linux-kernel - Re: [PATCH 1/6] prctl: introduce PR_THP_POLICY_DEFAULT

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Message-ID: <360bac52-2cda-41fd-a674-89b113fef918@redhat.com>
Date: Thu, 15 May 2025 17:57:57 +0200
From: David Hildenbrand <david@...hat.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Usama Arif <usamaarif642@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 hannes@...xchg.org, shakeel.butt@...ux.dev, riel@...riel.com,
 ziy@...dia.com, laoar.shao@...il.com, baolin.wang@...ux.alibaba.com,
 npache@...hat.com, ryan.roberts@....com, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 1/6] prctl: introduce PR_THP_POLICY_DEFAULT_HUGE for the
 process

On 15.05.25 17:45, Liam R. Howlett wrote:
> * David Hildenbrand <david@...hat.com> [250515 10:44]:
>> On 15.05.25 16:40, Lorenzo Stoakes wrote:
>>> Overall I feel this series should _DEFINITELY_ be an RFC. This is pretty
>>> outlandish stuff and needs discussion.
>>>
>>> You're basically making it so /sys/kernel/mm/transparent_hugepage/enabled =
>>> never is completely ignored and overridden.
>>
>> I thought I made it very clear during earlier discussions that never means
>> never.
> 
> I also thought so, but the comments later made here [1] seem to
> contradict that?

It's ... complicated.

> 
> It seems "never" means "default_no" and not actually "never"?

We should consider these system toggles a single set of toggles that define a
state, and not individual toggles that overwrite each other.

If you say
	/sys/kernel/mm/transparent_hugepage/enabled = never
and
	/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled = always

instead of the *default*

	/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled = inherit

the admin explicitly stats "I want the system behavior for 2048kB not to be configured using
/sys/kernel/mm/transparent_hugepage/enabled". That's an admin decision, not a
per-process overwrite or whatever.

> 
> Maybe the global/system toggles need to affect the state of each other?
> That is, if /sys/kernel/mm/transparent_hugepage/enabled is never and you
> set /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled to
> madvise, it should not leave /sys/kernel/mm/transparent_hugepage/enabled
> as never.

I recall we discussed that, but there was also a catch to that. :(

-- 
Cheers,

David / dhildenb