Re: [PATCH v4 08/12] khugepaged: skip collapsing mTHP to smaller orders

[Nico Pache <npache@redhat.com>]

On Thu, Apr 24, 2025 at 1:49 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/4/17 08:02, 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@arm.com/
> >
> > Co-developed-by: Dev Jain <dev.jain@arm.com>
> > Signed-off-by: Dev Jain <dev.jain@arm.com>
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >   mm/khugepaged.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index ece39fd71fe6..383aff12cd43 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -625,7 +625,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >               folio = page_folio(page);
> >               VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
> >
> > -             /* See hpage_collapse_scan_pmd(). */
> > +             if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) {
> > +                     result = SCAN_PTE_MAPPED_HUGEPAGE;
> > +                     goto out;
> > +             }
>
> Should we also add this check in hpage_collapse_scan_pmd() to abort the
> scan early?
No I dont think so, we can't abort there because we dont know the
attempted collapse order, and we dont want to miss potential mTHP
collapses (by bailing out early and not populating the bitmap).

-- Nico
>