[PATCH v4 02/19] selftests/mm: skip collapse_compound_extreme() where the PMD is too large

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

collapse_compound_extreme() builds a PTE table full of distinct PTE-mapped
compound pages by cycling hpage_pmd_nr fault-time THPs through mremap.  It
therefore needs hpage_pmd_nr PMD-order allocations in a row.  That is fine
at a 2M PMD (4K base pages) or a 32M one (16K).  A 512M PMD -- arm64 with
64K base pages -- makes each of those an order-13 allocation, which the
allocator cannot reliably hand out even once, let alone 8192 times.

The failure is not a quiet one: the case calls ksft_exit_fail_msg(), so the
whole binary stops and every case after it is lost.

Skip the case where the PMD is larger than 32M.  The MADV_COLLAPSE cases
still cover PMD-order collapse on those configurations, and 4K and 16K
PMDs are unaffected.

Assisted-by: Claude-Code:claude-opus-5
Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 tools/testing/selftests/mm/khugepaged.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 6cfec2b940ac..dd924edd8557 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -940,6 +940,16 @@ static void collapse_compound_extreme(struct collapse_context *c, struct mem_ops
 	void *p;
 	int i;
 
+	/*
+	 * The test needs hpage_pmd_nr PMD-order allocations, which is likely to
+	 * fail for large PMD sizes.  Skip if the PMD size is over 32M.
+	 */
+	if (hpage_pmd_size > (32UL << 20)) {
+		ksft_test_result_skip("%s: PMD too large for fault-time THP construction\n",
+				      __func__);
+		return;
+	}
+
 	p = ops->setup_area(1);
 	ksft_print_msg("Construct PTE page table full of different PTE-mapped compound pages\n");
 	for (i = 0; i < hpage_pmd_nr; i++) {
-- 
2.54.0