123 lines
4.5 KiB
ReStructuredText
123 lines
4.5 KiB
ReStructuredText
.. _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``:
|
|
|
|
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;
|
|
}
|
|
|
|
- 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 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.
|
|
|
|
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.
|