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

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Date:   Wed, 26 Oct 2022 14:21:13 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Waiman Long <longman@...hat.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>
Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

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.

[...]
-- 
Michal Hocko
SUSE Labs