Adding upstream version 6.1.76.upstream/6.1.76upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 122 insertions, 0 deletions

diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
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+.. _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.