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

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Message-ID: <51662e0b-4ca2-4841-b8ba-cbf32178eea2@redhat.com>
Date: Mon, 16 Jun 2025 16:26:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Bijan Tabatabai <bijan311@...il.com>
Cc: 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, ying.huang@...ux.alibaba.com, 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 16:16, Bijan Tabatabai wrote:
> On Mon, Jun 16, 2025 at 4:46 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> 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:
> [...]
>> Hi,
>>
>>>
>>> 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.
> 
> Sorry, I think I misunderstood you before. You are right, we should
> consider the VMA policy before using the task policy. I will do this
> in the next revision.
> 
>>
>>> 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.
>>
>> Which, you also have during the rmap walk.
> 
> There is another subtle dependency in get_vma_policy.
> It first checks if a VMA policy exists, and if it doesn't, it uses the
> task policy of the current task, which doesn't make sense when called
> by a kdamond thread.

Indeed, anything that depends on current task / nid / cpu might require 
care.

-- 
Cheers,

David / dhildenb