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

David Hildenbrand
2025-08-21T20:43:42Z

>
> 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.

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

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

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).

If any other value is set, essentially
pr_warn("Unsupported 'max_ptes_none' value for mTHP collapse");

for now and just disable it.

--
Cheers

David / dhildenb