linux-kernel - Re: [PATCH v10 00/13] khugepaged: mTHP support

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Message-ID: <bdbb5168-7657-4f11-a42d-b75cce7e0bca@lucifer.local>
Date: Fri, 22 Aug 2025 11:41:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Nico Pache <npache@...hat.com>, Dev Jain <dev.jain@....com>,
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Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support

On Thu, Aug 21, 2025 at 10:43:35PM +0200, David Hildenbrand wrote:
> >
> > The one thing we absolutely cannot have is a default that causes this
> > 'creeping' behaviour. This feels like shipping something that is broken and
> > alluding to it in the documentation.
> >
> > I spoke to David off-list and he gave some insight into this and perhaps
> > some reasonable means of avoiding an additional tunable.
> >
> > I don't want to rehash what he said as I think it's more productive for him
> > to reply when he has time but broadly I think how we handle this needs
> > careful consideration.
> >
> > To me it's clear that some sense of ratio is just immediately very very
> > confusing, but then again this interface is already confusing, as with much
> > of THP.
> >
> > Anyway I'll let David respond here so we don't loop around before he has a
> > chance to add his input.
> >
> > Cheers, Lorenzo
> >
>
> [Resending because Thunderbird decided to use the wrong smtp server]
>
> I've been summoned.

Welcome :)

>
> As raised in the past, I would initially only support specific values here like
>
> 0 				  : Never collapse with any pte_none/zeropage
> 511 (HPAGE_PMD_NR - 1) / default  : Always collapse, ignoring pte_none/zeropage
>

OK so if had effectively an off/on (I guess we have to keep this as it is for
legay purposes) and is forced to one or other of these values then fine (as long
as we don't have uAPI worries).

> Once could also easily support the value 255 (HPAGE_PMD_NR / 2- 1), but not sure
> if we have to add that for now.

Yeah not so sure about this, this is a 'just have to know' too, and yes you
might add it to the docs, but people are going to be mightily confused, esp if
it's a calculated value.

I don't see any other way around having a separate tunable if we don't just have
something VERY simple like on/off.

Also the mentioned issue sounds like something that needs to be fixed elsewhere
honestly in the algorithm used to figure out mTHP ranges (I may be wrong - and
happy to stand corrected if this is somehow inherent, but reallly feels that
way).

>
> Because, as raised in the past, I'm afraid nobody on this earth has a clue how
> to set this parameter to values different to 0 (don't waste memory with khugepaged)
> and 511 (page fault behavior).

Yup

>
>
> If any other value is set, essentially
> 	pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse");
>
> for now and just disable it.

Hmm but under what circumstances? I would just say unsupported value not mention
mTHP or people who don't use mTHP might find that confusing.

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo