memory_hotplug: introduce CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE

This patchset continues the work I started with commit 31bc3858ea3e
("memory-hotplug: add automatic onlining policy for the newly added
memory").

Initially I was going to stop there and bring the policy setting logic
to userspace. I met two issues on this way:

1) It is possible to have memory hotplugged at boot (e.g. with QEMU).
These blocks stay offlined if we turn the onlining policy on by
userspace.

2) My attempt to bring this policy setting to systemd failed, systemd
maintainers suggest to change the default in kernel or ... to use
tmpfiles.d to alter the policy (which looks like a hack to me):
https://github.com/systemd/systemd/pull/2938

Here I suggest to add a config option to set the default value for the
policy and a kernel command line parameter to make the override.

This patch (of 2):

Introduce config option to set the default value for memory hotplug
onlining policy (/sys/devices/system/memory/auto_online_blocks). The
reason one would want to turn this option on are to have early onlining
for hotpluggable memory available at boot and to not require any
userspace actions to make memory hotplug work.

[[e-mail]: tweak Kconfig text]
Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: David Vrabel <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Igor Mammedov <[e-mail]>
Cc: Lennart Poettering <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/memory-hotplug.txt

···
 
 % cat /sys/devices/system/memory/auto_online_blocks
 
-The default is "offline" which means the newly added memory is not in a
-ready-to-use state and you have to "online" the newly added memory blocks
-manually. Automatic onlining can be requested by writing "online" to
-"auto_online_blocks" file:
+The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config
+option. If it is disabled the default is "offline" which means the newly added
+memory is not in a ready-to-use state and you have to "online" the newly added
+memory blocks manually. Automatic onlining can be requested by writing "online"
+to "auto_online_blocks" file:
 
 % echo online > /sys/devices/system/memory/auto_online_blocks
 

mm/Kconfig

···
 	def_bool y
 	depends on SPARSEMEM && MEMORY_HOTPLUG
 
+config MEMORY_HOTPLUG_DEFAULT_ONLINE
+        bool "Online the newly added memory blocks by default"
+        default n
+        depends on MEMORY_HOTPLUG
+        help
+	  This option sets the default policy setting for memory hotplug
+	  onlining policy (/sys/devices/system/memory/auto_online_blocks) which
+	  determines what happens to newly added memory regions. Policy setting
+	  can always be changed at runtime.
+	  See Documentation/memory-hotplug.txt for more information.
+
+	  Say Y here if you want all hot-plugged memory blocks to appear in
+	  'online' state by default.
+	  Say N here if you want the default policy to keep all hot-plugged
+	  memory blocks in 'offline' state.
+
 config MEMORY_HOTREMOVE
 	bool "Allow for memory hot remove"
 	select MEMORY_ISOLATION

mm/memory_hotplug.c

···
 #define memhp_lock_acquire()      lock_map_acquire(&mem_hotplug.dep_map)
 #define memhp_lock_release()      lock_map_release(&mem_hotplug.dep_map)
 
+#ifndef CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE
 bool memhp_auto_online;
+#else
+bool memhp_auto_online = true;
+#endif
 EXPORT_SYMBOL_GPL(memhp_auto_online);
 
 void get_online_mems(void)