selftests/mm: skip guard_regions.uffd tests when uffd is not present

Patch series "Skip mm selftests instead when kernel features are not
present", v2.

Two guard_regions tests on userfaultfd fail when userfaultfd is not
present.  Skip them instead.

hugevm test reads kernel config to get page table level information and
fails when neither /proc/config.gz nor /boot/config-* is present.  Skip it
instead.


This patch (of 2):

When userfaultfd is not compiled into kernel, userfaultfd() returns -1,
causing guard_regions.uffd tests to fail.  Skip the tests instead.

Link: https://lkml.kernel.org/r/20250516132938.356627-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250516132938.356627-2-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Cc: Adam Sindelar <adam@wowsignal.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: x-y-z

tools/testing/selftests/mm/guard-regions.c

@@ -1453,8 +1453,21 @@ TEST_F(guard_regions, uffd)
 
 	/* Set up uffd. */
 	uffd = userfaultfd(0);
-	if (uffd == -1 && errno == EPERM)
-		ksft_exit_skip("No userfaultfd permissions, try running as root.\n");
+	if (uffd == -1) {
+		switch (errno) {
+		case EPERM:
+			SKIP(return, "No userfaultfd permissions, try running as root.");
+			break;
+		case ENOSYS:
+			SKIP(return, "userfaultfd is not supported/not enabled.");
+			break;
+		default:
+			ksft_exit_fail_msg("userfaultfd failed with %s\n",
+					   strerror(errno));
+			break;
+		}
+	}
+
 	ASSERT_NE(uffd, -1);
 
 	ASSERT_EQ(ioctl(uffd, UFFDIO_API, &api), 0);