Re: Guaranteed allocation of huge pages (1G) using movablecore=N doesn't seem to work at all

[Michal Hocko <mhocko@suse.com>]

On Tue 07-11-17 09:20:47, Vlastimil Babka wrote:
> On 11/06/2017 07:32 PM, Michal Hocko wrote:
> > On Mon 06-11-17 20:13:36, Maxim Levitsky wrote:
> >> Yes, I tested git head from mainline and few kernels from ubuntu repos
> >> since I was lazy to compile them too.
> > 
> > OK, so this hasn't worked realiably as I've suspected.
> > 
> >> Do you have an idea what can I do about this issue? Do you think its
> >> feasable to fix this?
> > 
> > Well, I think that giga pages need quite some love to be usable
> > reliably. The current implementation is more towards "make it work if
> > there is enough unused memory".
> > 
> >> And if not using moveable zone, how would it even be possible to have
> >> guaranreed allocation of 1g pages
> > 
> > Having a guaranteed giga pages is something the kernel is not yet ready
> > to offer.  Abusing zone movable might look like the right direction
> > but that is not really the case until we make sure those pages are
> > migratable.
> 
> Migratable where? It's very unlikely you will be able to migrate them
> away from a movable zone to a normal zone. So the use case is migration
> between hotplugable nodes, so one of them can be removed?

Yes, basically what we do for hugetlb pages normally. Smaller hugetlb
pages are more likely to succeed, though.

> That would
> probably be an improvement (even if you could not guarantee to offline
> all hotplugable nodes at once without admin intervention removing those
> giga pages). Right now the only scenario where giga pages are compatible
> with hot-remove is to put them on the already-limited non-removable node
> 0...

Yes, we will never be perfect, but I can see why people want to allocate
from movable zones so we definitely should work on making giga pages
more robust. The current state makes cry...
-- 
Michal Hocko
SUSE Labs

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