Re: [PATCH mm-unstable v15 06/13] mm/khugepaged: skip collapsing mTHP to smaller orders

[Nico Pache]

On Thu, Mar 12, 2026 at 3:00 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 2/26/26 04:24, Nico Pache wrote:
> > khugepaged may try to collapse a mTHP to a smaller mTHP, resulting in
> > some pages being unmapped. Skip these cases until we have a way to check
> > if its ok to collapse to a smaller mTHP size (like in the case of a
> > partially mapped folio).
> >
> > This patch is inspired by Dev Jain's work on khugepaged mTHP support [1].
> >
> > [1] https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@xxxxxxx/
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > Co-developed-by: Dev Jain <dev.jain@xxxxxxx>
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> > ---
> >  mm/khugepaged.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index fb3ba8fe5a6c..c739f26dd61e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -638,6 +638,14 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >                               goto out;
> >                       }
> >               }
> > +             /*
> > +              * TODO: In some cases of partially-mapped folios, we'd actually
> > +              * want to collapse.
> > +              */
> > +             if (!is_pmd_order(order) && folio_order(folio) >= order) {
> > +                     result = SCAN_PTE_MAPPED_HUGEPAGE;
> > +                     goto out;
> > +             }
> >
> >               if (folio_test_large(folio)) {
> >                       struct folio *f;
>
> Why aren't we doing the same in hpage_collapse_scan_pmd() ?

We can't do this in the scan phase because we are not yet aware of the
order we want to collapse to.

The scan phase builds the bitmap (if mthp sizes are enabled). I tried
to think if there were any similar checks we could perform; the only
one that came to mind is whether the current folio size exceeds the
highest enabled folio size. Is that worth checking?

Cheers,
-- Nico

>
> --
> Cheers,
>
> David
>