[RFC 09/12] selftests/mm: add PUD THP partial munmap test

[Usama Arif <usamaarif642@gmail.com>]

Add a test that allocates a PUD THP and unmaps a 2MB region from the
middle. Since the PUD can no longer cover the entire region, it must
be split. The test verifies that memory before and after the hole
remains accessible with correct data.

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 tools/testing/selftests/mm/pud_thp_test.c | 31 +++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/tools/testing/selftests/mm/pud_thp_test.c b/tools/testing/selftests/mm/pud_thp_test.c
index 27a509cd477d5..8d4cb0e60f7f7 100644
--- a/tools/testing/selftests/mm/pud_thp_test.c
+++ b/tools/testing/selftests/mm/pud_thp_test.c
@@ -225,4 +225,35 @@ TEST_F(pud_thp, fork_cow)
 	       self->split_before, split_after);
 }
 
+/*
+ * Test: Partial munmap triggers split
+ * Verifies that unmapping part of a PUD THP splits it correctly
+ */
+TEST_F(pud_thp, partial_munmap)
+{
+	unsigned long *ptr = (unsigned long *)self->aligned;
+	unsigned long *after_hole;
+	unsigned long split_after;
+	int ret;
+
+	/* Touch memory to allocate PUD THP */
+	memset(self->aligned, 0xDD, PUD_SIZE);
+
+	/* Unmap a 2MB region in the middle - should trigger PUD split */
+	ret = munmap((char *)self->aligned + PUD_SIZE / 2, PMD_SIZE);
+	ASSERT_EQ(ret, 0);
+
+	split_after = read_vmstat("thp_split_pud");
+
+	/* Verify memory before the hole is still accessible and correct */
+	ASSERT_EQ(ptr[0], 0xDDDDDDDDDDDDDDDDUL);
+
+	/* Verify memory after the hole is still accessible and correct */
+	after_hole = (unsigned long *)((char *)self->aligned + PUD_SIZE / 2 + PMD_SIZE);
+	ASSERT_EQ(*after_hole, 0xDDDDDDDDDDDDDDDDUL);
+
+	TH_LOG("Partial munmap completed (thp_split_pud: %lu -> %lu)",
+	       self->split_before, split_after);
+}
+
 TEST_HARNESS_MAIN
-- 
2.47.3