xen_balloon: support memory auto onlining policy

	  Memory could be hotplugged in following steps:

	    1) dom0: xl mem-max <domU> <maxmem>

	    1) target domain: ensure that memory auto online policy is in

	       effect by checking /sys/devices/system/memory/auto_online_blocks

	       file (should be 'online').

	    2) control domain: xl mem-max <target-domain> <maxmem>

	       where <maxmem> is >= requested memory size,

	    2) dom0: xl mem-set <domU> <memory>

	    3) control domain: xl mem-set <target-domain> <memory>

	       where <memory> is requested memory size; alternatively memory

	       could be added by writing proper value to

	       /sys/devices/system/xen_memory/xen_memory0/target or

	       /sys/devices/system/xen_memory/xen_memory0/target_kb on dumU,

	       /sys/devices/system/xen_memory/xen_memory0/target_kb on the

	       target domain.

	  Alternatively, if memory auto onlining was not requested at step 1

	  the newly added memory can be manually onlined in the target domain

	  by doing the following:

	    3) domU: for i in /sys/devices/system/memory/memory*/state; do \

		for i in /sys/devices/system/memory/memory*/state; do \

		  [ "`cat "$i"`" = offline ] && echo online > "$i"; done

	  Memory could be onlined automatically on domU by adding following line to udev rules:

	  or by adding the following line to udev rules:

	  SUBSYSTEM=="memory", ACTION=="add", RUN+="/bin/sh -c '[ -f /sys$devpath/state ] && echo online > /sys$devpath/state'"

	  In that case step 3 should be omitted.

config XEN_BALLOON_MEMORY_HOTPLUG_LIMIT

	int "Hotplugged memory limit (in GiB) for a PV guest"

	default 512 if X86_64

	}

#endif

	rc = add_memory_resource(nid, resource, false);

	/*

	 * add_memory_resource() will call online_pages() which in its turn

	 * will call xen_online_page() callback causing deadlock if we don't

	 * release balloon_mutex here. Unlocking here is safe because the

	 * callers drop the mutex before trying again.

	 */

	mutex_unlock(&balloon_mutex);

	rc = add_memory_resource(nid, resource, memhp_auto_online);

	mutex_lock(&balloon_mutex);

	if (rc) {

		pr_warn("Cannot add additional memory (%i)\n", rc);

		goto err;