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 67A0E39F190; Mon, 15 Jun 2026 08:08:57 +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=1781510938; cv=none; b=cJuQ4PGiJ5bbTH6r0xB5KRNsVtfkkgrXCnIciUNfNBsn1yl1P+HLMhrXWQBRtnWV0GHi+kpkZ2JMinir9gETmOJ9RHqua+BHpqomieqYT71qPLQ2gpd/wItl/sAxq6DbBMWEryGBgf0DJyRTwrqYjq65RBoejH2g6T/du40G6jg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781510938; c=relaxed/simple; bh=zhq3OdVOwzWfLf8gLEM0B6MUg2yZCaLsehZmcvAew1w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iehZMdPjTZC3V9J5iy760WPp8jEBRPKXiSApmCaQDFBJTkJV6kJWn7/MlyZCgTZip9brRYwZQcbUNzloL8dSYZ5B/9ek91pQnU9qJ89GrAmAG6up3R+ab6N9/SaxHSiZtBnSo/HrwZb0aEUvvK55sCIMqwtwEZpkCmJtA7qMFvg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=E9uwC3no; 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="E9uwC3no" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5A1E91F000E9; Mon, 15 Jun 2026 08:08:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781510937; bh=czdigwlYZ8jdQimVMktoCHfmpDTgW1yiFk/SNIziXVU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=E9uwC3noUffTCjqknAzvm/sSeJXawABhkRsSILrPn17ih/UNq+c9+BkJD+Vkcn/hn N5bBuOUgg+FUmzKQnsE/oVMZYLCCx9hrW6Yl7KgZjHKHkRGIUqwXwExRpSdnFr1lmH CV9Fejyqa+F596cG/Rp0MUnKxO/YvIG6pWVKqXGu9IJR/Mbpn/1qF1W+iXNoEzYWu/ Fy51qJhpbToiLfgSvGECd+frmrnp9cXzd1yYGqL/Fg3Qlml0dOLSPHg7+a3XSASnH/ zvW0vmBxXA1GR+of9X+gR8TMmn6ZJGA8/W6aJIyJDqnhZRiL2xSqipW4HAshyA+atu Ief3OVsouqFIQ== Message-ID: <8d3b4561-92cd-4ebc-8462-5fb0fd659e8a@kernel.org> Date: Mon, 15 Jun 2026 10:08:51 +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: Farhad Alemi , Andrew Morton , Waiman Long Cc: Farhad Alemi , Gregory Price , 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: From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/14/26 15:25, Farhad Alemi wrote: Hi, thanks for your patch! For the future, please don't submit new revisions as reply to previous submissions. > Creating a child cpuset where cpuset.mems is never set leads to a div/0 > when a VMA mempolicy with MPOL_F_RELATIVE_NODES rebinds in response to a > CPU hotplug event. > > Reproduction steps: > 1) Create a cgroup w/ cpuset controls (do not set cpuset.mems) > 2) Move the task into the child cpuset > 3) Create a VMA mempolicy for that task with MPOL_F_RELATIVE_NODES > 4) unplug and hotplug a cpu > echo 0 > /sys/devices/system/cpu/cpu1/online > echo 1 > /sys/devices/system/cpu/cpu1/online > 5) mempolicy rebind does a div/0 in mpol_relative_nodemask on the > call to __nodes_fold() > > The cpuset code passes (cs->mems_allowed) which is not guaranteed to have > nodes to the rebind routine. Use cs->effective_mems instead, which is > guaranteed to have a non-empty nodemask. Probably worth mentioning here that this makes the linked reproducer happy. > > Link: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/ This should be a Closes: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/ > Link: https://lore.kernel.org/all/CA+0ovCiEz6SP_sn3kN4Tb+_oC=eHMXy_Ffj=usV3wREdQrUtww@mail.gmail.com/ > Fixes: ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}") > Suggested-by: Gregory Price > Suggested-by: Waiman Long > Signed-off-by: Farhad Alemi > Cc: stable@vger.kernel.org > --- > v2: rebind to cs->effective_mems instead of newmems (Waiman Long); > condense the changelog. > > kernel/cgroup/cpuset.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -2649,7 +2649,7 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs) > > migrate = is_memory_migrate(cs); > > - mpol_rebind_mm(mm, &cs->mems_allowed); > + mpol_rebind_mm(mm, &cs->effective_mems); God this is confusing. So, we obtain newmems from guarantee_online_mems(), which guarantees that newmems is non-empty. In cpuset_change_task_nodemask(), we set tsk->mems_allowed to newmems, and call mpol_rebind_task(tsk, newmems). So at least tsk->mems_allowed should be non-empty. Then we call mpol_rebind_mm(mm, &cs->mems_allowed); Naturally I wonder: Why are we not using "task->mems_allowed" (maybe cs vs. tsk was the original bug?), which is effectively just newmems? guarantee_online_mems() computes newmems as "cs->effective_mems & node_states[N_MEMORY]", but walks up to the parent if it would be empty. -- Cheers, David