Re: [PATCH v2 2/2] kselftest: mm: fix intermittent failure khugepaged test

[Baolin Wang]

On 9/21/26 6:35 PM, 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.
>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> ---
>   tools/testing/selftests/mm/khugepaged.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index c32244b565658..1aad4bb427ece 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -578,6 +578,9 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
>   		usleep(TICK);
>   	}
>   
> +	if (!strncmp(ops->name, "anon", 4))

We usually use the 'if (ops == &__anon_ops)' check in this file to 
identify anonymous test cases.

With that, LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

> +		madvise(p, len, MADV_NOHUGEPAGE);
> +
>   	return timeout == -1;
>   }
>