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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/16/26 05:43, Waiman Long wrote:
> On 6/15/26 10:26 PM, Waiman Long wrote:
>>
>> On 6/15/26 5:38 AM, Gregory Price wrote:
>>> 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.

newmems is "obviously" correct, so I really don't see why we should add
something that needs half a page of text to explain why it is fine -- if newmems
just does the trick?

Please enlighten me.

-- 
Cheers,

David