[PATCH] selftests: cgroup: fall back to an unlinked file without O_TMPFILE

[Sasha Levin <sashal@kernel.org>]

get_temp_fd() creates its page cache file with O_TMPFILE in the current
directory. On filesystems without O_TMPFILE support, such as NFS, the
open fails with EOPNOTSUPP, and with it every test that needs page cache:
test_memcg_current_peak, test_memcg_min, test_memcg_low, test_memcg_high,
test_memcg_max and test_memcg_reclaim. Boards that boot with an NFS root
fail all six.

Create a regular file with mkstemp() and unlink it in that case.

Fixes: 84092dbcf901 ("selftests: cgroup: add memory controller self-tests")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Found with KernelCI's NVIDIA lab, whose boards boot from NFS. Tested on a
144-CPU Grace with KernelCI's arm64 build of 05441d697f7f9: run from an
NFSv3 mount, all six tests fail without this patch. With it, they behave
as they do from ext4 on that machine: the other four pass, while
test_memcg_min fails and test_memcg_low sometimes fails there too.

 tools/testing/selftests/cgroup/test_memcontrol.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index bb673012dcf08..7dc9c26628675 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -33,7 +33,17 @@ static int page_size;
 
 int get_temp_fd(void)
 {
-	return open(".", O_TMPFILE | O_RDWR | O_EXCL, 0600);
+	char name[] = "./cgroup_test_XXXXXX";
+	int fd;
+
+	fd = open(".", O_TMPFILE | O_RDWR | O_EXCL, 0600);
+	if (fd >= 0 || errno != EOPNOTSUPP)
+		return fd;
+
+	fd = mkstemp(name);
+	if (fd >= 0)
+		unlink(name);
+	return fd;
 }
 
 int alloc_pagecache(int fd, size_t size)
-- 
2.53.0