Re: [PATCH v2 for v5.9] mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs

[Michal Hocko]

On Tue 29-09-20 17:38:43, Joonsoo Kim wrote:
> 2020년 9월 29일 (화) 오후 5:08, Michal Hocko <mhocko@xxxxxxxx>님이 작성:
> >
> > On Mon 28-09-20 17:50:46, Joonsoo Kim wrote:
> > > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > >
> > > memalloc_nocma_{save/restore} APIs can be used to skip page allocation
> > > on CMA area, but, there is a missing case and the page on CMA area could
> > > be allocated even if APIs are used. This patch handles this case to fix
> > > the potential issue.
> > >
> > > Missing case is an allocation from the pcplist. MIGRATE_MOVABLE pcplist
> > > could have the pages on CMA area so we need to skip it if ALLOC_CMA isn't
> > > specified.
> > >
> > > Fixes: 8510e69c8efe (mm/page_alloc: fix memalloc_nocma_{save/restore} APIs)
> > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > > ---
> > >  mm/page_alloc.c | 13 ++++++++++---
> > >  1 file changed, 10 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index fab5e97..104d2e1 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -3367,9 +3367,16 @@ struct page *rmqueue(struct zone *preferred_zone,
> > >       struct page *page;
> > >
> > >       if (likely(order == 0)) {
> > > -             page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,
> > > +             /*
> > > +              * MIGRATE_MOVABLE pcplist could have the pages on CMA area and
> > > +              * we need to skip it when CMA area isn't allowed.
> > > +              */
> > > +             if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
> > > +                             migratetype != MIGRATE_MOVABLE) {
> > > +                     page = rmqueue_pcplist(preferred_zone, zone, gfp_flags,
> > >                                       migratetype, alloc_flags);
> > > -             goto out;
> > > +                     goto out;
> > > +             }
> > >       }
> >
> > This approach looks definitely better than the previous version.
> 
> Thanks!
> 
> > >
> > >       /*
> > > @@ -3381,7 +3388,7 @@ struct page *rmqueue(struct zone *preferred_zone,
> > >
> > >       do {
> > >               page = NULL;
> > > -             if (alloc_flags & ALLOC_HARDER) {
> > > +             if (order > 0 && alloc_flags & ALLOC_HARDER) {
> > >                       page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> > >                       if (page)
> > >                               trace_mm_page_alloc_zone_locked(page, order, migratetype);
> >
> > But this condition is not clear to me. __rmqueue_smallest doesn't access
> > pcp lists. Maybe I have missed the point in the original discussion but
> > this deserves a comment at least.
> 
> Before the pcplist skipping is applied, order-0 request can not reach here.
> But, now, an order-0 request can reach here. Free memory on
> MIGRATE_HIGHATOMIC is reserved for high-order atomic allocation
> so an order-0 request should skip it.

OK, I see. Thanks for the clarification.

> I will add a code comment on the next version.

Thanks, that would be indeed helpful. With that, feel free to add
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

-- 
Michal Hocko
SUSE Labs