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Tue, 02 Jun 2026 01:44:43 -0700 (PDT) Date: Tue, 2 Jun 2026 09:44:41 +0100 From: Gregory Price To: "David Hildenbrand (Arm)" Cc: 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, Farhad Alemi , Waiman Long , Rasmus Villemoes , cgroups@vger.kernel.org Subject: Re: [PATCH] mm: don't allow empty relative nodemask in mpol_relative_nodemask() Message-ID: References: <20260528124133.c88c27b11a8ea0ef05e494f7@linux-foundation.org> <20260529152616.2308736-1-joshua.hahnjy@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: B257E20008 X-Rspam-User: X-Stat-Signature: 6uofu9in5nxjaxnwqwmbcdxjo1c1fe97 X-HE-Tag: 1780389885-874770 X-HE-Meta: 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 10mQyz4n 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 01, 2026 at 04:32:25PM +0200, David Hildenbrand (Arm) wrote: > >> > >> Thank you for taking a shot at fixing the bug report, please let me know what > >> you think! Have a great day : -) > > > > Hi Joshua. > > > > Indeed, quick and dirty shot. > > > > The problem is that nodes_fold() can't work with the sz == 0. In > > other words, folding to a 0-bit bitmap is an error. We don't check > > that on bitmaps level because it's an internal helper, and it's a > > caller's responsibility to validate the parameters. > > > > nodes_onto(), or more specifically bitmap_onto(), is a different > > story. In case of empty relmap, the function actually clears all the > > bits in dst and returns. > > It's very weird that mpol_new_nodemask() (->create() callback) disallows empty > nodemasks, but mpol_rebind_nodemask() (->rebind() callback) would allow empty > nodemasks. > Was this actually observed? mpol_rebind_nodemask() happens when cgroup.cpuset changes, and cgroup.cpuset cannot be empty. cpuset only changes with sysfs twiddles or offlining. In either case, cpuset *guarantees* that cpuset.mems will never be empty. So... is this an observed bug or just a statically discovered "bug" that can't actually be reached? ~Gregory