Re: [PATCH v3 1/2] kselftest: mm: prevent random failure of huge page split for khugepaged

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Fri, Aug 28, 2026 at 09:11:33AM +0100, Yeoreum Yun wrote:
> There're some random failure for split_huge_page_test when khugepaged
> collapses pages into pmd again which had split by the test.
>
> Prevent the khugepaged's collapses for split page by setting the
> mapped pmd-huge-page with MADV_NOHUGEPAGE before split.
>
> Reported-by: Kevin Brodsky <kevin.brodsky@arm.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
>  tools/testing/selftests/mm/split_huge_page_test.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index 86a603692826..c718269a291f 100644
> --- a/tools/testing/selftests/mm/split_huge_page_test.c
> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> @@ -180,6 +180,10 @@ static void verify_rss_anon_split_huge_page_all_zeroes(char *one_page, int nr_hp
>  	if (!rss_anon_before)
>  		ksft_exit_fail_msg("No RssAnon is allocated before split\n");
>
> +	/* Prevent khugepaged from collapsing the pages. */
> +	if (madvise(one_page, len, MADV_NOHUGEPAGE))
> +		ksft_print_msg("madvise(MADV_NOHUGEPAGE) failed\n");

This should probably terminate the test no? There's no reason to expect this to
fail and it's better to fail then to risk a flake :)

> +
>  	/* split all THPs */
>  	write_debugfs(PID_FMT, getpid(), (uint64_t)one_page,
>  		      (uint64_t)one_page + len, 0);
> @@ -227,6 +231,10 @@ static void split_pmd_thp_to_order(int order)
>  	if (!check_huge_anon(one_page, 4 * pmd_pagesize, 4, pmd_pagesize))
>  		ksft_exit_fail_msg("No THP is allocated\n");
>
> +	/* Prevent khugepaged from collapsing the pages. */
> +	if (madvise(one_page, len, MADV_NOHUGEPAGE))
> +		ksft_print_msg("madvise(MADV_NOHUGEPAGE) failed\n");
> +

Same comment as above, also since this is a repeated pattern, I think it's worth
abstracting it like:

	static void madv_nohuge(char *ptr, size_t len)
	{
		if (!madvise(ptr, len, MADV_NOHUGEPAGE))
			return;

		ksft_exit_fail_msg("MADV_NOHUGEPAGE failed, err=%d\n", errno);
	}

>  	/* split all THPs */
>  	write_debugfs(PID_FMT, getpid(), (uint64_t)one_page,
>  		(uint64_t)one_page + len, order);
> @@ -313,6 +321,10 @@ static void split_pte_mapped_thp(void)
>  		goto out;
>  	}
>
> +	/* Prevent khugepaged from collapsing the pages. */
> +	if (madvise(thp_area, thp_area_size, MADV_NOHUGEPAGE))
> +		ksft_print_msg("madvise(MADV_NOHUGEPAGE) failed\n");
> +
>  	/* Split all THPs through the remapped pages. */
>  	write_debugfs(PID_FMT, getpid(), (uint64_t)page_area,
>  		      (uint64_t)page_area + page_area_size, 0);
> @@ -542,6 +554,9 @@ static int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size,
>  		ksft_test_result_skip("Pagecache folio split skipped\n");
>  		return -2;
>  	}
> +	/* Prevent khugepaged from collapsing the pages. */
> +	if (madvise(*addr, fd_size, MADV_NOHUGEPAGE))
> +		ksft_print_msg("madvise(MADV_NOHUGEPAGE) failed\n");

Obviously same comments re: this and above

>  	return 0;
>  err_out_close:
>  	close(*fd);
>
> --
> 2.43.0
>

--
Cheers, Lorenzo