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

Michal Hocko
2019-04-03T10:46:09Z

On Wed 03-04-19 11:40:54, 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.

And just to be more specific. The api shouldn't really care about this
implementation detail. So ideally the caller of add_pages just picks up
the proper allocator and the rest is completely transparent.

> 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.

THis is a good point. We do have means to tell a default zone for the
regular memory hotplug so this can be reused based on the pfn range
(default_zone_for_pfn).
--
Michal Hocko
SUSE Labs