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 check is also not done during the scan phase as the current collapse order is unknown at that time.
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]> Co-developed-by: Dev Jain <[email protected]> Signed-off-by: Dev Jain <[email protected]> Signed-off-by: Nico Pache <[email protected]> --- mm/khugepaged.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index ff6f9f1883ed..8740d379882e 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -686,6 +686,14 @@ 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; -- 2.53.0
