1.. _memory_hotplug: 2 3============== 4Memory hotplug 5============== 6 7Memory hotplug event notifier 8============================= 9 10Hotplugging events are sent to a notification queue. 11 12There are six types of notification defined in ``include/linux/memory.h``: 13 14MEM_GOING_ONLINE 15 Generated before new memory becomes available in order to be able to 16 prepare subsystems to handle memory. The page allocator is still unable 17 to allocate from the new memory. 18 19MEM_CANCEL_ONLINE 20 Generated if MEM_GOING_ONLINE fails. 21 22MEM_ONLINE 23 Generated when memory has successfully brought online. The callback may 24 allocate pages from the new memory. 25 26MEM_GOING_OFFLINE 27 Generated to begin the process of offlining memory. Allocations are no 28 longer possible from the memory but some of the memory to be offlined 29 is still in use. The callback can be used to free memory known to a 30 subsystem from the indicated memory block. 31 32MEM_CANCEL_OFFLINE 33 Generated if MEM_GOING_OFFLINE fails. Memory is available again from 34 the memory block that we attempted to offline. 35 36MEM_OFFLINE 37 Generated after offlining memory is complete. 38 39A callback routine can be registered by calling:: 40 41 hotplug_memory_notifier(callback_func, priority) 42 43Callback functions with higher values of priority are called before callback 44functions with lower values. 45 46A callback function must have the following prototype:: 47 48 int callback_func( 49 struct notifier_block *self, unsigned long action, void *arg); 50 51The first argument of the callback function (self) is a pointer to the block 52of the notifier chain that points to the callback function itself. 53The second argument (action) is one of the event types described above. 54The third argument (arg) passes a pointer of struct memory_notify:: 55 56 struct memory_notify { 57 unsigned long start_pfn; 58 unsigned long nr_pages; 59 int status_change_nid_normal; 60 int status_change_nid; 61 } 62 63- start_pfn is start_pfn of online/offline memory. 64- nr_pages is # of pages of online/offline memory. 65- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask 66 is (will be) set/clear, if this is -1, then nodemask status is not changed. 67- status_change_nid is set node id when N_MEMORY of nodemask is (will be) 68 set/clear. It means a new(memoryless) node gets new memory by online and a 69 node loses all memory. If this is -1, then nodemask status is not changed. 70 71 If status_changed_nid* >= 0, callback should create/discard structures for the 72 node if necessary. 73 74The callback routine shall return one of the values 75NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP 76defined in ``include/linux/notifier.h`` 77 78NOTIFY_DONE and NOTIFY_OK have no effect on the further processing. 79 80NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE, 81MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops 82further processing of the notification queue. 83 84NOTIFY_STOP stops further processing of the notification queue. 85 86Locking Internals 87================= 88 89When adding/removing memory that uses memory block devices (i.e. ordinary RAM), 90the device_hotplug_lock should be held to: 91 92- synchronize against online/offline requests (e.g. via sysfs). This way, memory 93 block devices can only be accessed (.online/.state attributes) by user 94 space once memory has been fully added. And when removing memory, we 95 know nobody is in critical sections. 96- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC) 97 98Especially, there is a possible lock inversion that is avoided using 99device_hotplug_lock when adding memory and user space tries to online that 100memory faster than expected: 101 102- device_online() will first take the device_lock(), followed by 103 mem_hotplug_lock 104- add_memory_resource() will first take the mem_hotplug_lock, followed by 105 the device_lock() (while creating the devices, during bus_add_device()). 106 107As the device is visible to user space before taking the device_lock(), this 108can result in a lock inversion. 109 110onlining/offlining of memory should be done via device_online()/ 111device_offline() - to make sure it is properly synchronized to actions 112via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type) 113 114When adding/removing/onlining/offlining memory or adding/removing 115heterogeneous/device memory, we should always hold the mem_hotplug_lock in 116write mode to serialise memory hotplug (e.g. access to global/zone 117variables). 118 119In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read 120mode allows for a quite efficient get_online_mems/put_online_mems 121implementation, so code accessing memory can protect from that memory 122vanishing. 123