On 11/02/2025 00:30, 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/[email protected]/ > > Signed-off-by: Nico Pache <[email protected]> > --- > mm/khugepaged.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e2ba18e57064..fc30698b8e6e 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -622,6 +622,11 @@ static int __collapse_huge_page_isolate(struct > vm_area_struct *vma, > folio = page_folio(page); > VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); > > + if (order != HPAGE_PMD_ORDER && folio_order(folio) >= order) { > + result = SCAN_PTE_MAPPED_HUGEPAGE; > + goto out; > + }
One of the key areas where we want to benefit from khugepaged collapsing to mTHP is when a COW event happens. If the original folio was large, then it becomes partially mapped due to COW and we want to collapse it again. I think this will prevent that? I made some fairly detailed suggestions for what I think is the right approach in the context of Dev's series. It would be good to get your thoughts on that: https://lore.kernel.org/lkml/[email protected]/ Thanks, Ryan > + > /* See khugepaged_scan_pmd(). */ > if (folio_likely_mapped_shared(folio)) { > ++shared;
