[RFC v2 18/21] selftests/mm: add partial_mprotect test for change_pmd_range

[Usama Arif <usama.arif@linux.dev>]

Add test for partial mprotect on THP which exercises change_pmd_range().
This verifies that partial mprotect correctly splits the PMD, applies
protection only to the requested portion, and leaves the rest of the
mapping writable.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 .../testing/selftests/mm/thp_pmd_split_test.c | 31 +++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/tools/testing/selftests/mm/thp_pmd_split_test.c b/tools/testing/selftests/mm/thp_pmd_split_test.c
index 0f54ac04760d5..4944a5a516da9 100644
--- a/tools/testing/selftests/mm/thp_pmd_split_test.c
+++ b/tools/testing/selftests/mm/thp_pmd_split_test.c
@@ -146,4 +146,35 @@ TEST_F(thp_pmd_split, partial_munmap)
 		self->split_pmd_failed_before);
 }
 
+/*
+ * Partial mprotect on THP (change_pmd_range)
+ *
+ * Tests that partial mprotect of a THP correctly splits the PMD and
+ * applies protection only to the requested portion. This exercises
+ * the mprotect path which now handles split failures.
+ */
+TEST_F(thp_pmd_split, partial_mprotect)
+{
+	volatile unsigned char *ptr = (volatile unsigned char *)self->aligned;
+	int ret;
+
+	ret = allocate_thp(self->aligned, self->pmdsize);
+	if (ret)
+		SKIP(return, "Failed to allocate THP");
+
+	/* Partial mprotect - make middle page read-only */
+	ret = mprotect((char *)self->aligned + self->pagesize, self->pagesize, PROT_READ);
+	ASSERT_EQ(ret, 0);
+
+	/* Verify we can still write to non-protected pages */
+	ptr[0] = 0xDD;
+	ptr[self->pmdsize - 1] = 0xEE;
+
+	ASSERT_EQ(ptr[0], (unsigned char)0xDD);
+	ASSERT_EQ(ptr[self->pmdsize - 1], (unsigned char)0xEE);
+
+	log_and_check_pmd_split(_metadata, self->split_pmd_before,
+		self->split_pmd_failed_before);
+}
+
 TEST_HARNESS_MAIN
-- 
2.47.3