123 lines
4.5 KiB
ReStructuredText
123 lines
4.5 KiB
ReStructuredText
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.. _memory_hotplug:
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==============
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Memory hotplug
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==============
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Memory hotplug event notifier
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=============================
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Hotplugging events are sent to a notification queue.
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There are six types of notification defined in ``include/linux/memory.h``:
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MEM_GOING_ONLINE
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Generated before new memory becomes available in order to be able to
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prepare subsystems to handle memory. The page allocator is still unable
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to allocate from the new memory.
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MEM_CANCEL_ONLINE
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Generated if MEM_GOING_ONLINE fails.
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MEM_ONLINE
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Generated when memory has successfully brought online. The callback may
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allocate pages from the new memory.
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MEM_GOING_OFFLINE
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Generated to begin the process of offlining memory. Allocations are no
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longer possible from the memory but some of the memory to be offlined
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is still in use. The callback can be used to free memory known to a
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subsystem from the indicated memory block.
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MEM_CANCEL_OFFLINE
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Generated if MEM_GOING_OFFLINE fails. Memory is available again from
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the memory block that we attempted to offline.
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MEM_OFFLINE
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Generated after offlining memory is complete.
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A callback routine can be registered by calling::
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hotplug_memory_notifier(callback_func, priority)
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Callback functions with higher values of priority are called before callback
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functions with lower values.
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A callback function must have the following prototype::
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int callback_func(
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struct notifier_block *self, unsigned long action, void *arg);
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The first argument of the callback function (self) is a pointer to the block
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of the notifier chain that points to the callback function itself.
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The second argument (action) is one of the event types described above.
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The third argument (arg) passes a pointer of struct memory_notify::
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struct memory_notify {
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unsigned long start_pfn;
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unsigned long nr_pages;
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int status_change_nid_normal;
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int status_change_nid;
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}
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- start_pfn is start_pfn of online/offline memory.
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- nr_pages is # of pages of online/offline memory.
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- status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask
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is (will be) set/clear, if this is -1, then nodemask status is not changed.
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- status_change_nid is set node id when N_MEMORY of nodemask is (will be)
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set/clear. It means a new(memoryless) node gets new memory by online and a
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node loses all memory. If this is -1, then nodemask status is not changed.
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If status_changed_nid* >= 0, callback should create/discard structures for the
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node if necessary.
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The callback routine shall return one of the values
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NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP
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defined in ``include/linux/notifier.h``
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NOTIFY_DONE and NOTIFY_OK have no effect on the further processing.
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NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE,
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MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops
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further processing of the notification queue.
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NOTIFY_STOP stops further processing of the notification queue.
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Locking Internals
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=================
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When adding/removing memory that uses memory block devices (i.e. ordinary RAM),
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the device_hotplug_lock should be held to:
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- synchronize against online/offline requests (e.g. via sysfs). This way, memory
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block devices can only be accessed (.online/.state attributes) by user
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space once memory has been fully added. And when removing memory, we
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know nobody is in critical sections.
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- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC)
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Especially, there is a possible lock inversion that is avoided using
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device_hotplug_lock when adding memory and user space tries to online that
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memory faster than expected:
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- device_online() will first take the device_lock(), followed by
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mem_hotplug_lock
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- add_memory_resource() will first take the mem_hotplug_lock, followed by
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the device_lock() (while creating the devices, during bus_add_device()).
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As the device is visible to user space before taking the device_lock(), this
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can result in a lock inversion.
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onlining/offlining of memory should be done via device_online()/
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device_offline() - to make sure it is properly synchronized to actions
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via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type)
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When adding/removing/onlining/offlining memory or adding/removing
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heterogeneous/device memory, we should always hold the mem_hotplug_lock in
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write mode to serialise memory hotplug (e.g. access to global/zone
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variables).
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In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read
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mode allows for a quite efficient get_online_mems/put_online_mems
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implementation, so code accessing memory can protect from that memory
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vanishing.
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