selftests/mm: hugetlb_fault_after_madv: improve test output

Let's improve the test output. For example, print the proper test result.
Install a SIGBUS handler to catch any SIGBUS instead of crashing the test
on failure.

With unsuitable hugetlb page count:
$ ./hugetlb_fault_after_madv
TAP version 13
1..1
# [INFO] detected default hugetlb page size: 2048 KiB
ok 2 # SKIP This test needs one and only one page to execute. Got 0
# Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0

On a failure:
$ ./hugetlb_fault_after_madv
TAP version 13
1..1
not ok 1 SIGBUS behavior
Bail out! 1 out of 1 tests failed

On success:
$ ./hugetlb_fault_after_madv
TAP version 13
1..1
# [INFO] detected default hugetlb page size: 2048 KiB
ok 1 SIGBUS behavior
# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Reviewed-by: Breno Leitao <[e-mail]>
Tested-by: Mario Casquero <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/hugetlb_fault_after_madv.c

···
 #include <sys/mman.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <setjmp.h>
+#include <signal.h>
 
 #include "vm_util.h"
 #include "../kselftest.h"
···
 static char *huge_ptr;
 static size_t huge_page_size;
 
+static sigjmp_buf sigbuf;
+static bool sigbus_triggered;
+
+static void signal_handler(int signal)
+{
+	if (signal == SIGBUS) {
+		sigbus_triggered = true;
+		siglongjmp(sigbuf, 1);
+	}
+}
+
 /* Touch the memory while it is being madvised() */
 void *touch(void *unused)
 {
 	char *ptr = (char *)huge_ptr;
+
+	if (sigsetjmp(sigbuf, 1))
+		return NULL;
 
 	for (int i = 0; i < INLOOP_ITER; i++)
 		ptr[0] = '.';
···
 	 * interactions
 	 */
 	int max = 10000;
+	int err;
+
+	ksft_print_header();
+	ksft_set_plan(1);
 
 	srand(getpid());
+
+	if (signal(SIGBUS, signal_handler) == SIG_ERR)
+		ksft_exit_skip("Could not register signal handler.");
 
 	huge_page_size = default_huge_page_size();
 	if (!huge_page_size)
 		ksft_exit_skip("Could not detect default hugetlb page size.");
+
+	ksft_print_msg("[INFO] detected default hugetlb page size: %zu KiB\n",
+		       huge_page_size / 1024);
 
 	free_hugepages = get_free_hugepages();
 	if (free_hugepages != 1) {
···
 		munmap(huge_ptr, huge_page_size);
 	}
 
-	return KSFT_PASS;
+	ksft_test_result(!sigbus_triggered, "SIGBUS behavior\n");
+
+	err = ksft_get_fail_cnt();
+	if (err)
+		ksft_exit_fail_msg("%d out of %d tests failed\n",
+				   err, ksft_test_num());
+	ksft_exit_pass();
 }