[v3 21/24] selftests/mm: add partial_mprotect test for change_pmd_range

Usama Arif
2026-03-27T02:17:09Z

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
--
2.52.0