> 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
Nit: the code comment phrases it better: "so kernel refuses these collapses". > -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]> > > diff --git a/tools/testing/selftests/mm/khugepaged.c > b/tools/testing/selftests/mm/khugepaged.c > index a5ada78d90ee..b436b401e2ea 100644 > --- a/tools/testing/selftests/mm/khugepaged.c > +++ b/tools/testing/selftests/mm/khugepaged.c > @@ -1391,6 +1391,30 @@ 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. > + */ > + 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 (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; Reviewed-by: Mike Rapoport (Microsoft) <[email protected]> -- Sincerely yours, Mike.
