Re: [PATCH v10 07/13] khugepaged: skip collapsing mTHP to smaller orders

[Lorenzo Stoakes]

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
>