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

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

On 2026/6/10 01:04, Nico Pache wrote:
> On Tue, Jun 9, 2026 at 7:16 AM David Hildenbrand (Arm) <david@kernel.org> 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.
>>>
>>> [1] https://lore.kernel.org/linux-mm/CAA1CXcD7WAiA1b9GTLAuNZ+kHaFx0SzZwpBkqAZ=s+RHsTUaow@mail.gmail.com/
>>> [2] https://lore.kernel.org/linux-mm/20260605161422.213817-12-npache@redhat.com/
>>>
>>>   mm/khugepaged.c | 8 ++++++++
>>>   1 file changed, 8 insertions(+)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index b12187709f6d..617bca76db49 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -1508,6 +1508,14 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
>>>                nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset,
>>>                                                      offset + nr_ptes);
>>>
>>> +             /*
>>> +              * Swap PTEs accepted during the scan are counted in @unmapped,
>>> +              * not in the present-PTE bitmap. Account them for the PMD-order
>>> +              * candidate.
>>> +              */
>>> +             if (is_pmd_order(order))
>>> +                     nr_occupied_ptes += unmapped;
>>> +
>>
>> LGTM, there is a bit of opportunity for cleanup in the future :)
>>
>> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>>
>>
>> For example, as we no longer have the VMA here, collapse_max_ptes_none is
>> imprecise in uffd VMAs. We might try collapsing where there sure is nothing to
>> collapse.
>>
>> We could likely handle the userfaultfd_armed() part easier: some indication that
>> we must not have any pte_none() would be sufficient.
>>
>> Also, I don't see a good reason why uffd would not be allowed to collapse with
>> zeropages ... it's really just about missing faults due to pte_none().
> 
> I have some patches exactly for this :) so far 2-3 patches for better
> uffd handling

Neat, I'll leave that to you then :) Looking forward to seeing
those patches!

Cheers, Lance