Re: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for ZONE_MOVABLE request

[Michal Hocko]

On Fri 25-08-17 09:15:43, Joonsoo Kim wrote:
> On Thu, Aug 24, 2017 at 11:30:50AM +0200, Michal Hocko wrote:
> > On Thu 24-08-17 14:45:46, Joonsoo Kim wrote:
> > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > 
> > > Freepage on ZONE_HIGHMEM doesn't work for kernel memory so it's not that
> > > important to reserve. When ZONE_MOVABLE is used, this problem would
> > > theorectically cause to decrease usable memory for GFP_HIGHUSER_MOVABLE
> > > allocation request which is mainly used for page cache and anon page
> > > allocation. So, fix it.
> > 
> > I do not really understand what is the problem you are trying to fix.
> > Yes the memory is reserved for a higher priority consumer and that is
> > deliberate AFAICT. Just consider that an OOM victim wants to make
> > further progress and rely on memory reserve while doing
> > GFP_HIGHUSER_MOVABLE request.
> > 
> > So what is the real problem you are trying to address here?
> 
> If the system has the both, ZONE_HIGHMEM and ZONE_MOVABLE,
> ZONE_HIGHMEM will reserve the memory for ZONE_MOVABLE request.

Ohh, right. I forgot that __GFP_MOVABLE doesn't really enforce the
movable zone. It does so only if __GFP_HIGHMEM is specified as well when
ZONE_HIGHMEM is enabled. So indeed reserving memory in both is somehow
awkward. So why don't we simply remove reserves from the movable zone
when the highmem zone is enabled?
-- 
Michal Hocko
SUSE Labs