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

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Message-ID: <22590f74-ec91-e673-32df-8a04b4ab3931@linux.ibm.com>
Date:   Wed, 26 Oct 2022 18:05:46 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Michal Hocko <mhocko@...e.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 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. 

> 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.
> 
> [...]


-aneesh