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diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst new file mode 100644 index 000000000000..8fc97c2379de --- /dev/null +++ b/Documentation/core-api/memory-hotplug.rst @@ -0,0 +1,195 @@ +.. _memory_hotplug: + +============== +Memory hotplug +============== + +Memory hotplug event notifier +============================= + +Hotplugging events are sent to a notification queue. + +Memory notifier +---------------- + +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; + } + +- start_pfn is start_pfn of online/offline memory. +- nr_pages is # of pages of online/offline memory. + +It is possible to get notified for MEM_CANCEL_ONLINE without having been notified +for MEM_GOING_ONLINE, and the same applies to MEM_CANCEL_OFFLINE and +MEM_GOING_OFFLINE. +This can happen when a consumer fails, meaning we break the callchain and we +stop calling the remaining consumers of the notifier. +It is then important that users of memory_notify make no assumptions and get +prepared to handle such cases. + +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. + +Numa node notifier +------------------ + +There are six types of notification defined in ``include/linux/node.h``: + +NODE_ADDING_FIRST_MEMORY + Generated before memory becomes available to this node for the first time. + +NODE_CANCEL_ADDING_FIRST_MEMORY + Generated if NODE_ADDING_FIRST_MEMORY fails. + +NODE_ADDED_FIRST_MEMORY + Generated when memory has become available fo this node for the first time. + +NODE_REMOVING_LAST_MEMORY + Generated when the last memory available to this node is about to be offlined. + +NODE_CANCEL_REMOVING_LAST_MEMORY + Generated when NODE_CANCEL_REMOVING_LAST_MEMORY fails. + +NODE_REMOVED_LAST_MEMORY + Generated when the last memory available to this node has been offlined. + +A callback routine can be registered by calling:: + + hotplug_node_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 node_notify:: + + struct node_notify { + int nid; + } + +- nid is the node we are adding or removing memory to. + +It is possible to get notified for NODE_CANCEL_ADDING_FIRST_MEMORY without +having been notified for NODE_ADDING_FIRST_MEMORY, and the same applies to +NODE_CANCEL_REMOVING_LAST_MEMORY and NODE_REMOVING_LAST_MEMORY. +This can happen when a consumer fails, meaning we break the callchain and we +stop calling the remaining consumers of the notifier. +It is then important that users of node_notify make no assumptions and get +prepared to handle such cases. + +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 NODE_ADDING_FIRST_MEMORY, +NODE_REMOVING_LAST_MEMORY, NODE_ADDED_FIRST_MEMORY or +NODE_REMOVED_LAST_MEMORY action to cancel hotplugging. +It stops further processing of the notification queue. + +NOTIFY_STOP stops further processing of the notification queue. + +Please note that we should not fail for NODE_ADDED_FIRST_MEMORY / +NODE_REMOVED_FIRST_MEMORY, as memory_hotplug code cannot rollback at that +point anymore. + +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 +variables). + +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 +vanishing. |