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

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

[Lance Yang]

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;
+
 		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
 			enum scan_result ret;
 
-- 
2.49.0

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

[David Hildenbrand (Arm)]

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().

-- 
Cheers,

David

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

[Lorenzo Stoakes]

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@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 :)

From my point of view, accepting the mTHP khugepaged changes was essentially a
big compromise on how much it adds to the mess of the existing code base, and
AFAIC we shouldn't accept any further major changes until we actually sort this
mess out :)

>
> 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().

Ugh uffd.

>
> --
> Cheers,
>
> David

Cheers, Lorenzo

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

[Lance Yang]

On 2026/6/9 21:16, 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.
>>
>> [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 :)

Yes, follow-up cleanup material :)

> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks!

> 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.

Oh, good catch. We may end up trying a collapse that cannot really
go anywhere ... One for a follow-up.

> We could likely handle the userfaultfd_armed() part easier: some indication that
> we must not have any pte_none() would be sufficient.

Right. By the time we get to mthp_collapse(), we probably only need
to carry that as a small "no pte_none" constraint for the candidate
range.

> 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().

Makes sense to me. I'll take a look when I get a chance. And yeah,
as Lorenzo said, better to clean up the khugepaged mess first
before piling more on top :)

Cheers, Lance

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

[Nico Pache]

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

>
> --
> Cheers,
>
> David
>

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

[David Hildenbrand (Arm)]

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?

-- 
Cheers,

David

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

[Lance Yang]

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

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

[Andrew Morton]

On Tue, 9 Jun 2026 21:56:13 +0800 Lance Yang <lance.yang@linux.dev> wrote:

> >> 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.

No probs.

> Should be soon, I guess :P

I move the second batch into mm-stable after Linus has merged the first
batch, usually.

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

[Lorenzo Stoakes]

On Tue, Jun 09, 2026 at 08:04:43PM +0800, 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>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
> 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;
> +
>  		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
>  			enum scan_result ret;
>
> --
> 2.49.0
>

Thanks, Lorenzo

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

[Nico Pache]

On Tue, Jun 9, 2026 at 6:05 AM Lance Yang <lance.yang@linux.dev> 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.

Thanks for sending the fixup :)

Acked-by: Nico Pache <npache@redhat.com>

>
> [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;
> +
>                 if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
>                         enum scan_result ret;
>
> --
> 2.49.0
>