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

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Date:	Wed, 11 May 2016 16:44:07 +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 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
> 
> A more elaborate rework might make this more readable.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 13cff7b81275 ("mm, compaction: simplify __alloc_pages_direct_compact feedback interface")

Please do not use SHA for mmotm commits because they are unstable and
change each linux-next release.

--- 
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;
 }
 
-- 
Michal Hocko
SUSE Labs