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.. _memory_hotplug_notifier:

=============================
Memory hotplug event notifier
=============================

Hotplugging events are sent to a notification queue.

There are six types of notification defined in ``include/linux/memory.h``:

MEM_GOING_ONLINE
  Generated before new memory becomes available in order to be able to
  prepare subsystems to handle memory. The page allocator is still unable
  to allocate from the new memory.

MEM_CANCEL_ONLINE
  Generated if MEM_GOING_ONLINE fails.

MEM_ONLINE
  Generated when memory has successfully brought online. The callback may
  allocate pages from the new memory.

MEM_GOING_OFFLINE
  Generated to begin the process of offlining memory. Allocations are no
  longer possible from the memory but some of the memory to be offlined
  is still in use. The callback can be used to free memory known to a
  subsystem from the indicated memory block.

MEM_CANCEL_OFFLINE
  Generated if MEM_GOING_OFFLINE fails. Memory is available again from
  the memory block that we attempted to offline.

MEM_OFFLINE
  Generated after offlining memory is complete.

A callback routine can be registered by calling::

  hotplug_memory_notifier(callback_func, priority)

Callback functions with higher values of priority are called before callback
functions with lower values.

A callback function must have the following prototype::

  int callback_func(
    struct notifier_block *self, unsigned long action, void *arg);

The first argument of the callback function (self) is a pointer to the block
of the notifier chain that points to the callback function itself.
The second argument (action) is one of the event types described above.
The third argument (arg) passes a pointer of struct memory_notify::

	struct memory_notify {
		unsigned long start_pfn;
		unsigned long nr_pages;
		int status_change_nid_normal;
		int status_change_nid_high;
		int status_change_nid;
	}

- start_pfn is start_pfn of online/offline memory.
- nr_pages is # of pages of online/offline memory.
- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
  is (will be) set/clear, if this is -1, then nodemask status is not changed.
- status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask
  is (will be) set/clear, if this is -1, then nodemask status is not changed.
- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
  set/clear. It means a new(memoryless) node gets new memory by online and a
  node loses all memory. If this is -1, then nodemask status is not changed.

  If status_changed_nid* >= 0, callback should create/discard structures for the
  node if necessary.

The callback routine shall return one of the values
NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
defined in ``include/linux/notifier.h``

NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.

NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
further processing of the notification queue.

NOTIFY_STOP stops further processing of the notification queue.