Re: [PATCH v3 3/6] mm: apply per-task gfp constraints in fast path

[Pavel Tatashin <pasha.tatashin@soleen.com>]

On Tue, Dec 15, 2020 at 3:25 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 15-12-20 00:20:39, Pavel Tatashin wrote:
> > > Ack to this.
> >
> > Thank you.
> >
> > >
> > > But I do not really understand this. All allocation contexts should have
> > > a proper gfp mask so why do we have to call current_gfp_context here?
> > > In fact moving the current_gfp_context in the allocator path should have
> > > made all this games unnecessary. Memcg reclaim path might need some
> > > careful check because gfp mask is used more creative there but the
> > > general reclaim paths should be ok.
> > >
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > >
> > > Again, why do we need this when the gfp_mask
> > > >       };
> > > >
> > --
> >
> > Hi Michal,
> >
> > Beside from __alloc_pages_nodemask(), the current_gfp_context() is
> > called from the following six functions:
> >
> > try_to_free_pages()
> > try_to_free_mem_cgroup_pages()
> > __node_reclaim()
> > __need_fs_reclaim()
> > alloc_contig_range()
> > pcpu_alloc()
> >
> > As I understand, the idea is that because the allocator now honors
> > gfp_context values for all paths, the call can be removed from some of
> > the above functions. I think you are correct. But, at least from a
> > quick glance, this is not obvious, and is not the case for all of the
> > above functions.
> >
> > For example:
> >
> > alloc_contig_range()
> >   __alloc_contig_migrate_range
> >    isolate_migratepages_range
> >      isolate_migratepages_block
> >         /*
> >          * Only allow to migrate anonymous pages in GFP_NOFS context
> >          * because those do not depend on fs locks.
> >          */
> >        if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
> >           goto isolate_fail;
> >
> > If we remove current_gfp_context() from alloc_contig_range(), the
> > cc->gfp_mask will not be updated with proper __GFP_FS flag.
>
> I do not think I was proposing to drop current_gfp_context from
> alloc_contig_range. ACR needs some work to be properly scoped gfp mask
> aware. This should be addressed but I do not think think the code
> works properly now so I wouldn't lose sleep over it in this series. At
> least __alloc_contig_migrate_range should follow the gfp mask given to
> alloc_contig_range.
>
> > I have studied some other paths, and they are also convoluted.
> > Therefore, I am worried about performing this optimization in this
> > series.
>
> Dropping current_gfp_context from the reclaim context should be done in
> a separate patch. I didn't mean to push for this here. All I meant was
> to simply not touch gfp/zone_idx in the reclaim path. The changelog
> should call out that the page allocator always provides proper gfp mask.

I see what you mean. I will remove reclaim changes, and will add a
note to changelog.

Thank you,
Pasha

>
> --
> Michal Hocko
> SUSE Labs