Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 10/26/22 2:49 PM, Michal Hocko wrote:
> On Wed 26-10-22 16:00:13, Feng Tang wrote:
>> On Wed, Oct 26, 2022 at 03:49:48PM +0800, Aneesh Kumar K V wrote:
>>> On 10/26/22 1:13 PM, Feng Tang wrote:
>>>> In page reclaim path, memory could be demoted from faster memory tier
>>>> to slower memory tier. Currently, there is no check about cpuset's
>>>> memory policy, that even if the target demotion node is not allowd
>>>> by cpuset, the demotion will still happen, which breaks the cpuset
>>>> semantics.
>>>>
>>>> So add cpuset policy check in the demotion path and skip demotion
>>>> if the demotion targets are not allowed by cpuset.
>>>>
>>>
>>> What about the vma policy or the task memory policy? Shouldn't we respect
>>> those memory policy restrictions while demoting the page? 
>>  
>> Good question! We have some basic patches to consider memory policy
>> in demotion path too, which are still under test, and will be posted
>> soon. And the basic idea is similar to this patch.
> 
> For that you need to consult each vma and it's owning task(s) and that
> to me sounds like something to be done in folio_check_references.
> Relying on memcg to get a cpuset cgroup is really ugly and not really
> 100% correct. Memory controller might be disabled and then you do not
> have your association anymore.
> 

I was looking at this recently and I am wondering whether we should worry about VM_SHARE
vmas. 

ie, page_to_policy() can just reverse lookup just one VMA and fetch the policy right?
if it VM_SHARE it will be a shared policy we can find using vma->vm_file? 

For non anonymous and anon vma not having any policy set  it is owning task vma->vm_mm->owner task policy ? 
We don't worry about multiple tasks that can be possibly sharing that page right? 

> This all can get quite expensive so the primary question is, does the
> existing behavior generates any real issues or is this more of an
> correctness exercise? I mean it certainly is not great to demote to an
> incompatible numa node but are there any reasonable configurations when
> the demotion target node is explicitly excluded from memory
> policy/cpuset?

I guess vma policy is important. Applications want to make sure that they don't
have variable performance and they go to lengths to avoid that by using MEM_BIND.
So if they access the memory they always know access is satisfied from a specific
set of NUMA nodes. Swapin can cause performance impact but then all continued
access will be from a specific NUMA nodes. With slow memory demotion that is
not going to be the case. Large in-memory database applications are observed to
be sensitive to these access latencies. 


-aneesh