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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.