[PATCH v2 3/3] selftests/mm: fix exclusive_cow test fork() handling

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

The test ignores the return value of fork(), so both the parent and
the (newly created) child run the COW verification loops and then
call hmm_buffer_free() before returning into the kselftest harness,
which _exit()s each side.  This duplicated teardown sequence has
been observed to manifest as a SIGSEGV in the test child, e.g.:

  hmm-tests[360141]: segfault (11) at 0 nip 10006964 lr 1000ac3c code 1
  in hmm-tests[6964,10000000+30000]

Fix this by adopting the same fork()-then-wait pattern already used
by the nearby anon_write_child / anon_write_child_shared tests in
this file: the child performs the COW verification and then _exit(0)s
so it does not run the test teardown, while the parent independently
verifies COW, waits for the child, and only then frees the buffer.

Fixes: b659baea75469 ("mm: selftests for exclusive device memory")
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
---
 tools/testing/selftests/mm/hmm-tests.c | 30 +++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
index 1d49d9e919c1f..48834e7d1d984 100644
--- a/tools/testing/selftests/mm/hmm-tests.c
+++ b/tools/testing/selftests/mm/hmm-tests.c
@@ -1868,6 +1868,8 @@ 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);
@@ -1896,14 +1898,36 @@ 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: fault pages back and verify COW, then _exit().
+		 * On ASSERT failure the harness calls abort(); the parent
+		 * reports it via the WIFEXITED/WEXITSTATUS checks below.
+		 */
+		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);
 }
-- 
2.54.0