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b=KukOqma5oU78wHocXEMOB2C3meoqCN9qVUAmBuHpOYbPa1W5lJXyz4jBph1wca+/+ EIi4evOjLPYjh+lzE9XwByrE3EEuQYdjrCBxiRoSY+T6uDQbAfYXE3Fff634b8EFa3 4eeqW3IQY3lw+9RJw99pGheVqXPAMgMIqIjsOliOsQboqN7rUYlOsvDEfau7zyfesJ ZzFoJitqUB7U1p27fsq+YPN8vxiwf9cckV+OeS2ikYQkSbQc2AJU1CQlHdA9OwC01k BQnGTBy+FWLnkyWa+d42BccbE8sv8EgUhKdxEE5eTEg2G3ERQ0E37m1kZGigTVbPCN 2aXtF5sVsE87w== Message-ID: Date: Mon, 15 Jun 2026 13:08:16 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed To: Gregory Price Cc: Farhad Alemi , Andrew Morton , Waiman Long , 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> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: jpsrzfrptu3pj556chjesu4s51xgumpm X-Rspamd-Queue-Id: 99EAC140004 X-HE-Tag: 1781521702-430833 X-HE-Meta: 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 nJj963vy e7xYbDfmC6BRQ57SXhgIRFS886naXNPPE392r0iYKmSTG/y9nPFMiDgtIGJh1G9eNuJn18W67XVcGn+HNv7CV5O2bqrzPLfajPEg4+BnYFdwrn+6nsaLAHX5GhBbObB9oCEjSdwGHyb83usaSpg98MtKFmgMztGN9/GWkIaZKbQGS0ZoME7uyL1uMAbhV9CTLBD9CDg8crM78caVQcSUWlI+5NzjZOtlk8J+pH4xanKZNmUGgplYoO+hr2SSZfJnBA8yaXyjQsYYZjQSuZoPtGQljUvCb+3HMw9E063UJVxuUBZki/3+0qe3L5WHR6oTnLPEV2KR3y7Nks/ldNHBM4gCD6xQc3Gfk3Cq47+c3yw1nmBrTH+bhBK9Ze+aF9v4HZk3z Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/15/26 11:38, Gregory Price wrote: > On Mon, Jun 15, 2026 at 10:08:51AM +0200, David Hildenbrand (Arm) wrote: >> On 6/14/26 15:25, Farhad Alemi wrote: >>> >>> 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. >> > > 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. > >> Naturally I wonder: Why are we not using "task->mems_allowed" (maybe cs vs. tsk >> was the original bug?), which is effectively just newmems? >> > > Short answer: task->mems_allowed is protected by the task lock and we > don't hold the task lock for a foreign task (not-current) over mm > operations. Well, we can just use newmems, which cannot change? Again, that is based on cs->effective_mems but is guaranteed to return something non-empty. AI was not able to convince me (neither was I able to convince AI) that there is not some obscure cgroup v1 scenario where the current fix would also be wrong. With newmems it's clear that it is guaranteed to not be empty. -- Cheers, David