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 2BA2835BDDB; Fri, 5 Jun 2026 15:18:14 +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=1780672696; cv=none; b=mz6+ohfbpHgpWVgaCdYFDeNCNotOVh+u1Ho/6/aa4OwpJTHMQodDTnaSZzQTI6ZZCZ9s1an5+ET1+mte/5erWL/jsO0A96UK1QN32pTf0lGecgT1qOqE2b9JFlCB0IfEg63SCuvhVKgxJVAla+UrAkZ1IKx1xBKR1lUSXoUinDc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780672696; c=relaxed/simple; bh=vn/5cXhuJlOMn30ZTneglH3cHmpvMV952zzgBo9k3TQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PHIu2y41jAa7ACfYYFZj3vTkCE8L2KP5jAWd8WGpfki2df7Yhn1iixtJUaF0rZsYG8xzi2NJUhQvJzM66PC204K1P+Fh2w0nfKhyz6r5L7ozWRVc5lr/MKABfKwvnwKJ7Zr5n+MtnbOD80cydgj+iE7Gx+6qOkQR+38avqA4Yvg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GSPu40WW; 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="GSPu40WW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 696671F00893; Fri, 5 Jun 2026 15:18:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780672694; bh=bpBz50L7GInctas+/Il3i3Hx7c4Pv4RIfM6k49q0DxM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=GSPu40WWv9onXNY6VO0dlcw7gsX0kg6588NMgROX+7us5AEUFjsK+9/iM1JALiQve s9FIHVxGScwUx0snSzqh7890Rvu3diNQ15ki1tIq4j85wka8H7p6WucwIqbQyOM2I2 hBnkXoJI7ZJAVkpkIbJHfqCEh/o6znOhql5PeV2JEp1IfSIOLjl5P7nQ3iis3Uzq+P 9u60D0CPpvy8fLYJp7JncpQfH9t2EKVDtKWBGdJkd2OBdXVDzj6rGRatKgUGb8/23Z 3WlhRQF0kKRXyR0Kptz8QJfDNfZ3JHMDu8VjLy2Y3RB4RxB6Mo2Y/6NFLb9Ra1VK57 7YcT54/CLRijQ== Message-ID: <25c4bc47-b65d-4c04-8a8f-18eef2b5566a@kernel.org> Date: Fri, 5 Jun 2026 17:18:07 +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] mm: don't allow empty relative nodemask in mpol_relative_nodemask() To: Farhad Alemi , Gregory Price Cc: falemi@asu.edu, Yury Norov , Joshua Hahn , Andrew Morton , Zi Yan , Matthew Brost , Rakie Kim , Byungchul Park , Ying Huang , Alistair Popple , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Waiman Long , Rasmus Villemoes , cgroups@vger.kernel.org References: <20260528124133.c88c27b11a8ea0ef05e494f7@linux-foundation.org> <20260529152616.2308736-1-joshua.hahnjy@gmail.com> 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 6/2/26 17:01, Farhad Alemi wrote: > Confirmed, with a standalone reproducer (attached); it panics linus/master > at e8c2f9fdadee. cs->mems_allowed can legitimately be empty > on v2 -- a freshly created cpuset child that never had cpuset.mems > written keeps mems_allowed empty (never initialized) while effective_mems > is inherited non-empty in cpuset_css_online(), and v2 allows attaching > tasks to it (the empty-mems guard in cpuset_can_attach_check() is gated > on !is_in_v2_mode()). So the non-empty guarantee holds for effective_mems, > not for the configured cs->mems_allowed; forbidding empty cpuset.mems > would break v2's inherit-from-parent semantics. > > The reproducer enables +cpuset, mkdirs a child without writing > cpuset.mems, moves a task in, mbind()s a VMA with > MPOL_BIND | MPOL_F_RELATIVE_NODES, and offlines a CPU; the hotplug walk > then calls mpol_rebind_mm(mm, &cs->mems_allowed) with the empty mask and > folds modulo nodes_weight(*rel) == 0 (console logs attached). > > The newmems instinct looks right: it's the effective, online mask the > task is actually allowed to use, guarantee_online_mems() keeps it > non-empty, and it matches cpuset_attach(), which already rebinds against > cs->effective_mems. The fix this implies: > > - mpol_rebind_mm(mm, &cs->mems_allowed); > + mpol_rebind_mm(mm, &newmems); > > I built the current base (e8c2f9fdadee) with and without this one-liner: > the unpatched kernel panics on the first cpu1 offline, while the patched > kernel runs the reproducer's 8 offline/online cycles cleanly, with no > divide error. > > This regressed in ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}", > v3.17), which moved cpuset_attach() to the effective mask but left this > rebind on cs->mems_allowed. > > Happy to send this as a proper patch (Fixes: ae1c802382f7, Cc: stable, > reproducer) if you agree the cpuset side is right, or to test a > mempolicy-side fix if not. Yes, please send a patch, including a high-level explanation of what you analyzed above! -- Cheers, David