linux-kernel - Re: [PATCH] mm, compaction: avoid uninitialized variable use

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Date:	Wed, 11 May 2016 18:13:18 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Taku Izumi <izumi.taku@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, compaction: avoid uninitialized variable use

On Wed 11-05-16 16:52:41, Arnd Bergmann wrote:
> On Wednesday 11 May 2016 16:44:07 Michal Hocko wrote:
> > On Wed 11-05-16 15:24:44, Arnd Bergmann wrote:
> > > A recent rework of the compaction code introduced a warning about
> > > an uninitialized variable when CONFIG_COMPACTION is disabled and
> > > __alloc_pages_direct_compact() does not set its 'compact_result'
> > > output argument:
> > > 
> > > mm/page_alloc.c: In function '__alloc_pages_nodemask':
> > > mm/page_alloc.c:3651:6: error: 'compact_result' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> > > 
> > > This adds another check for CONFIG_COMPACTION to ensure we never
> > > evaluate the uninitialized variable in this configuration, which
> > > is probably the simplest way to avoid the warning.
> > 
> > I think that hiding this into __alloc_pages_direct_compact is a better
> > idea. See the diff below
> 
> Ok, sounds good.
> 
> > --- 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 4950d01ff935..14e3b4d93adc 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3300,6 +3300,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> >                 unsigned int alloc_flags, const struct alloc_context *ac,
> >                 enum migrate_mode mode, enum compact_result *compact_result)
> >  {
> > +       *compact_result = COMPACT_DEFERRED;
> >         return NULL;
> >  }
> > 
> 
> I thought about this but didn't know which COMPACT_* value was appropriate here.
> 
> The behavior then changes a bit with your approach compared to mine,
> because 
> 
>                 if (compact_result == COMPACT_DEFERRED)
>                         goto nopage;
> 
> is true now. I assume this is what we want though.

No, we actually want to try to reclaim, that's why COMPACT_SKIPPED is
more appropriate. COMPACT_SKIPPED is even correct semantically because
the compaction wasn't invoked.

-- 
Michal Hocko
SUSE Labs