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

[David Hildenbrand <david@redhat.com>]

On 20.01.25 14:37, Ryan Roberts wrote:
> On 20/01/2025 12:54, David Hildenbrand wrote:
>>>> 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" ? :)
>>
> 
> That sounds like a great simplification in principle!

And certainly a much easier to start with :)

If we ever get the request to support something else, maybe that's also 
where we can learn *why*, and what we would actually want to do with mTHP.

> We would need to consider
> the swap and shared tunables too though. Perhaps we can pull a similar trick
> with those?

Swapped and shared are a bit more challenging, because they are set to 
"/ 2" or "/ 8" heuristics.


One simple starting point here is of course to say "when collapsing 
mTHP, all have to be unshared and all have to be swapped in", so to 
essentially ignore both tunables (in a memory friendly way, as if they 
are set to 0) for mTHP collapse and worry about that later, when really 
required.

Two alternatives I discussed with Nico for these (not sure which is 
implemented here) is to calculate it proportionally to the folio order 
we are collapsing:

Assuming max_ptes_swap = 64 (PMD: 512 PTEs) and we are collapsing a 1 
MiB mTHP (256 PTEs), 32 PTEs would be allowed to be swapped out.

-- 
Cheers,

David / dhildenb