The process hangs during memory-hotplug.

[Yang Yanchao <yangyanchao6@huawei.com>]

Hello,

I find a hanging issue during memory-hotplug on kernel-4.18.
Repetition steps:
1. malloc for all system memory, write 'x', then free
2. for each removable memory block:
echo offline > /sys/devices/system/memory/memoryXXX/state
Then during the offline process, there is a high probability of being stuck for more than 20 minutes to five hours.
cat /sys/ Device/system/Memory/memoryXXX/state
The status is "going-offline"
I try to understand it by adding some print to the kernel.The discovery process can't exit in this loop:
__offline_pages
    do_migrate_range
        migrate_pages
            unmap_and_move
                move_to_new_page
                    fallback_migrate_page   --> return EAGAIN
I try to clear the cache, but it don't seems to solve the problem.
echo 3 > /proc/sys/vm/drop_caches
Can I fix this problem with other Settings? Or can I see why it's stuck?
System configuration information:
Use physical machines instead of virtual machines
[root@localhost ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:          502Gi       201Gi       300Gi        10Mi       172Mi       299Gi
Swap:         4.0Gi          0B       4.0Gi
[root@localhost ~]# uname -i
x86_64

Regards,
Yang Yanchao