Re: [PATCH 4/8] mm/memory_hotplug: introduce MEM_PHYS_ONLINE/OFFLINE memory notifiers

[Sumanth Korikkar]

On Tue, Nov 14, 2023 at 07:27:35PM +0100, David Hildenbrand wrote:
> On 14.11.23 19:02, Sumanth Korikkar wrote:
> > Add new memory notifiers to mimic the dynamic ACPI event triggered logic
> > for memory hotplug on platforms that do not generate such events. This
> > will be used to implement "memmap on memory" feature for s390 in a later
> > patch.
> > 
> > Platforms such as x86 can support physical memory hotplug via ACPI. When
> > there is physical memory hotplug, ACPI event leads to the memory
> > addition with the following callchain:
> > acpi_memory_device_add()
> >    -> acpi_memory_enable_device()
> >       -> __add_memory()
> > 
> > After this, the hotplugged memory is physically accessible, and altmap
> > support prepared, before the "memmap on memory" initialization in
> > memory_block_online() is called.
> > 
> > On s390, memory hotplug works in a different way. The available hotplug
> > memory has to be defined upfront in the hypervisor, but it is made
> > physically accessible only when the user sets it online via sysfs,
> > currently in the MEM_GOING_ONLINE notifier. This requires calling
> > add_memory() during early memory detection, in order to get the sysfs
> > representation, but we cannot use "memmap on memory" altmap support at
> > this stage, w/o having it physically accessible.
> > 
> > Since no ACPI or similar events are generated, there is no way to set up
> > altmap support, or even make the memory physically accessible at all,
> > before the "memmap on memory" initialization in memory_block_online().
> > 
> > The new MEM_PHYS_ONLINE notifier allows to work around this, by
> > providing a hook to make the memory physically accessible, and also call
> > __add_pages() with altmap support, early in memory_block_online().
> > Similarly, the MEM_PHYS_OFFLINE notifier allows to make the memory
> > inaccessible and call __remove_pages(), at the end of
> > memory_block_offline().
> > 
> > Calling __add/remove_pages() requires mem_hotplug_lock, so move
> > mem_hotplug_begin/done() to include the new notifiers.
> > 
> > All architectures ignore unknown memory notifiers, so this patch should
> > not introduce any functional changes.
> 
> Sorry to say, no. No hacks please, and this is a hack for memory that has
> already been added to the system.
> 
> If you want memory without an altmap to suddenly not have an altmap anymore,
> then look into removing and readding that memory, or some way to convert
> offline memory.

Sorry, I couldnt get the context. Could you please give me more details?

Thanks