linux-kernel - Re: [PATCH v11 00/15] khugepaged: mTHP support

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Message-ID: <1312fa3e-88dd-490a-9e07-9749c625933b@redhat.com>
Date: Mon, 15 Sep 2025 13:10:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On 15.09.25 12:59, Lorenzo Stoakes wrote:
> On Mon, Sep 15, 2025 at 12:52:53PM +0200, David Hildenbrand wrote:
>> On 15.09.25 12:48, Lorenzo Stoakes wrote:
>>> On Mon, Sep 15, 2025 at 12:44:34PM +0200, David Hildenbrand wrote:
>>>>>>> Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy.
>>>>>>
>>>>>> And at different mTHP levels too right?
>>>>>>
>>>>>
>>>>> Another point here, since we have to keep:
>>>>>
>>>>> /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
>>>>>
>>>>> Around, and users will try to set values there, presumably we will now add:
>>>>>
>>>>> /sys/kernel/mm/transparent_hugepage/khugepaged/eagerness
>>>>>
>>>>> How will we map <-> the two tunables?
>>>>
>>>> Well, the easy case if someone updates eagerness, then we simply et it to
>>>> whatever magic value we compute and document.
>>>>
>>>> The other direction is more problematic, likely we'll simply warn and do
>>>> something reasonable (map it to whatever eagerness scale is closest or
>>>> simply indicate it as "-1" -- user intervened or sth like that)
>>>
>>> I don't love the idea of a -1 situation, as that's going to create some
>>> confusion.
>>
>> swapiness also has a "max" parameter, so we could just say "override" /"
>> disabled" / whatever?
> 
> I don't love the user being able to override this though, let's just nuke their
> ability to set this pleeeease.
> 
> Because if they can override it, then we have to do some deeply nasty scaling
> for mTHP again.

There are ways to have it working internally, just using a different 
"scale" instead of the 100 -> 50 -> 25 etc.

I am afraid we cannot change the parameter to ignore other values 
because of the interaction with the shrinker that easily .... we might 
be able to detracted at wait a bunch of kernel releases probably.

-- 
Cheers

David / dhildenb