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+.. _memory_hotplug:
+Memory hotplug
+Memory hotplug event notifier
+Hotplugging events are sent to a notification queue.
+There are six types of notification defined in ``include/linux/memory.h``:
+ 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.
+ Generated if MEM_GOING_ONLINE fails.
+ Generated when memory has successfully brought online. The callback may
+ allocate pages from the new memory.
+ 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.
+ Generated if MEM_GOING_OFFLINE fails. Memory is available again from
+ the memory block that we attempted to 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
+defined in ``include/linux/notifier.h``
+NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
+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.
+Locking Internals
+When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
+the device_hotplug_lock should be held to:
+- synchronize against online/offline requests (e.g. via sysfs). This way, memory
+ block devices can only be accessed (.online/.state attributes) by user
+ space once memory has been fully added. And when removing memory, we
+ know nobody is in critical sections.
+- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
+Especially, there is a possible lock inversion that is avoided using
+device_hotplug_lock when adding memory and user space tries to online that
+memory faster than expected:
+- device_online() will first take the device_lock(), followed by
+ mem_hotplug_lock
+- add_memory_resource() will first take the mem_hotplug_lock, followed by
+ the device_lock() (while creating the devices, during bus_add_device()).
+As the device is visible to user space before taking the device_lock(), this
+can result in a lock inversion.
+onlining/offlining of memory should be done via device_online()/
+device_offline() - to make sure it is properly synchronized to actions
+via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
+When adding/removing/onlining/offlining memory or adding/removing
+heterogeneous/device memory, we should always hold the mem_hotplug_lock in
+write mode to serialise memory hotplug (e.g. access to global/zone
+In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
+mode allows for a quite efficient get_online_mems/put_online_mems
+implementation, so code accessing memory can protect from that memory