Re: [PATCH v2] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed

[Waiman Long <longman@redhat.com>]

On 6/15/26 10:26 PM, Waiman Long wrote:
>
> On 6/15/26 5:38 AM, 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.
>
> That is what I believe why mpol_rebind_mm() a bit differently from the 
> others and it is historically done this way a long time ago before 
> cgroup v2.
>
> For cgroup v1, mems_allowed can't be empty or you can't put any task 
> into the cpuset. Also effective_mems is the same as mems_allowed. 
> cgroup v2 is quite different in how it handles memory nodes and CPUs. 
> Users can isn't forced to set mems_allowed and cpus_allowed as 
> effective_mems and effective_cpus will inherit parent version if 
> mems_allowed and cpus_allowed are not set. IOW, effective_mems will 
> never be empty. Yes, it is a bug with the introduction of cpuset v2 
> that we should have replaced mems_allowed by effective_mems at that 
> time. With v2, effective_mems should contain only online nodes. The 
> only exception is during the short transition period when a memory 
> node hotunplug operation is in progress when a write to cpuset.mems is 
> happening at the same time. With v1, it is theoretically possible that 
> none of the nodes in mems_allowed is online.
>
> The reason why I am suggesting to use cs->effective_mems to keep the 
> old cgroup v1 behavior. If the consensus is to use the output of 
> guarantee_online_mems() for mpol_rebind_mm(), I will not be against 
> that but it will be a slight change in user-visible behavior. 

BTW, I still prefer the v2 patch. If it is decided we should use the 
guarantee_online_mems() value instead, it will have to be a separate 
patch with changes in the relevant documentation like 
Documentation/admin-guide/cgroup-v1/cpuset.rst.

Cheers,
Longman