Re: [PATCH 1/4] mm/page_alloc: fix non cma alloc context

[Joonsoo Kim <js1304@gmail.com>]

2020년 7월 15일 (수) 오후 5:24, Vlastimil Babka <vbabka@suse.cz>님이 작성:
>
> On 7/15/20 7:05 AM, js1304@gmail.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >
> > Currently, preventing cma area in page allocation is implemented by using
> > current_gfp_context(). However, there are two problems of this
> > implementation.
> >
> > First, this doesn't work for allocation fastpath. In the fastpath,
> > original gfp_mask is used since current_gfp_context() is introduced in
> > order to control reclaim and it is on slowpath.
> > Second, clearing __GFP_MOVABLE has a side effect to exclude the memory
> > on the ZONE_MOVABLE for allocation target.
> >
> > To fix these problems, this patch changes the implementation to exclude
> > cma area in page allocation. Main point of this change is using the
> > alloc_flags. alloc_flags is mainly used to control allocation so it fits
> > for excluding cma area in allocation.
>
> Agreed, should have been done with ALLOC_CMA since the beginning.
>
> > Fixes: d7fefcc (mm/cma: add PF flag to force non cma alloc)
>
> More digits please.
> Fixes: d7fefcc8de91 ("mm/cma: add PF flag to force non cma alloc")

Will do.

> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > ---
> >  include/linux/sched/mm.h |  4 ----
> >  mm/page_alloc.c          | 27 +++++++++++++++------------
> >  2 files changed, 15 insertions(+), 16 deletions(-)
> >
> > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> > index 44ad5b7..a73847a 100644
> > --- a/include/linux/sched/mm.h
> > +++ b/include/linux/sched/mm.h
> > @@ -191,10 +191,6 @@ static inline gfp_t current_gfp_context(gfp_t flags)
> >                       flags &= ~(__GFP_IO | __GFP_FS);
> >               else if (pflags & PF_MEMALLOC_NOFS)
> >                       flags &= ~__GFP_FS;
>
> Above this hunk you should also remove PF_MEMALLOC_NOCMA from the test.

Will do.

> > -#ifdef CONFIG_CMA
> > -             if (pflags & PF_MEMALLOC_NOCMA)
> > -                     flags &= ~__GFP_MOVABLE;
> > -#endif
> >       }
> >       return flags;
> >  }
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 6416d08..cd53894 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2791,7 +2791,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
> >        * allocating from CMA when over half of the zone's free memory
> >        * is in the CMA area.
> >        */
> > -     if (migratetype == MIGRATE_MOVABLE &&
> > +     if (alloc_flags & ALLOC_CMA &&
> >           zone_page_state(zone, NR_FREE_CMA_PAGES) >
> >           zone_page_state(zone, NR_FREE_PAGES) / 2) {
> >               page = __rmqueue_cma_fallback(zone, order);
> > @@ -2802,7 +2802,7 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
> >  retry:
> >       page = __rmqueue_smallest(zone, order, migratetype);
> >       if (unlikely(!page)) {
> > -             if (migratetype == MIGRATE_MOVABLE)
> > +             if (alloc_flags & ALLOC_CMA)
> >                       page = __rmqueue_cma_fallback(zone, order);
> >
> >               if (!page && __rmqueue_fallback(zone, order, migratetype,
> > @@ -3502,11 +3502,9 @@ static inline long __zone_watermark_unusable_free(struct zone *z,
> >       if (likely(!alloc_harder))
> >               unusable_free += z->nr_reserved_highatomic;
> >
> > -#ifdef CONFIG_CMA
> >       /* If allocation can't use CMA areas don't use free CMA pages */
> > -     if (!(alloc_flags & ALLOC_CMA))
> > +     if (IS_ENABLED(CONFIG_CMA) && !(alloc_flags & ALLOC_CMA))
> >               unusable_free += zone_page_state(z, NR_FREE_CMA_PAGES);
> > -#endif
> >
> >       return unusable_free;
> >  }
> > @@ -3693,6 +3691,16 @@ alloc_flags_nofragment(struct zone *zone, gfp_t gfp_mask)
> >       return alloc_flags;
> >  }
> >
> > +static inline void current_alloc_flags(gfp_t gfp_mask,
> > +                             unsigned int *alloc_flags)
> > +{
> > +     unsigned int pflags = READ_ONCE(current->flags);
> > +
> > +     if (!(pflags & PF_MEMALLOC_NOCMA) &&
> > +             gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> > +             *alloc_flags |= ALLOC_CMA;
> > +}
>
> I don't like the modification through parameter, would just do what
> current_gfp_context() does and return the modified value.
> Also make it a no-op (including no READ_ONCE(current->flags)) if !CONFIG_CMA,
> please.

Okay.

> >  /*
> >   * get_page_from_freelist goes through the zonelist trying to allocate
> >   * a page.
> > @@ -3706,6 +3714,8 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
> >       struct pglist_data *last_pgdat_dirty_limit = NULL;
> >       bool no_fallback;
> >
> > +     current_alloc_flags(gfp_mask, &alloc_flags);
>
> I don't see why to move the test here? It will still be executed in the
> fastpath, if that's what you wanted to avoid.

I want to execute it on the fastpath, too. Reason that I moved it here
is that alloc_flags could be reset on slowpath. See the code where
__gfp_pfmemalloc_flags() is on. This is the only place that I can apply
this option to all the allocation paths at once.

Thanks.

> > +
> >  retry:
> >       /*
> >        * Scan zonelist, looking for a zone with enough free.
> > @@ -4339,10 +4349,6 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> >       } else if (unlikely(rt_task(current)) && !in_interrupt())
> >               alloc_flags |= ALLOC_HARDER;
> >
> > -#ifdef CONFIG_CMA
> > -     if (gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> > -             alloc_flags |= ALLOC_CMA;
> > -#endif
>
> I would just replace this here with:
> alloc_flags = current_alloc_flags(gfp_mask, alloc_flags);
>
> >       return alloc_flags;
> >  }
> >
> > @@ -4808,9 +4814,6 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
> >       if (should_fail_alloc_page(gfp_mask, order))
> >               return false;
> >
> > -     if (IS_ENABLED(CONFIG_CMA) && ac->migratetype == MIGRATE_MOVABLE)
> > -             *alloc_flags |= ALLOC_CMA;
>
> And same here... Ah, I see. current_alloc_flags() should probably take a
> migratetype parameter instead of gfp_mask then.
>
> > -
> >       return true;
> >  }
> >
> >
>