include/linux/memory.h: implement register_hotmemory_notifier()

When CONFIG_MEMORY_HOTPLUG=n, we don't want the memory-hotplug notifier
handlers to be included in the .o files, for space reasons.

The existing hotplug_memory_notifier() tries to handle this but testing
with gcc-4.4.4 shows that it doesn't work - the hotplug functions are
still present in the .o files.

So implement a new register_hotmemory_notifier() which is a copy of
register_hotcpu_notifier(), and which actually works as desired.
hotplug_memory_notifier() and register_memory_notifier() callsites
should be converted to use this new register_hotmemory_notifier().

While we're there, let's repair the existing hotplug_memory_notifier():
it simply stomps on the register_memory_notifier() return value, so
well-behaved code cannot check for errors.  Apparently non of the
existing callers were well-behaved :(

Cc: Andrew Shewmaker <agshew@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memory.h

@@ -18,6 +18,7 @@
 #include <linux/node.h>
 #include <linux/compiler.h>
 #include <linux/mutex.h>
+#include <linux/notifier.h>
 
 #define MIN_MEMORY_BLOCK_SIZE     (1UL << SECTION_SIZE_BITS)
 
@@ -127,13 +128,18 @@ enum mem_add_context { BOOT, HOTPLUG };
 #endif /* CONFIG_MEMORY_HOTPLUG_SPARSE */
 
 #ifdef CONFIG_MEMORY_HOTPLUG
-#define hotplug_memory_notifier(fn, pri) {			\
+#define hotplug_memory_notifier(fn, pri) ({		\
 	static __meminitdata struct notifier_block fn##_mem_nb =\
 		{ .notifier_call = fn, .priority = pri };\
 	register_memory_notifier(&fn##_mem_nb);			\
-}
+})
+#define register_hotmemory_notifier(nb)		register_memory_notifier(nb)
+#define unregister_hotmemory_notifier(nb) 	unregister_memory_notifier(nb)
 #else
-#define hotplug_memory_notifier(fn, pri) do { } while (0)
+#define hotplug_memory_notifier(fn, pri)	(0)
+/* These aren't inline functions due to a GCC bug. */
+#define register_hotmemory_notifier(nb)    ({ (void)(nb); 0; })
+#define unregister_hotmemory_notifier(nb)  ({ (void)(nb); })
 #endif
 
 /*

include/linux/notifier.h

@@ -47,8 +47,11 @@
  * runtime initialization.
  */
 
+typedef	int (*notifier_fn_t)(struct notifier_block *nb,
+			unsigned long action, void *data);
+
 struct notifier_block {
-	int (*notifier_call)(struct notifier_block *, unsigned long, void *);
+	notifier_fn_t notifier_call;
 	struct notifier_block __rcu *next;
 	int priority;
 };