH+IYiQoAlWFeglzqoe6Ojkwdfs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Vit6hou76Pr582DIhmm1r+3MqLDTBVVV70tHLFKt343D8W+HATDggoGM5YZ+NkQWi 6PMEUBEvbfP/k4unJJn9xUjFCLbEB07rPwgfbog6ZnS27hAsp6ifVAKTw1Rjge9tbU Ei+CHSJra4nXJh/3KdVz2h6sGvUAU1FQb8EBqi0n2VByyzkE4pBt+F2Kcb/sp8Fp32 4HPMKSqBMNJ4e276wsxEuuetcDXGm+nPsQLQcbwv7kkUclNQOcVcFy1Lgatcc+Cj9r u25ERhSbH0N3O/F1xWHrm1Fq5lQ9UgPofMdrmywkwCP/gjgYm/Wm1fxn+/OxPiCmOD DbSJZsVuQ0OPA== Message-ID: <8d611a0a-4ada-4536-875d-061492773e05@kernel.org> Date: Thu, 15 Oct 2026 09:59:31 +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 (Meta)" , linux-mm@kvack.org, cgroups@vger.kernel.org Cc: 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> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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? > --- > kernel/cgroup/cpuset.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > index 3f52717c19654..8ee239d5c6b48 100644 > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -4354,10 +4354,21 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk) > * @nodemask: the nodemask to be checked > * > * Are any of the nodes in the nodemask allowed in current->mems_allowed? > + * > + * A cpuset update rebinds the task's mempolicy and changes mems_allowed in > + * separate steps, so retry if an update raced with the check. > */ > int cpuset_nodemask_valid_mems_allowed(const nodemask_t *nodemask) > { > - return nodes_intersects(*nodemask, current->mems_allowed); > + unsigned int seq; > + int ret; > + > + do { > + seq = read_mems_allowed_begin(); > + ret = nodes_intersects(*nodemask, current->mems_allowed); > + } while (read_mems_allowed_retry(seq)); > + > + return ret; > } > > /*