On Sat, Aug 15, 2026 at 02:58:46AM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
> ---
> 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
