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

[Michal Hocko]

On Wed 26-10-22 17:38:06, Aneesh Kumar K V wrote:
> On 10/26/22 4:32 PM, Michal Hocko wrote:
> > On Wed 26-10-22 16:12:25, Aneesh Kumar K V wrote:
> >> 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?
> > 
> > How would that help for private mappings shared between parent/child?
> 
> 
> this is MAP_PRIVATE | MAP_SHARED?

This is not a valid combination IIRC. What I meant is a simple
MAP_PRIVATE|MAP_ANON that is CoW shared between parent and child.

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-- 
Michal Hocko
SUSE Labs