selftests/mm/kugepaged: restore thp settings at exit

Previously, the saved thp settings would be restored upon a signal or at
the natural end of the test suite. But there are some tests that directly
call exit() upon failure. In this case, the thp settings were not being
restored, which could then influence other tests.

Fix this by installing an atexit() handler to do the actual restore. The
signal handler can now just call exit() and the atexit handler is invoked.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ryan Roberts <[e-mail]>
Reviewed-by: Alistair Popple <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Tested-by: Kefeng Wang <[e-mail]>
Tested-by: John Hubbard <[e-mail]>
Cc: Anshuman Khandual <[e-mail]>
Cc: Barry Song <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: "Huang, Ying" <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Itaru Kitayama <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Luis Chamberlain <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Yang Shi <[e-mail]>
Cc: Yin Fengwei <[e-mail]>
Cc: Yu Zhao <[e-mail]>
Cc: Zi Yan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/khugepaged.c

···
 	write_settings(current_settings());
 }
 
-static void restore_settings(int sig)
+static void restore_settings_atexit(void)
 {
 	if (skip_settings_restore)
-		goto out;
+		return;
 
 	printf("Restore THP and khugepaged settings...");
 	write_settings(&saved_settings);
 	success("OK");
-	if (sig)
-		exit(EXIT_FAILURE);
-out:
-	exit(exit_status);
+
+	skip_settings_restore = true;
+}
+
+static void restore_settings(int sig)
+{
+	/* exit() will invoke the restore_settings_atexit handler. */
+	exit(sig ? EXIT_FAILURE : exit_status);
 }
 
 static void save_settings(void)
···
 
 	success("OK");
 
+	atexit(restore_settings_atexit);
 	signal(SIGTERM, restore_settings);
 	signal(SIGINT, restore_settings);
 	signal(SIGHUP, restore_settings);