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 42C8B2701B8; Tue, 16 Jun 2026 06:59:11 +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=1781593153; cv=none; b=gqnDttoWFo2+cbcvv8QqWjnBflqM3Lm+QNRapU2qN7hVwNXQS28v0BbfnxKAAvrxzb7gdi/KbWInuH3xXSM9UBpbm8lc3LwGZZTBMIxSgA8Kpz+ccs5chQyGPO9o+6zO3KGV2XkA/ATh1xanXsCbYn99oNYM1Oay8PvpxkV8zMU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781593153; c=relaxed/simple; bh=OJoQ4rwTfeQVczcs9E8yczOtJC57CIFV7E/9WEiHJqQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=KbYHN0s1hj8AR2n8f7LaXFFjutE7CPlU4WWTce78HerT10Gb8X4VoWOP+lOQ760Xw4yptW88xXMmplf1MRfmC7YaP2b728Em1OIxAjNaEoPI2nX77h+Y4lRhQMtGelTABdVfPjeAYuIuMtvJ9fxFyqVY+iBSFuwO8NFWW17qtqw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TLqP6U4G; 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="TLqP6U4G" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AB2E11F000E9; Tue, 16 Jun 2026 06:59:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781593151; bh=2HeJtcoXm4ZF32QqrGWVmTih8pCwYMbsPqyap9P6bfc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=TLqP6U4GA/skhRUO8ixnBrnDMvd+SoxBCxjD22o1RpnT8HCnGPeQGzyUtP9SlvUD0 OcJkQe1Sr4lwC4ZW+2T94fH0t9VrebYbGGv/B+4TdbhdAbew6pFaDFh9fG9tt7kbs3 18COFZHX03SXr3+24A6d+JiRRKUkRR7w7szZ0V/SyvQ6+92cEgpIQl9Tgt1JB0PZtx w/xKg9OQubIQ4MPWFa356swMKFUTLU/KJw6JXHnMzZfF6LDZb/zG/tN73axyHP/3NE nFDd3pSkjy/3ahdvgudr/K/WXijbDQF+eD6gH3ZesoxLnQwnXIbKcA8fcue3lQjpHi 01futx6HIAjsg== Message-ID: <51eafe6c-6622-479b-b391-6d3ff9350e75@kernel.org> Date: Tue, 16 Jun 2026 08:59:07 +0200 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed To: Waiman Long , Gregory Price Cc: Farhad Alemi , Andrew Morton , Farhad Alemi , Yury Norov , Joshua Hahn , Zi Yan , Matthew Brost , Rakie Kim , Byungchul Park , Ying Huang , Alistair Popple , Rasmus Villemoes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, stable@vger.kernel.org References: <8d3b4561-92cd-4ebc-8462-5fb0fd659e8a@kernel.org> <70f486ce-5ef6-4d72-8cc3-7086f4eea930@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/16/26 05:43, Waiman Long wrote: > On 6/15/26 10:26 PM, Waiman Long wrote: >> >> On 6/15/26 5:38 AM, Gregory Price wrote: >>> All interactions between mempolicy and cpuset are horrible and >>> confusing.  Much like Lorenzo's anon_vma work, I have to keep >>> notes on how this whole thing doesn't just spew SIGBUS constantly. >>> >>> The short answer is: mempolicy is advisory and cpuset is strictly >>> followed - in a dispute cpuset wins... except for file backed memory, >>> then everyon loses and nothing is consistent. >> >> That is what I believe why mpol_rebind_mm() a bit differently from the others >> and it is historically done this way a long time ago before cgroup v2. >> >> For cgroup v1, mems_allowed can't be empty or you can't put any task into the >> cpuset. Also effective_mems is the same as mems_allowed. cgroup v2 is quite >> different in how it handles memory nodes and CPUs. Users can isn't forced to >> set mems_allowed and cpus_allowed as effective_mems and effective_cpus will >> inherit parent version if mems_allowed and cpus_allowed are not set. IOW, >> effective_mems will never be empty. Yes, it is a bug with the introduction of >> cpuset v2 that we should have replaced mems_allowed by effective_mems at that >> time. With v2, effective_mems should contain only online nodes. The only >> exception is during the short transition period when a memory node hotunplug >> operation is in progress when a write to cpuset.mems is happening at the same >> time. With v1, it is theoretically possible that none of the nodes in >> mems_allowed is online. >> >> The reason why I am suggesting to use cs->effective_mems to keep the old >> cgroup v1 behavior. If the consensus is to use the output of >> guarantee_online_mems() for mpol_rebind_mm(), I will not be against that but >> it will be a slight change in user-visible behavior. > > BTW, I still prefer the v2 patch. If it is decided we should use the > guarantee_online_mems() value instead, it will have to be a separate patch with > changes in the relevant documentation like Documentation/admin-guide/cgroup-v1/ > cpuset.rst. newmems is "obviously" correct, so I really don't see why we should add something that needs half a page of text to explain why it is fine -- if newmems just does the trick? Please enlighten me. -- Cheers, David