Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings

[Michal Hocko]

On Tue 09-10-18 14:00:34, Mel Gorman wrote:
> On Tue, Oct 09, 2018 at 02:27:45PM +0200, Michal Hocko wrote:
> > [Sorry for being slow in responding but I was mostly offline last few
> >  days]
> > 
> > On Tue 09-10-18 10:48:25, Mel Gorman wrote:
> > [...]
> > > This goes back to my point that the MADV_HUGEPAGE hint should not make
> > > promises about locality and that introducing MADV_LOCAL for specialised
> > > libraries may be more appropriate with the initial semantic being how it
> > > treats MADV_HUGEPAGE regions.
> > 
> > I agree with your other points and not going to repeat them. I am not
> > sure madvise s the best API for the purpose though. We are talking about
> > memory policy here and there is an existing api for that so I would
> > _prefer_ to reuse it for this purpose.
> > 
> 
> I flip-flopped on that one in my head multiple times on the basis of
> how strict it should be. Memory policies tend to be black or white --
> bind here, interleave there, etc. It wasn't clear to me what the best
> policy would be to describe "allocate local as best as you can but allow
> fallbacks if necessary".

I was thinking about MPOL_NODE_PROXIMITY with the following semantic:
- try hard to allocate from a local or very close numa node(s) even when
that requires expensive operations like the memory reclaim/compaction
before falling back to other more distant numa nodes.


> Hence, I started leaning towards advise as it is
> really about advice that the kernel can ignore if necessary. That said,
> I don't feel as strongly about the "how" as I do about the fact that
> applications and libraries should not depend on side-effects of the
> MADV_HUGEPAGE implementation that relate to locality.

completely agreed on this.
-- 
Michal Hocko
SUSE Labs