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

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Message-ID: <87plf43q6s.fsf@DESKTOP-5N7EMDA>
Date: Mon, 16 Jun 2025 19:02:03 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: David Hildenbrand <david@...hat.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

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()).

This needs to be called by the NUMA page fault handler.

> Which, you also have during the rmap walk.
>

[snip]

---
Best Regards,
Huang, Ying