Re: [PATCH 5/6] mm: honor PF_MEMALLOC_NOMOVABLE for all allocations

[Michal Hocko <mhocko@suse.com>]

On Thu 03-12-20 10:15:41, Pavel Tatashin wrote:
> On Thu, Dec 3, 2020 at 4:17 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 02-12-20 00:23:29, Pavel Tatashin wrote:
> > [...]
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 611799c72da5..7a6d86d0bc5f 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -3766,20 +3766,25 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
> > >       return alloc_flags;
> > >  }
> > >
> > > -static inline unsigned int current_alloc_flags(gfp_t gfp_mask,
> > > -                                     unsigned int alloc_flags)
> > > +static inline unsigned int cma_alloc_flags(gfp_t gfp_mask,
> > > +                                        unsigned int alloc_flags)
> > >  {
> > >  #ifdef CONFIG_CMA
> > > -     unsigned int pflags = current->flags;
> > > -
> > > -     if (!(pflags & PF_MEMALLOC_NOMOVABLE) &&
> > > -         gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> > > +     if (gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> > >               alloc_flags |= ALLOC_CMA;
> > > -
> > >  #endif
> > >       return alloc_flags;
> > >  }
> > >
> > > +static inline gfp_t current_gfp_checkmovable(gfp_t gfp_mask)
> > > +{
> > > +     unsigned int pflags = current->flags;
> > > +
> > > +     if ((pflags & PF_MEMALLOC_NOMOVABLE))
> > > +             return gfp_mask & ~__GFP_MOVABLE;
> > > +     return gfp_mask;
> > > +}
> > > +
> >
> > It sucks that we have to control both ALLOC and gfp flags. But wouldn't
> > it be simpler and more straightforward to keep current_alloc_flags as is
> > (module PF rename) and hook the gfp mask evaluation into current_gfp_context
> > and move it up before the first allocation attempt?
> 
> We could do that, but perhaps as a separate patch? I am worried about
> hidden implication of adding extra scope (GFP_NOIO|GFP_NOFS) to the
> fast path.

Why?

> Also, current_gfp_context() is used elsewhere, and in some
> places removing __GFP_MOVABLE from gfp_mask means that we will need to
> also change other things. For example [1], in try_to_free_pages() we
> call current_gfp_context(gfp_mask) which can reduce the maximum zone
> idx, yet we simply set it to: reclaim_idx = gfp_zone(gfp_mask), not to
> the newly determined gfp_mask.

Yes and the direct reclaim should honor the movable zone restriction.
Why should we reclaim ZONE_MOVABLE when the allocation cannot really
allocate from it? Or have I misunderstood your concern?

> 
> [1] https://soleen.com/source/xref/linux/mm/vmscan.c?r=2da9f630#3239
> 
> 
>  All scope flags
> > should be applicable to the hot path as well. It would add few cycles to
> > there but the question is whether that would be noticeable over just
> > handling PF_MEMALLOC_NOMOVABLE on its own. The cache line would be
> > pulled in anyway.
> 
> Let's try it in a separate patch? I will add it in the next version of
> this series.

Separate patch or not is up to you. But I do not see a strong reason why
this cannot be addressed in a single one.

-- 
Michal Hocko
SUSE Labs