mm,memory_hotplug: implement numa node notifier

There are at least six consumers of hotplug_memory_notifier that what they
really are interested in is whether any numa node changed its state, e.g:
going from having memory to not having memory and vice versa.

Implement a specific notifier for numa nodes when their state gets
changed, which will later be used by those consumers that are only
interested in numa node state changes.

Add documentation as well.

[dan.carpenter@linaro.org: set failure reason in offline_pages()]
  Link: https://lkml.kernel.org/r/be4fd31b-7d09-46b0-8329-6d0464ffa7a5@sabinyo.mountain
Link: https://lkml.kernel.org/r/20250616135158.450136-4-osalvador@suse.de
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 83 insertions, 0 deletions

diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst
index d1b8eb9add8a..fb84e78968b2 100644
--- a/Documentation/core-api/memory-hotplug.rst
+++ b/Documentation/core-api/memory-hotplug.rst
@@ -9,6 +9,9 @@ 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
@@ -68,6 +71,14 @@ The third argument (arg) passes a pointer of struct memory_notify::
   If status_changed_nid* >= 0, callback should create/discard structures for the
   node if necessary.
 
+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``
@@ -80,6 +91,78 @@ 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
 =================