diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
index 8f4f82467043..e4c49699f3f7 100644
--- a/tools/testing/selftests/mm/hmm-tests.c
+++ b/tools/testing/selftests/mm/hmm-tests.c
@@ -1884,6 +1884,8 @@ TEST_F(hmm, exclusive_cow)
 	unsigned long i;
 	int *ptr;
 	int ret;
+	pid_t pid;
+	int status;
 
 	npages = ALIGN(HMM_BUFFER_SIZE, self->page_size) >> self->page_shift;
 	ASSERT_NE(npages, 0);
@@ -1912,14 +1914,37 @@ TEST_F(hmm, exclusive_cow)
 	ASSERT_EQ(ret, 0);
 	ASSERT_EQ(buffer->cpages, npages);
 
-	fork();
+	pid = fork();
+	if (pid == -1)
+		ASSERT_EQ(pid, 0);
 
-	/* Fault pages back to system memory and check them. */
+	if (pid == 0) {
+		/*
+		 * Child verifies COW independently, then _exit(0)s so it does
+		 * not run the test teardown.  A failed ASSERT_* here makes the
+		 * harness abort() the child, so the parent sees
+		 * !WIFEXITED(status) below and fails in turn.
+		 */
+		for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
+			ASSERT_EQ(ptr[i]++, i);
+
+		for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
+			ASSERT_EQ(ptr[i], i + 1);
+
+		_exit(0);
+	}
+
+	/* Parent: also increment to verify COW works for both processes. */
 	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
 		ASSERT_EQ(ptr[i]++, i);
 
 	for (i = 0, ptr = buffer->ptr; i < size / sizeof(*ptr); ++i)
-		ASSERT_EQ(ptr[i], i+1);
+		ASSERT_EQ(ptr[i], i + 1);
+
+	/* Parent: wait for child and then free the buffer. */
+	ASSERT_EQ(waitpid(pid, &status, 0), pid);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
 
 	hmm_buffer_free(buffer);
 }