Known problems:
1. memory can't be offlined when CONFIG_MEMCG is selected.
   For example: there is a memory device on node 1. The address range
   is [1G, 1.5G). You will find 4 new directories memory8, memory9, memory10,
   and memory11 under the directory /sys/devices/system/memory/.
   If CONFIG_MEMCG is selected, we will allocate memory to store page cgroup
   when we online pages. When we online memory8, the memory stored page cgroup
   is not provided by this memory device. But when we online memory9, the memory
   stored page cgroup may be provided by memory8. So we can't offline memory8
   now. We should offline the memory in the reversed order.
   When the memory device is hotremoved, we will auto offline memory provided
   by this memory device. But we don't know which memory is onlined first, so
   offlining memory may fail. In such case, you should offline the memory by
   hand before hotremoving the memory device.

Just iterate twice. 1st iterate: offline every non primary memory
block. 2nd iterate:
offline primary (i.e. first added) memory block. It may work.

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