memory-hotplug: document and enable CONFIG_MOVABLE_NODE

Add help info for CONFIG_MOVABLE_NODE and permit its selection.

This option allows the user to online all memory of a node as movable
memory.  So that the whole node can be hotplugged.  Users who don't use
the hotplug feature are also fine with this option on since they won't
online memory as movable.

Signed-off-by: Tang Chen <[email protected]>
Reviewed-by: Yasuaki Ishimatsu <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Cc: Wen Congyang <[email protected]>
Cc: Ingo Molnar <[email protected]>
[[email protected]: tweak help text]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/Kconfig

@@ -149,7 +149,18 @@ config MOVABLE_NODE
 	depends on NO_BOOTMEM
 	depends on X86_64
 	depends on NUMA
-	depends on BROKEN
+	default n
+	help
+	  Allow a node to have only movable memory.  Pages used by the kernel,
+	  such as direct mapping pages cannot be migrated.  So the corresponding
+	  memory device cannot be hotplugged.  This option allows users to
+	  online all the memory of a node as movable memory so that the whole
+	  node can be hotplugged.  Users who don't use the memory hotplug
+	  feature are fine with this option on since they don't online memory
+	  as movable.
+
+	  Say Y here if you want to hotplug a whole node.
+	  Say N here if you want kernel to use memory on all nodes evenly.
 
 # eventually, we can have this option just 'select SPARSEMEM'
 config MEMORY_HOTPLUG