[RFC v2 21/21] selftests/mm: add madv_dontneed_partial test

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

Add test for partial MADV_DONTNEED on THP. This verifies that
MADV_DONTNEED correctly triggers a PMD split, discards only the
requested page (which becomes zero-filled), and preserves data
in the surrounding pages.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 .../testing/selftests/mm/thp_pmd_split_test.c | 34 +++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/tools/testing/selftests/mm/thp_pmd_split_test.c b/tools/testing/selftests/mm/thp_pmd_split_test.c
index 1f29296759a5b..060ca1e341b75 100644
--- a/tools/testing/selftests/mm/thp_pmd_split_test.c
+++ b/tools/testing/selftests/mm/thp_pmd_split_test.c
@@ -253,4 +253,38 @@ TEST_F(thp_pmd_split, partial_mremap)
 		self->split_pmd_failed_before);
 }
 
+/*
+ * MADV_DONTNEED on THP
+ *
+ * Tests that MADV_DONTNEED on a partial THP correctly handles
+ * the PMD split and discards only the requested pages.
+ */
+TEST_F(thp_pmd_split, partial_madv_dontneed)
+{
+	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");
+
+	/* Write pattern */
+	memset(self->aligned, 0xDD, self->pmdsize);
+
+	/* Partial MADV_DONTNEED - discard middle page */
+	ret = madvise((char *)self->aligned + self->pagesize, self->pagesize, MADV_DONTNEED);
+	ASSERT_EQ(ret, 0);
+
+	/* Verify non-discarded pages still have data */
+	ASSERT_EQ(ptr[0], (unsigned char)0xDD);
+	ASSERT_EQ(ptr[2 * self->pagesize], (unsigned char)0xDD);
+	ASSERT_EQ(ptr[self->pmdsize - 1], (unsigned char)0xDD);
+
+	/* Discarded page should be zero */
+	ASSERT_EQ(ptr[self->pagesize], (unsigned char)0x00);
+
+	log_and_check_pmd_split(_metadata, self->split_pmd_before,
+		self->split_pmd_failed_before);
+}
+
 TEST_HARNESS_MAIN
-- 
2.47.3