Enhance the guest_memfd indexing selftest to also verify the precision of
memory conversions between private and shared.

The existing test converted a single page within a multi-page mapping but
did not explicitly check the state of the surrounding pages after the
conversion loop.

Add checks to confirm that converting a single page from shared to private
only affects the target page. Iterate through all other pages in the
guest_memfd region to ensure they remain in their original shared state,
thus verifying that the conversion operation is precise and does not have
unintended side effects.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 .../selftests/kvm/guest_memfd_conversions_test.c    | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kvm/guest_memfd_conversions_test.c b/tools/testing/selftests/kvm/guest_memfd_conversions_test.c
index 9dc47316112f..b109f078bc6b 100644
--- a/tools/testing/selftests/kvm/guest_memfd_conversions_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_conversions_test.c
@@ -227,7 +227,8 @@ GMEM_CONVERSION_TEST_INIT_SHARED(init_shared)
 
 /*
  * Test indexing of pages within guest_memfd, using test data that is a multiple
- * of page index.
+ * of page index.  Also test the precision of conversion, that it does not
+ * affect surrounding pages.
  */
 GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED(indexing, 4)
 {
@@ -247,12 +248,20 @@ GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED(indexing, 4)
 			test_shared(t, i, i * 2, i * 3, i * 4);
 	}
 
+	/* Confirm that only one page was converted */
 	for (i = 0; i < nr_pages; ++i) {
 		if (i == test_page)
-			test_convert_to_shared(t, i, i * 4, i * 5, i * 6);
+			test_private(t, i, i * 4, i * 6);
 		else
 			test_shared(t, i, i * 4, i * 5, i * 6);
 	}
+
+	for (i = 0; i < nr_pages; ++i) {
+		if (i == test_page)
+			test_convert_to_shared(t, i, i * 6, i * 7, i * 8);
+		else
+			test_shared(t, i, i * 6, i * 7, i * 8);
+	}
 }
 
 /*
-- 
2.53.0.rc1.225.gd81095ad13-goog