On 03/17/20 at 11:49am, David Hildenbrand wrote:

Distributions nowadays use udev rules ([1] [2]) to specify if and
how to online hotplugged memory. The rules seem to get more complex with
many special cases. Due to the various special cases,
CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE cannot be used. All memory hotplug
is handled via udev rules.

Everytime we hotplug memory, the udev rule will come to the same
conclusion. Especially Hyper-V (but also soon virtio-mem) add a lot of
memory in separate memory blocks and wait for memory to get onlined by user
space before continuing to add more memory blocks (to not add memory faster
than it is getting onlined). This of course slows down the whole memory
hotplug process.

To make the job of distributions easier and to avoid udev rules that get
more and more complicated, let's extend the mechanism provided by
- /sys/devices/system/memory/auto_online_blocks
- "memhp_default_state=" on the kernel cmdline
to be able to specify also "online_movable" as well as "online_kernel"

This patch series looks good, thanks. Since Andrew has merged it to -mm again,
I won't add my Reviewed-by to bother. 

Hi David, Vitaly

There are several things unclear to me.

So, these improved interfaces are used to alleviate the burden of the 
existing udev rules, or try to replace it? As you know, we have been
using udev rules to interact between kernel and user space on bare metal,
and guests who want to hot add/remove.

And also the OOM issue in hyperV when onlining pages after adding memory
block. I am not a virt devel expert, could this happen on bare metal
system?

Thanks
Baoquan