chain should be updated to provide notifications of these events.
This patch adds the notifications for memory hot add and hot remove.
-Memory hotplug has event notifier. There are 6 types of notification.
+  Generated prior to the process of hot removing memory.
+  Generated if MEMORY_BEING_HOT_REMOVED fails.
+  Generated when memory has been successfully hot removed.
+  Generated prior to the process of hot adding memory.
+  Generated if MEMORY_BEING_HOT_ADDED fails.
+  Generated when memory has successfully been hot added.
-start_pfn is start_pfn of online/offline memory.
-nr_pages is # of pages of online/offline memory.
+start_pfn is start_pfn of online/offline/add/remove memory.
+nr_pages is # of pages of online/offline/add/remove memory.