memory_hotplug: introduce memhp_default_state= command line parameter

CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE specifies the default value for the
memory hotplug onlining policy.  Add a command line parameter to make it
possible to override the default.  It may come handy for debug and
testing purposes.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index fcafbfc..caf2a14 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -85,6 +85,17 @@
 #endif
 EXPORT_SYMBOL_GPL(memhp_auto_online);
 
+static int __init setup_memhp_default_state(char *str)
+{
+	if (!strcmp(str, "online"))
+		memhp_auto_online = true;
+	else if (!strcmp(str, "offline"))
+		memhp_auto_online = false;
+
+	return 1;
+}
+__setup("memhp_default_state=", setup_memhp_default_state);
+
 void get_online_mems(void)
 {
 	might_sleep();