selftests: clarify common error when running gup_test

The gup_test binary will fail showing only the output of perror("open") in
the case that /sys/kernel/debug/gup_test is not found. This will almost
always be due to CONFIG_GUP_TEST not being set, which enables
compilation of a kernel that provides this file.

Add a short error message to clarify this failure and point the user to
the solution.

Link: https://lkml.kernel.org/r/20220502224942.995427-1-jsavitz@redhat.com
Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/tools/testing/selftests/vm/gup_test.c b/tools/testing/selftests/vm/gup_test.c
index 593262555e18ceae818f1c287c0aaf1f6b44dd33..6bb36ca71cb5074a58fce72fa121de62d26f055f 100644 (file)
--- a/tools/testing/selftests/vm/gup_test.c
+++ b/tools/testing/selftests/vm/gup_test.c
@@ -21,6 +21,8 @@
 #define FOLL_WRITE     0x01    /* check pte is writable */
 #define FOLL_TOUCH     0x02    /* mark page accessed */
 
+#define GUP_TEST_FILE "/sys/kernel/debug/gup_test"
+
 static unsigned long cmd = GUP_FAST_BENCHMARK;
 static int gup_fd, repeats = 1;
 static unsigned long size = 128 * MB;