Re: [PATCH 0/4] Support dynamic (de)configuration of memory

Sumanth Korikkar
2025-10-08T06:05:55Z

> Care to share an example output? I only have a s390x VM with 2 CPUs and no
> way to configure/deconfigure.

lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2 ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0 yes yes vert-medium 0
1 0 0 0 0 0 1:1:1 yes yes vert-medium 1
2 0 0 0 0 1 2:2:2 yes yes vert-low 2
3 0 0 0 0 1 3:3:3 yes yes vert-low 3

# chcpu -d 2-3
CPU 2 disabled
CPU 3 disabled
# chcpu -g 2
CPU 2 deconfigured
# chcpu -c 2
CPU 2 configured
# chcpu -e 2-3
CPU 2 enabled
CPU 3 enabled

> > chmem changes would look like:
> > chmem -c 128M -m 1 : configure memory with memmap-on-memory enabled
> > chmem -g 128M : deconfigure memory
>
> I wonder if the above two are really required. I would expect most/all users
> to simply keep using -e / -d.
>
> Sure, there might be some corner cases, but I would assume most people to
> not want to care about memmap-on-memory with the new model.

I believe this remains very beneficial for customers in the following
scenario:

1) Initial memory layout:
4 GB configured online
512 GB standby

If memory_hotplug.memmap_on_memory=Y is set in the kernel command line:
Suppose user requires more memory and onlines 256 GB. With memmap-on-memory
enabled, this likely succeeds by default.

Later, the user needs 256 GB of contiguous physical memory across memory
blocks. Then, the user can still configure those memory blocks with
memmap-on-memory disabled and online it.

2) If the administrator forgets to configure
memory_hotplug.memmap_on_memory=Y, the following steps can be taken:
Rescue from OOM situations: configure with memmap-on-memory enabled, online it.

Thank you,
Sumanth