Re: [RFC 00/11] khugepaged: mTHP support

David Hildenbrand
2025-01-20T12:54:25Z

>> I think the 1 problem that emerged during review of Dev's series, which we don't
>> have a proper solution to yet, is the issue of "creep", where regions can be
>> collapsed to progressively higher orders through iterative scans. At each
>> collapse, the required thresholds (e.g. max_ptes_none) are met, and the collapse
>> effectively adds more non-none ptes so the next scan will then collapse to even
>> higher order. Does your solution suffer from this (theoretical/edge case) issue?
>> If not, how did you solve?
>
> Yes sadly it suffers from the same issue. bringing max_ptes_none much
> lower as a default would "help".

Can we just keep it simple and only support max_ptes_none = 511
("pagefault behavior" -- PMD_NR_PAGES - 1) or max_ptes_none = 0
("deferred behavior") and document that the other weird configurations
will make mTHP skip, because "weird and unexpetced" ? :)

--
Cheers,

David / dhildenb