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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=fSAB7NW2; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B6BD5606A31A; Mon, 19 Oct 2026 16:41:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=fSAB7NW2 Received: by smtp.kernel.org (Postfix) with ESMTPSA id E71F71F000C4; Mon, 19 Oct 2026 16:41:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792428114; bh=x95JnSTAGKgQoctI7x0v2xKm15KaSPE+6GLlSALF+i4=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=fSAB7NW20JCAiLixJGlTa7CjLWqMJDWZgD12mgzeun5xFvFsWyDR4GqfEVYbvrC/I iEaKWjR+VSugmhXEWPc4hQEATJoflD5rXgGsRgBfnvOYuvFNYDG5EyqlDLXsvSBCk0 1Ga79NvuSti/pKjA4DyNTyQyjZ2QNjOy/ktmfcPbBLoDlzFNO5+ERlL9SY/xhzZ3sY ldcWLD89xFA/9xXjarUgBK6hR2JhiUjG//L5CECQs6tF5Rh2g3grJy07Xuq7bp1nKn lEPDLUOHiuBjmlAGPzS/NbnBZFx/dl/WLpcBbhItHFJtPVuIRyXs+X9OCeX6BHtGe6 EW0/Ue3RbSTRw== Message-ID: <805cfff2-1856-4b48-9afc-174a56405283@kernel.org> Date: Mon, 19 Oct 2026 18:41:50 +0200 X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] 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, mgorman@suse.de References: <20261019162138.3271947-1-gourry@gourry.net> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/19/26 18:21, 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. And since this can have only performance implications and not cause a functional issue, we don't need Cc: stable. (unlike violating a __GFP_THISNODE allocation which could be quite bad and has happened in the past) > 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") > Reviewed-by: Vlastimil Babka (SUSE) > Reviewed-by: Waiman Long > Signed-off-by: Gregory Price (Meta) > --- > v2: there is no real update here, i'm breaking it off of the page_alloc > series so the page_alloc fixes backport collectively cleaner. > > 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; > } > > /*