Re: [PATCH 16/16] selftests/mm: skip khugepaged shmem cases without a PMD page cache folio

[Zi Yan]

On Sun Aug 2, 2026 at 3:52 PM EDT, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
>
> The page cache caps folio order at MAX_PAGECACHE_ORDER, and
> xas_split_alloc() puts that cap below the PMD order where a PMD is 512M --
> arm64 with 64K base pages, as include/linux/pagemap.h says outright.
> shmem_huge_global_enabled() then offers no PMD order at all, so
> MADV_COLLAPSE of a shmem range answers -EINVAL and khugepaged passes over
> it.

IIRC, after READ_ONLY_THP_FOR_FS is removed, all pagecache folios are
split using non uniform split, xas_try_split(), so does shmem (except
shmem in swapcache not splittable). In theory, we can get rid of the
cap, since xas_try_split() does not split more than one level like
one can try to make xas_split_alloc() split more than two level (e.g.,
512MB to 64KB on arm64 with 64KB base page).

>
> The shmem cases nonetheless ask for a PMD-sized shmem folio, so on such a
> configuration four of them fail and the run bails out in the middle:
>
> not ok 2 collapse_full not ok 4 collapse_single_pte_entry # Allocate huge
> page...Bail out! madvise(MADV_COLLAPSE): Invalid argument (22)
>
> That is the kernel declining something it deliberately does not support,
> not a collapse defect.  Skip those cases where the PMD order is not a shmem
> order, which thp_shmem_supported_orders() already reports -- it reads the
> same per-size shmem_enabled controls the kernel only publishes for orders
> the page cache can hold.  A tmpfs-backed file argument is skipped on the
> same grounds, and a run left with nothing to collapse into skips outright.
>
> Anonymous collapse is unaffected: its orders are not capped this way, and
> the anonymous cases pass at a 512M PMD.
>
> Assisted-by: Claude-Code:claude-opus-5
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> ---
>  tools/testing/selftests/mm/khugepaged.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 9213ce1658d0..c5a3c3922581 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -1584,6 +1584,27 @@ int main(int argc, char **argv)
>  	hpage_pmd_nr = hpage_pmd_size / page_size;
>  	hpage_pmd_order = __builtin_ctz(hpage_pmd_nr);
>  
> +	/*
> +	 * The page cache caps folio order at MAX_PAGECACHE_ORDER, which
> +	 * xas_split_alloc() puts below the PMD order on arm64 with 64K pages.
> +	 * A PMD-sized page cache folio is then impossible, so the kernel
> +	 * refuses these collapses by design and there is nothing to test.
> +	 */
> +	if (!(thp_shmem_supported_orders() & (1UL << hpage_pmd_order))) {
> +		if (shmem_ops) {
> +			ksft_print_msg("no PMD-order page cache folio: skipping shmem\n");
> +			shmem_ops = NULL;
> +		}
> +		if (finfo.type == VMA_SHMEM && read_only_file_ops) {
> +			ksft_print_msg("no PMD-order page cache folio: skipping tmpfs 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");
> +	}
> +
>  	if (anon_target_order &&
>  	    !(thp_supported_orders() & (1UL << anon_target_order)))
>  		ksft_exit_skip("Order %d is not a supported anon THP order\n",




-- 
Best Regards,
Yan, Zi