Re: [PATCH v4 2/2] kselftest: mm: fix intermittent failure khugepaged test

[Lorenzo Stoakes (ARM)]

On Tue, Sep 29, 2026 at 11:33:19AM +0100, Yeoreum Yun wrote:
> There are intermittent failures in collapse_max_ptes_swap() and
> collapse_max_ptes_shared() when using the khugepaged_context:
>
>   // while running ./khugepaged -s 2
>
>   # Run test: collapse_max_ptes_shared (khugepaged:anon)
>   # Allocate huge page... OK
>   # Share huge page over fork()... OK
>   # Trigger CoW on page 1023 of 2048... OK
>   # Maybe collapse with max_ptes_shared exceeded.... OK
>   # Trigger CoW on page 1024 of 2048... Fail
>   Bail out! Unexpected huge page
>   # Planned tests != run tests (26 != 23)
>   # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0
>
>   # Run test: collapse_max_ptes_swap (khugepaged:anon)
>   # Swapout 257 of 2048 pages... OK
>   # Maybe collapse with max_ptes_swap exceeded.... OK
>   # Swapout 256 of 2048 pages... OK
>   Bail out! Unexpected huge page
>   # Planned tests != run tests (26 != 17)
>   # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0
>
> This happens because khugepaged may collapse the pages before wait_for_scan()
> is called, causing a sanity check that expects uncollapsed pages to fail.
>
> For example, in collapse_max_ptes_swap(), after faulting the pages back in
> and paging out up to max_ptes_swap pages, khugepaged may collapse them again
> before c->collapse() is called.
>
> To prevent this, mark the VMA with MADV_NOHUGEPAGE after it has been
> collapsed by wait_for_scan() for anon. This prevents khugepaged from
> collapsing it again before c->collapse() is called.
>
> This failure was observed on NVIDIA Spark with 16KB page.
>
> Reviewed-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>

Looks sensible to me so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>

> ---
>  tools/testing/selftests/mm/khugepaged.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 2aa7c9197158..4e888b7bf310 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -618,6 +618,16 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
>  		usleep(TICK);
>  	}
>
> +	/*
> +	 * The file and shmem tests rely on refaults to install PMD mappings
> +	 * after collapse. MADV_NOHUGEPAGE would prevent those mappings.
> +	 *
> +	 * Apply MADV_NOHUGEPAGE only to anonymous VMAs to prevent khugepaged
> +	 * from unexpectedly collapsing pages during the test.
> +	 */
> +	if (is_anon(ops))
> +		madvise(p, len, MADV_NOHUGEPAGE);
> +
>  	return timeout == -1;
>  }
>
>
> --
> 2.43.0
>

--
Cheers, Lorenzo