On Tue, Aug 19, 2025 at 07:41:59AM -0600, 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/ > > Acked-by: David Hildenbrand <david@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> Other than comment below, LGTM so: Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > --- > mm/khugepaged.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 1ad7e00d3fd6..6a4cf7e4a7cc 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -611,6 +611,15 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, > folio = page_folio(page); > VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); > > + /* > + * TODO: In some cases of partially-mapped folios, we'd actually > + * want to collapse. > + */ Not a fan of adding todo's in code, they have a habit of being left forever. I'd maybe put a more written out comment something similar to the commit message. > + if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) { > + result = SCAN_PTE_MAPPED_HUGEPAGE; > + goto out; > + } > + > /* See collapse_scan_pmd(). */ > if (folio_maybe_mapped_shared(folio)) { > ++shared; > -- > 2.50.1 >
