Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[David Hildenbrand <david@redhat.com>]

On 25.06.25 10:12, Lorenzo Stoakes wrote:
> On Wed, Jun 25, 2025 at 08:55:28AM +0100, Lorenzo Stoakes wrote:
>> I suppose the least awful way of addressing Baolin's concerns re: mTHP
>> while simultaneosly keeping existing semantics is:
>>
>> 1. Introduce deny to mean what never should have meant.
> 
> To fix Baolin's issue btw we'd have to add 'deny' to both 'global' settings
> _and_ each page size setting.
> 
> Because otherwise we'd end up in a weird case where say:
> 
> global 'deny'
> 
>   2 MiB 'never'
> 64 KiB 'inherit'
> 
> And err... get 2 MiB THP pages from MADV_COLLAPSE :)
> 
> Or:
> 
> global 'deny'
> 
>   2 MiB 'never'
> 64 KiB 'always'
> 
> Or:
> 
> global 'never'
> 
>   2 MiB 'never'
> 64 KiB 'always'
> 
> Or:
> 
> global 'never'
> 
>   2 MiB 'madvise'
> 64 KiB 'always'
> 
> All doing the same. Not very clear is it?
> 
> We have sowed the seeds of something terrible here, truly.

Fully agreed. "Deny" is nasty. Maybe if we really need a way to disable 
"madv_collapse", it should be done differently, not using this toggle here.

Regarding MADV_COLLAPSE, I strongly assume that we should not change it 
to collapse smaller mTHPs as part of the khugepaged mTHP work. For now, 
it will simply always collapse to PMD THPs.

Once we want to support other sizes, likely MADV_COLLAPSE users want to 
have better control over which size to use, at which point it all gets 
nasty.

-- 
Cheers,

David / dhildenb