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

[Andrea Arcangeli]

On Tue, Oct 09, 2018 at 04:25:10PM +0200, Michal Hocko wrote:
> 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".

MPOL_PREFERRED is not black and white. In fact I asked David earlier
if MPOL_PREFERRED could check if it would already be a good fit for
this. Still the point is it requires privilege (and for a good
reason).

> 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.

If MPOL_PREFERRED can't work something like this could be added.

I think "madvise vs mbind" is more an issue of "no-permission vs
permission" required. And if the processes ends up swapping out all
other process with their memory already allocated in the node, I think
some permission is correct to be required, in which case an mbind
looks a better fit. MPOL_PREFERRED also looks a first candidate for
investigation as it's already not black and white and allows spillover
and may already do the right thing in fact if set on top of
MADV_HUGEPAGE.

Thanks,
Andrea