Re: [PATCH 2/2] mm/slub: don't use reserved highatomic pageblock for optimistic try

[Michal Hocko]

On Thu 31-08-17 10:42:41, Joonsoo Kim wrote:
> On Tue, Aug 29, 2017 at 09:33:44AM +0900, Joonsoo Kim wrote:
> > On Mon, Aug 28, 2017 at 03:08:29PM +0200, Michal Hocko wrote:
> > > On Mon 28-08-17 13:29:29, Vlastimil Babka wrote:
> > > > On 08/28/2017 03:11 AM, js1304@xxxxxxxxx wrote:
> > > > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > > > 
> > > > > High-order atomic allocation is difficult to succeed since we cannot
> > > > > reclaim anything in this context. So, we reserves the pageblock for
> > > > > this kind of request.
> > > > > 
> > > > > In slub, we try to allocate higher-order page more than it actually
> > > > > needs in order to get the best performance. If this optimistic try is
> > > > > used with GFP_ATOMIC, alloc_flags will be set as ALLOC_HARDER and
> > > > > the pageblock reserved for high-order atomic allocation would be used.
> > > > > Moreover, this request would reserve the MIGRATE_HIGHATOMIC pageblock
> > > > > ,if succeed, to prepare further request. It would not be good to use
> > > > > MIGRATE_HIGHATOMIC pageblock in terms of fragmentation management
> > > > > since it unconditionally set a migratetype to request's migratetype
> > > > > when unreserving the pageblock without considering the migratetype of
> > > > > used pages in the pageblock.
> > > > > 
> > > > > This is not what we don't intend so fix it by unconditionally setting
> > > > > __GFP_NOMEMALLOC in order to not set ALLOC_HARDER.
> > > > 
> > > > I wonder if it would be more robust to strip GFP_ATOMIC from alloc_gfp.
> > > > E.g. __GFP_NOMEMALLOC does seem to prevent ALLOC_HARDER, but not
> > > > ALLOC_HIGH. Or maybe we should adjust __GFP_NOMEMALLOC implementation
> > > > and document it more thoroughly? CC Michal Hocko
> > > 
> > > Yeah, __GFP_NOMEMALLOC is rather inconsistent. It has been added to
> > > override __GFP_MEMALLOC resp. PF_MEMALLOC AFAIK. In this particular
> > > case I would agree that dropping __GFP_HIGH and __GFP_ATOMIC would
> > > be more precise. I am not sure we want to touch the existing semantic of
> > > __GFP_NOMEMALLOC though. This would require auditing all the existing
> > > users (something tells me that quite some of those will be incorrect...)
> > 
> > Hmm... now I realize that there is another reason that we need to use
> > __GFP_NOMEMALLOC. Even if this allocation comes from PF_MEMALLOC user,
> > this optimistic try should not use the reserved memory below the
> > watermark. That is, it should not use ALLOC_NO_WATERMARKS. It can
> > only be accomplished by using __GFP_NOMEMALLOC.
> 
> Michal, Vlastimil, Any thought?

Hmm, I would go with a helper like below and use it in slub

gfp_t gfp_drop_reserves(gfp_t mask)
{
	mask &= ~(__GFP_HIGH|__GFP_ATOMIC)
	mask |= __GFP_NOMEMALLOC;

	return mask;
}
-- 
Michal Hocko
SUSE Labs