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

[David Hildenbrand]

On 07.10.25 16:30, Sumanth Korikkar wrote:
> On Fri, Sep 26, 2025 at 03:15:23PM +0200, Sumanth Korikkar wrote:
> > Hi,
> >
> > Patchset provides a new interface for dynamic configuration and
> > deconfiguration of hotplug memory on s390, allowing with/without
> > memmap_on_memory support. It is a follow up on the discussion with David
> > when introducing memmap_on_memory support for s390 and support dynamic
> > (de)configuration of memory:
> > https://lore.kernel.org/all/ee492da8-74b4-4a97-8b24-73e07257f01d@xxxxxxxxxx/
> > https://lore.kernel.org/all/20241202082732.3959803-1-sumanthk@xxxxxxxxxxxxx/
> >
> > The original motivation for introducing memmap_on_memory on s390 was to
> > avoid using online memory to store struct pages metadata, particularly
> > for standby memory blocks. This became critical in cases where there was
> > an imbalance between standby and online memory, potentially leading to
> > boot failures due to insufficient memory for metadata allocation.
> >
> > To address this, memmap_on_memory was utilized on s390. However, in its
> > current form, it adds struct pages metadata at the start of each memory
> > block at the time of addition (only standby memory), and this
> > configuration is static. It cannot be changed at runtime  (When the user
> > needs continuous physical memory).
> >
> > Inorder to provide more flexibility to the user and overcome the above
> > limitation, add an option to dynamically configure and deconfigure
> > hotpluggable memory block with/without memmap_on_memory.
> >
> > With the new interface, s390 will not add all possible hotplug memory in
> > advance, like before, to make it visible in sysfs for online/offline
> > actions. Instead, before memory block can be set online, it has to be
> > configured via a new interface in /sys/firmware/memory/memoryX/config,
> > which makes s390 similar to others.  i.e. Adding of hotpluggable memory is
> > controlled by the user instead of adding it at boottime.
>
> Hi David,
>
> Looking forward to your feedback to proceed further.

Thanks for bumping it up in my inbox, will comment today :)

--
Cheers

David / dhildenb