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

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.
+	 */
+	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;
-- 
2.54.0