linux-kernel - Re: [RFC PATCH 1/4] mm/mempolicy: Expose policy

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Message-ID: <5a331053-0777-4426-b0da-4d813cbe0a9a@redhat.com>
Date: Mon, 16 Jun 2025 13:11:24 +0200
From: David Hildenbrand <david@...hat.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: Bijan Tabatabai <bijan311@...il.com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, sj@...nel.org,
 akpm@...ux-foundation.org, corbet@....net, ziy@...dia.com,
 matthew.brost@...el.com, joshua.hahnjy@...il.com, rakie.kim@...com,
 byungchul@...com, gourry@...rry.net, apopple@...dia.com,
 bijantabatab@...ron.com, venkataravis@...ron.com, emirakhur@...ron.com,
 ajayjoshi@...ron.com, vtavarespetr@...ron.com, damon@...ts.linux.dev
Subject: Re: [RFC PATCH 1/4] mm/mempolicy: Expose policy_nodemask() in
 include/linux/mempolicy.h

On 16.06.25 13:02, Huang, Ying wrote:
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 13.06.25 18:33, Bijan Tabatabai wrote:
>>> On Fri, Jun 13, 2025 at 8:45 AM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> On 12.06.25 20:13, Bijan Tabatabai wrote:
>>>>> From: Bijan Tabatabai <bijantabatab@...ron.com>
>>>>>
> 
> [snip]
> 
>>> I did not use get_vma_policy or mpol_misplaced, which I believe is
>>> the
>>> closest function that exists for what I want in this patch, because
>>> those functions
>>
>> I think what you mean is, that you are performing an rmap walk. But
>> there, you do have a VMA + MM available (stable).
>>
>>> seem to assume they are called inside of the task that the folio/vma
>>> is mapped to.
>>
>> But, we do have a VMA at hand, so why would we want to ignore any set
>> policy? (I think VMA policies so far only apply to shmem, but still).
>>
>> I really think you want to use get_vma_policy() instead of the task policy.
>>
>>
>>> More specifically, mpol_misplaced assumes it is being called within a
>>> page fault.
>>> This doesn't work for us, because we call it inside of a kdamond process.
>>
>> Right.
>>
>> But it uses the vmf only for ...
>>
>> 1) Obtaining the VMA
>> 2) Sanity-checking that the ptlock is held.
> 
> 3) update NUMA balancing per-folio cpupid state (via should_numa_migrate_memory()).

How is the vmf used for that ("it uses the vmf only for ...")?

But yes, anything that relies on "thiscpu" and "thisnid" might be 
fault-specific as well (what I mentioned further below)


-- 
Cheers,

David / dhildenb