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

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Date:	Wed, 11 May 2016 16:52:41 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Michal Hocko <mhocko@...nel.org>
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 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.

	Arnd