Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Usama Arif]

On Wed, 25 Feb 2026 20:26:31 -0700 Nico Pache <npache@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 1c3711ed4513..388d3f2537e2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1492,9 +1492,42 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address,
>  			ret = collapse_huge_page(mm, collapse_address, referenced,
>  						 unmapped, cc, mmap_locked,
>  						 order);
> -			if (ret == SCAN_SUCCEED) {
> +
> +			switch (ret) {
> +			/* Cases were we continue to next collapse candidate */
> +			case SCAN_SUCCEED:
>  				collapsed += nr_pte_entries;
> +				fallthrough;
> +			case SCAN_PTE_MAPPED_HUGEPAGE:
>  				continue;
> +			/* Cases were lower orders might still succeed */
> +			case SCAN_LACK_REFERENCED_PAGE:
> +			case SCAN_EXCEED_NONE_PTE:
> +			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;
> +			/* Cases were no further collapse is possible */
> +			case SCAN_CGROUP_CHARGE_FAIL:

The only one that stands out to me is SCAN_CGROUP_CHARGE_FAIL. memcg charging
of higher order folio might fail, but a lower order folio might pass?
That said, if the cgroup is that tight, continuing collapse work may not
be productive.

Acked-by: Usama Arif <usama.arif@xxxxxxxxx>

> +			case SCAN_COPY_MC:
> +			case SCAN_ADDRESS_RANGE:
> +			case SCAN_NO_PTE_TABLE:
> +			case SCAN_ANY_PROCESS:
> +			case SCAN_VMA_NULL:
> +			case SCAN_VMA_CHECK:
> +			case SCAN_SCAN_ABORT:
> +			case SCAN_PAGE_ANON:
> +			case SCAN_PMD_MAPPED:
> +			case SCAN_FAIL:
> +			default:
> +				return collapsed;
>  			}
>  		}
>  
> -- 
> 2.53.0
> 
>