linux-kernel - Re: [PATCH 2/2] mm/slub: don't use reserved memory for optimistic try

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Date:   Wed, 6 Sep 2017 11:20:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 2/2] mm/slub: don't use reserved memory for optimistic try

On Wed 06-09-17 10:10:22, Vlastimil Babka wrote:
> On 09/06/2017 06:37 AM, js1304@...il.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > High-order atomic allocation is difficult to succeed since we cannot
> > reclaim anything in this context. So, we reserves the pageblock for
> > this kind of request.
> > 
> > In slub, we try to allocate higher-order page more than it actually
> > needs in order to get the best performance. If this optimistic try is
> > used with GFP_ATOMIC, alloc_flags will be set as ALLOC_HARDER and
> > the pageblock reserved for high-order atomic allocation would be used.
> > Moreover, this request would reserve the MIGRATE_HIGHATOMIC pageblock
> > ,if succeed, to prepare further request. It would not be good to use
> > MIGRATE_HIGHATOMIC pageblock in terms of fragmentation management
> > since it unconditionally set a migratetype to request's migratetype
> > when unreserving the pageblock without considering the migratetype of
> > used pages in the pageblock.
> > 
> > This is not what we don't intend so fix it by unconditionally masking
> > out __GFP_ATOMIC in order to not set ALLOC_HARDER.
> > 
> > And, it is also undesirable to use reserved memory for optimistic try
> > so mask out __GFP_HIGH. This patch also adds __GFP_NOMEMALLOC since
> > we don't want to use the reserved memory for optimistic try even if
> > the user has PF_MEMALLOC flag.
> > 
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > ---
> >  include/linux/gfp.h | 1 +
> >  mm/page_alloc.c     | 8 ++++++++
> >  mm/slub.c           | 6 ++----
> >  3 files changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index f780718..1f5658e 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -568,6 +568,7 @@ extern gfp_t gfp_allowed_mask;
> >  
> >  /* Returns true if the gfp_mask allows use of ALLOC_NO_WATERMARK */
> >  bool gfp_pfmemalloc_allowed(gfp_t gfp_mask);
> > +gfp_t gfp_drop_reserves(gfp_t gfp_mask);
> >  
> >  extern void pm_restrict_gfp_mask(void);
> >  extern void pm_restore_gfp_mask(void);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 6dbc49e..0f34356 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3720,6 +3720,14 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
> >  	return !!__gfp_pfmemalloc_flags(gfp_mask);
> >  }
> >  
> > +gfp_t gfp_drop_reserves(gfp_t gfp_mask)
> > +{
> > +	gfp_mask &= ~(__GFP_HIGH | __GFP_ATOMIC);
> > +	gfp_mask |= __GFP_NOMEMALLOC;
> > +
> > +	return gfp_mask;
> > +}
> > +
> 
> I think it's wasteful to do a function call for this, inline definition
> in header would be better (gfp_pfmemalloc_allowed() is different as it
> relies on a rather heavyweight __gfp_pfmemalloc_flags().

Agreed. If you do that, feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>
-- 
Michal Hocko
SUSE Labs