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

[Wei Yang]

On Tue, Jun 09, 2026 at 03:16:10PM +0200, David Hildenbrand (Arm) wrote:
>On 6/9/26 14:04, Lance Yang wrote:
>> From: Lance Yang <lance.yang@xxxxxxxxx>
>> 
>> 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@xxxxxxxxx>
>> ---
>> 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@xxxxxxxxxxxxxx/
>> [2] https://lore.kernel.org/linux-mm/20260605161422.213817-12-npache@xxxxxxxxxx/
>> 
>>  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@xxxxxxxxxx>
>
>
>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.

I mentioned it in [1] too.

[1]: https://lore.kernel.org/linux-mm/20260512154431.jxcs632mqqatqtsw@master/

>
>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().
>
>-- 
>Cheers,
>
>David

-- 
Wei Yang
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