Re: [PATCH v11 00/15] khugepaged: mTHP support

[David Hildenbrand <david@redhat.com>]

On 12.09.25 14:19, Kiryl Shutsemau wrote:
> On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote:
>> The following series provides khugepaged with the capability to collapse
>> anonymous memory regions to mTHPs.
>>
>> To achieve this we generalize the khugepaged functions to no longer depend
>> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
>> pages that are occupied (!none/zero). After the PMD scan is done, we do
>> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD
>> range. The restriction on max_ptes_none is removed during the scan, to make
>> sure we account for the whole PMD range. When no mTHP size is enabled, the
>> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
>> by the attempted collapse order to determine how full a mTHP must be to be
>> eligible for the collapse to occur. If a mTHP collapse is attempted, but
>> contains swapped out, or shared pages, we don't perform the collapse. It is
>> now also possible to collapse to mTHPs without requiring the PMD THP size
>> to be enabled.
>>
>> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
>> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
>> mTHP collapses to prevent collapse "creep" behavior. This prevents
>> constantly promoting mTHPs to the next available size, which would occur
>> because a collapse introduces more non-zero pages that would satisfy the
>> promotion condition on subsequent scans.
> 
> Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
> all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.

BTW, I thought further about this and I agree: if we count zero-filled 
pages towards none_or_zero one we can avoid the "creep" problem.

The scanning-for-zero part is rather nasty, though.

-- 
Cheers

David / dhildenb