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

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Date:   Thu, 27 Oct 2022 11:02:19 +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 18:05:46, Aneesh Kumar K V wrote:
> On 10/26/22 5:51 PM, Michal Hocko wrote:
> > 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?
> > 
> 
> Sorry, I meant MAP_ANONYMOUS | MAP_SHARED. 

I am still not sure where you are targeting to be honest. MAP_SHARED or
MAP_PRIVATE both can have page shared between several vmas.
-- 
Michal Hocko
SUSE Labs