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

Lorenzo Stoakes
2025-08-20T10:44:43Z

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@redhat.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
>