Re: [PATCH v4 04/19] selftests/mm: skip khugepaged page cache cases without a PMD folio

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

On Sat, Aug 15, 2026 at 02:58:46AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
>
> The page cache caps folio order at MAX_PAGECACHE_ORDER, which is smaller
> than the PMD order on arm64 with 64K pages, where a PMD is 512M.  A
> PMD-sized page cache folio is impossible there, so MADV_COLLAPSE answers
> -EINVAL and khugepaged passes over the range.  The shmem cases ask for a
> PMD-sized folio anyway, so four of them fail and the run bails out in the
> middle.
>
> MAX_PAGECACHE_ORDER is not shmem-specific: it caps every file folio.  Skip
> both mem types where the cap is below the PMD order.
>
> Anonymous collapse is unaffected: its orders are not capped this way.
>
> Assisted-by: Claude-Code:claude-opus-5
> Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> ---
>  tools/testing/selftests/mm/khugepaged.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index c499804a0ec4..ec5c36a19d92 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -1357,6 +1357,30 @@ int main(int argc, char **argv)
>
>  	setbuf(stdout, NULL);
>
> +	/*
> +	 * The page cache caps folio order at MAX_PAGECACHE_ORDER, which is
> +	 * below the PMD order on arm64 with 64K pages.  A PMD-sized page cache
> +	 * folio is impossible there, so the kernel refuses these collapses by
> +	 * design and there is nothing to test.  The cap is not shmem-specific:
> +	 * it rules out regular files too, and the per-order shmem_enabled
> +	 * controls exist for exactly the orders it allows, which is what makes
> +	 * them readable here.
> +	 */

This is a very schloppy comment. Can you trim it please?

> +	if (!(thp_shmem_supported_orders() & (1UL << hpage_pmd_order))) {

Is this inferring file-backed khugepaged behaviour from shmem? That seems iffy.

> +		if (shmem_ops) {
> +			ksft_print_msg("no PMD-order page cache folio: skipping shmem\n");
> +			shmem_ops = NULL;
> +		}
> +		if (read_only_file_ops) {
> +			ksft_print_msg("no PMD-order page cache folio: skipping file\n");
> +			read_only_file_ops = NULL;
> +			read_write_file_read_ops = NULL;
> +			read_write_file_write_ops = NULL;
> +		}
> +		if (!anon_ops && !shmem_ops && !read_only_file_ops)
> +			ksft_exit_skip("Nothing left to collapse into\n");
> +	}
> +
>  	default_settings.khugepaged.max_ptes_none = hpage_pmd_nr - 1;
>  	default_settings.khugepaged.max_ptes_swap = hpage_pmd_nr / 8;
>  	default_settings.khugepaged.max_ptes_shared = hpage_pmd_nr / 2;
> --
> 2.54.0
>

--
Cheers, Lorenzo