Re: [PATCH 0/4] mm,memory_hotplug: allocate memmap from hotadded memory

Vlastimil Babka
2019-04-04T10:25:32Z

On 4/3/19 11:40 AM, Oscar Salvador wrote:
> On Wed, Apr 03, 2019 at 10:37:57AM +0200, Michal Hocko wrote:
>> That being said it should be the caller of the hotplug code to tell
>> the vmemmap allocation strategy. For starter, I would only pack vmemmaps
>> for "regular" kernel zone memory. Movable zones should be more careful.
>> We can always re-evaluate later when there is a strong demand for huge
>> pages on movable zones but this is not the case now because those pages
>> are not really movable in practice.
>
> I agree that makes sense to let the caller specify if it wants to allocate
> vmemmaps per memblock or per memory-range, so we are more flexible when it
> comes to granularity in hot-add/hot-remove operations.

Please also add possibility to not allocate from hotadded memory (i.e.
allocate from node 0 as it is done now). I know about some MCDRAM users
that want to expose as much as possible to userspace, and not even
occupy those ~4% for memmap.

> But the thing is that the zones are picked at onling stage, while
> vmemmaps are created at hot-add stage, so I am not sure we can define
> the strategy depending on the zone.
>