Re: [RFC PATCH] memory_hotplug: disable the functionality for 32b

[Robin Murphy <robin.murphy@arm.com>]

On 2020-02-19 9:46 pm, Andrew Morton wrote:
> On Tue, 18 Feb 2020 11:05:32 +0100 Michal Hocko <mhocko@kernel.org> wrote:
> 
>> Subject: [PATCH] memory_hotplug: disable the functionality for 32b
>>
>> Memory hotlug is broken for 32b systems at least since c6f03e2903c9
>> ("mm, memory_hotplug: remove zone restrictions") which has considerably
>> reworked how can be memory associated with movable/kernel zones. The
>> same is not really trivial to achieve in 32b where only lowmem is the
>> kernel zone. While we can tweak this immediate problem around there are
>> likely other land mines hidden at other places.
>>
>> It is also quite dubious that there is a real usecase for the memory
>> hotplug on 32b in the first place. Low memory is just too small to be
>> hotplugable (for hot add) and generally unusable for hotremove. Adding
>> more memory to highmem is also dubious because it would increase the
>> low mem or vmalloc space pressure for memmaps.
>>
>> Restrict the functionality to 64b systems. This will help future
>> development to focus on usecases that have real life application.  We
>> can remove this restriction in future in presence of a real life usecase
>> of course but until then make it explicit that hotplug on 32b is broken
>> and requires a non trivial amount of work to fix.
> 
> (cc linux-arch)
> 
> (and linux-arm-kernel, as ARM is a major 32-bit user)
> 
> Does anyone see a problem with disabling memory hotplug on 32-bit builds?

32-bit Arm doesn't support memory hotplug, and as far as I'm aware 
there's little likelihood of it ever wanting to. FWIW it looks like 
SuperH is the only pure-32-bit architecture to have hotplug support at all.

Robin.