[PATCH 0/10] memory-hotplug: hot-remove physical memory

[KOSAKI Motohiro]

> 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.