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]/ Reviewed-by: Lorenzo Stoakes <[email protected]> Reviewed-by: Baolin Wang <[email protected]> Acked-by: David Hildenbrand <[email protected]> Co-developed-by: Dev Jain <[email protected]> Signed-off-by: Dev Jain <[email protected]> Signed-off-by: Nico Pache <[email protected]> --- mm/khugepaged.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 248947e78a30..ebcc0c85a0d6 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -610,6 +610,15 @@ 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. + */ + 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; -- 2.51.0
