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

David Hildenbrand
2025-10-08T09:33:58Z

On 08.10.25 11:13, Sumanth Korikkar wrote:
>>>> 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.
>>
>> That's my point: I don't consider either very likely to be used by actual
>> admins.
>>
>> I guess in (1) it really only is a problem with very big memory blocks.
>> Assuming a memory block is just 128 MiB (or even 1 GiB), you can add+online
>> them individually. Once you succeeded with the first one (very likely), the
>> other ones will follow.
>>
>> Sure, if you are so low on memory that you cannot even a single memory
>> block, then memmap-on-memory makes sense.
>>
>> But note that memmap-on-memory was added to handle hotplug of large chunks
>> of memory (large DIMM/NVDIMM, large CXL device) in one go, without the
>> chance to add+online individual memory blocks incrementally.
>
> Interesting. Thanks David.
>
> Heiko suggested that memory increment size could also be upto
> 64GB. In that case, it might be useful.

Yeha, rings a bell. But that would not be your 4GiB scenario you shared :)

--
Cheers

David / dhildenb