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

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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

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_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

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.

But certainly not on the online/offline path triggered by sysfs.


David / dhildenb

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