Re: [PATCH v12 mm-new 13/15] khugepaged: avoid unnecessary mTHP collapse attempts

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Oct 22, 2025 at 12:37:15PM -0600, Nico Pache wrote:
> There are cases where, if an attempted collapse fails, all subsequent
> orders are guaranteed to also fail. Avoid these collapse attempts by
> bailing out early.
>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index e2319bfd0065..54f5c7888e46 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1431,10 +1431,39 @@ static int collapse_scan_bitmap(struct mm_struct *mm, unsigned long address,
>  			ret = collapse_huge_page(mm, address, referenced,
>  						 unmapped, cc, mmap_locked,
>  						 order, offset);
> -			if (ret == SCAN_SUCCEED) {
> +
> +			/*
> +			 * Analyze failure reason to determine next action:
> +			 * - goto next_order: try smaller orders in same region
> +			 * - continue: try other regions at same order

The stack is a DFS, so this isn't correct, you may have pushed a bunch of higher
order candidate mTHPs (I don't like plain 'region') which you will also true.

> +			 * - break: stop all attempts (system-wide failure)
> +			 */

This comment isn't hugely helpful, just put the relevant comments next to each
of the goto, continue, break (soon to be return re: review below) statements
please.

> +			switch (ret) {
> +			/* Cases were we should continue to the next region */
> +			case SCAN_SUCCEED:
>  				collapsed += 1UL << order;
> +				fallthrough;
> +			case SCAN_PTE_MAPPED_HUGEPAGE:
>  				continue;

Would we not run into trouble potentially in the previous patch's implementation
of this examing candidate mTHPs that are part of an already existing huge page,
or would a folio check in the collapse just make this wasted work?

> +			/* Cases were lower orders might still succeed */
> +			case SCAN_LACK_REFERENCED_PAGE:
> +			case SCAN_EXCEED_NONE_PTE:

How can we, having checked the max_pte_none, still fail due to this?

> +			case SCAN_EXCEED_SWAP_PTE:
> +			case SCAN_EXCEED_SHARED_PTE:
> +			case SCAN_PAGE_LOCK:
> +			case SCAN_PAGE_COUNT:
> +			case SCAN_PAGE_LRU:
> +			case SCAN_PAGE_NULL:
> +			case SCAN_DEL_PAGE_LRU:
> +			case SCAN_PTE_NON_PRESENT:
> +			case SCAN_PTE_UFFD_WP:
> +			case SCAN_ALLOC_HUGE_PAGE_FAIL:
> +				goto next_order;
> +			/* All other cases should stop collapse attempts */

I don't love us having a catch-all, plase spell out all cases.

> +			default:
> +				break;
>  			}
> +			break;

_Hate_ this double break. Just return collapsed please.

>  		}
>
>  next_order:
> --
> 2.51.0
>