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

[Lance Yang]

On 2026/6/10 01:04, Nico Pache wrote:
> On Tue, Jun 9, 2026 at 7:16 AM David Hildenbrand (Arm) <david@xxxxxxxxxx> 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.
> >
> > 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