Re: [RFC PATCH 1/2] mm, memory_hotplug: drop artificial restriction on online/offline

[Reza Arbab]

On Wed, May 24, 2017 at 02:24:10PM +0200, Michal Hocko wrote:
> 74d42d8fe146 ("memory_hotplug: ensure every online node has NORMAL
> memory") has added can_offline_normal which checks the amount of
> memory in !movable zones as long as CONFIG_MOVABLE_NODE is disable.
> It disallows to offline memory if there is nothing left with a
> justification that "memory-management acts bad when we have nodes which
> is online but don't have any normal memory".
>
> 74d42d8fe146 ("memory_hotplug: ensure every online node has NORMAL
> memory") has introduced a restriction that every numa node has to have
> at least some memory in !movable zones before a first movable memory
> can be onlined if !CONFIG_MOVABLE_NODE with the same justification
>
> While it is true that not having _any_ memory for kernel allocations on
> a NUMA node is far from great and such a node would be quite subotimal
> because all kernel allocations will have to fallback to another NUMA
> node but there is no reason to disallow such a configuration in
> principle.
>
> Besides that there is not really a big difference to have one memblock
> for ZONE_NORMAL available or none. With 128MB size memblocks the system
> might trash on the kernel allocations requests anyway. It is really
> hard to draw a line on how much normal memory is really sufficient so
> we have to rely on administrator to configure system sanely therefore
> drop the artificial restriction and remove can_offline_normal and
> can_online_high_movable altogether.

I'm really liking all this cleanup of the memory hotplug code. Thanks!  
Much appreciated.

Acked-by: Reza Arbab <arbab@xxxxxxxxxxxxxxxxxx>

--
Reza Arbab