selftests/mm: print some details when uffd-stress gets bad params

So this can be debugged more easily.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Brendan Jackman <[email protected]>
Cc: Dev Jain <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

tools/testing/selftests/mm/uffd-stress.c

@@ -457,7 +457,8 @@ int main(int argc, char **argv)
 
 	nr_pages_per_cpu = bytes / page_size / nr_parallel;
 	if (!nr_pages_per_cpu) {
-		_err("invalid MiB");
+		_err("pages_per_cpu = 0, cannot test (%lu / %lu / %lu)",
+			bytes, page_size, nr_parallel);
 		usage();
 	}