Re: [PATCH mm-unstable 1/1] mm/khugepaged: fix PMD collapse swap PTE accounting

[Lance Yang <lance.yang@linux.dev>]

On 2026/6/9 21:20, David Hildenbrand (Arm) wrote:
> On 6/9/26 14:04, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> mthp_collapse() uses mthp_present_ptes to decide whether a range has
>> enough occupied PTEs to try collapse. Swap PTEs accepted by
>> collapse_scan_pmd() are counted in unmapped, but are not represented in
>> mthp_present_ptes.
>>
>> When lower orders are enabled, collapse_scan_pmd() relaxes max_ptes_none
>> so the scan can cover the whole PMD and build the bitmap. mthp_collapse()
>> then checks the PMD-order candidate using the bitmap.
>>
>> With max_ptes_none set to 0, a range with 511 present PTEs and one swap
>> PTE no longer reaches collapse_huge_page(), even though PMD collapse can
>> handle swap PTEs up to max_ptes_swap.
>>
>> Account unmapped PTEs only for PMD order. PMD collapse supports swap PTEs
>> through max_ptes_swap, while lower-order mTHP collapse does not currently
>> support non-present PTEs. Keep non-present PTEs out of the lower-order
>> eligibility check.
>>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>> Sent separately, as discussed in [1], to spell out the PMD-order swap PTE
>> case. Patch [2] is still only in mm-unstable, so no Fixes: tag.
> 
> Right, probably we just want to add the Fixes: tag once Andrew moves the series
> to mm-stable?

Yep, hopefully Andrew can add the Fixes: tag when applying this, once
the series lands in mm-stable. Should be soon, I guess :P