linux-kernel - Re: [PATCH v10 08/13] khugepaged: avoid unnecessary mTHP collapse attempts

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Message-ID: <300be96e-6029-42eb-b284-e659d815ee8a@lucifer.local>
Date: Wed, 20 Aug 2025 11:38:50 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Nico Pache <npache@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
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Subject: Re: [PATCH v10 08/13] khugepaged: avoid unnecessary mTHP collapse
 attempts

On Tue, Aug 19, 2025 at 07:42:00AM -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@...hat.com>
> ---
>  mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6a4cf7e4a7cc..7d9b5100bea1 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1389,10 +1389,39 @@ 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 * KHUGEPAGED_MIN_MTHP_NR);
> -			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
> +			 * - break: stop all attempts (system-wide failure)
> +			 */
> +			switch (ret) {
> +			/* Cases were we should continue to the next region */
> +			case SCAN_SUCCEED:
>  				collapsed += (1 << order);

Yeah as bot noticed (and clang locally)

This needs a break or fallthrough.

> +			case SCAN_PAGE_RO:
> +			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;
> +			/* All other cases should stop collapse attempts */
> +			default:
> +				break;

Wouldn't it be better to not have this so the compiler asserts that you
have all cases listed here?

>  			}
> +			break;
>  		}
>
>  next_order:
> --
> 2.50.1
>