Re: [PATCH mm-unstable v15 06/13] mm/khugepaged: skip collapsing mTHP to smaller orders

[David Hildenbrand (Arm)]

On 2/26/26 04:24, 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/
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@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>
> ---
>  mm/khugepaged.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index fb3ba8fe5a6c..c739f26dd61e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -638,6 +638,14 @@ 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;

Why aren't we doing the same in hpage_collapse_scan_pmd() ?

-- 
Cheers,

David