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[71.191.145.202]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-55ad15eb7basm44690811cf.26.2026.11.14.09.41.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 14 Nov 2026 09:41:21 -0800 (PST) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: Re: [PATCH 2/2] mm/mempolicy: limit shared policy ranges per inode From: "Gregory Price (Meta)" To: "David Hildenbrand (Arm)" Cc: Hugh Dickins , Baolin Wang , Gregory Price , Zi Yan , Joshua Hahn , Rakie Kim , Ying Huang , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , Randy Dunlap , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org In-Reply-To: <20261113-mempolicy_limit-v1-2-16688b2644dc@kernel.org> References: <20261113-mempolicy_limit-v1-0-16688b2644dc@kernel.org> <20261113-mempolicy_limit-v1-2-16688b2644dc@kernel.org> Date: Sat, 14 Nov 2026 12:41:21 -0500 Message-Id: <179467808112.482102.13365594235443107281.b4-reply@b4> X-Mailer: b4 0.17-dev On 2026-11-13 22:36 +0100, David Hildenbrand (Arm) wrote: > Shared NUMA policy ranges can accumulate on an inode after its mappings > are removed. Currently, these nodes are effectively unbounded (even though > the allcoations are charged). > > An unbounded tree can cause long rwlock hold times when replacing or > freeing policies. Instead of trying to fix that, it sounds more like > having many nodes is actually only a scenario for processes trying to > cause trouble. Most use cases will probably only use a handful of them, > not millions. > > Add vm.max_shared_policy_count (inspired by vm.max_map_count), defaulting > to 1024 ranges per inode. Reject updates with -ENOMEM if they would > increase the count beyond the limit, taking merging, splitting etc. > into account. > > Signed-off-by: David Hildenbrand (Arm) This arguably breaks userspace. A single mapper that tries to set >1024 ranges would start getting -ENOMEMs unexpectedly with this change. Multi-task mappers were previously unlimited, but that seems like more of a nasty unintended corner case - not a real feature. A single process is already limited on the number of vmas/policies based on `max_map_count`, so it seems natural to just use max_map_count directly, but I took a brief look at some distros and most seem to ship a raised `max_map_count` (>1 million) by default). I tested that and even 1 million can cause a soft lockup. Not sure there's an easy solution here, we might have to rework the shm policy locks. -- ~Gregory