linux-kernel - Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes

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Message-ID: <1c0c89d3-0f72-4631-b1fc-41e1426c8bed@redhat.com>
Date: Fri, 31 Oct 2025 12:12:37 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce
 collapse_max_ptes_none helper function

>>>> "eagerness" would not be like swappiness, and we will really have to be
>>>> careful here. I don't know yet when I will have time to look into that.
>>>
>>> I guess I missed this part of the converastion, what do you mean?
>>
>> Johannes raised issues with that on the list and afterwards we had an
>> offline discussion about some of the details and why something unpredictable
>> is not good.
> 
> Could we get these details on-list so we can discuss them? This doesn't have to
> be urgent, but I would like to have a say in this or at least be part of the
> converastion please.

Sorry, I only found now time to reply on this point. Johannes raised the 
point in [1], and afterwards we went a bit into detail in a off-list 
discussion.

In essence, I think he is right that is something we have to be very 
careful about. So it turned out as something that will take a lot more 
time+effort on my side than I originally thought, turning it not 
feasible in the short term given how I already lack behind on so many 
other things.

So I concluded that it's probably best to have such and effort be 
independent of this series. And in some way it is either way, because 
max_ptes_none is just a horrible interface given the values are 
architecture dependent.

I'll be happy if we can focus in this series on the bare minimum initial 
support, and avoid any magic (scaling / capping) as it all turned out to 
be much more tricky (interaction with the deferred shrinker ...) than 
most of us initially thought.

But I think we're already on the same page here, just wanted to share a 
bit more details on the max_ptes_none vs. eagerness idea.

[1] https://lkml.kernel.org/r/20250915134359.GA827803@cmpxchg.org

-- 
Cheers

David / dhildenb