linux-kernel - [PATCH] mm: ensure alloc_flags in slow path are initialized

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Message-Id: <20170123121649.3180300-1-arnd@arndb.de>
Date:   Mon, 23 Jan 2017 13:16:12 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] mm: ensure alloc_flags in slow path are initialized

The __alloc_pages_slowpath() has gotten rather complex and gcc
is no longer able to follow the gotos and prove that the
alloc_flags variable is initialized at the time it is used:

mm/page_alloc.c: In function '__alloc_pages_slowpath':
mm/page_alloc.c:3565:15: error: 'alloc_flags' may be used uninitialized in this function [-Werror=maybe-uninitialized]

To be honest, I can't figure that out either, maybe it is or
maybe not, but moving the existing initialization up a little
higher looks safe and makes it obvious to both me and gcc that
the initialization comes before the first use.

Fixes: 74eaa4a97e8e ("mm: consolidate GFP_NOFAIL checks in the allocator slowpath")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 mm/page_alloc.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cf641932c015..d9fa4564524f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3591,6 +3591,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
 		gfp_mask &= ~__GFP_ATOMIC;
 
+	/*
+	 * The fast path uses conservative alloc_flags to succeed only until
+	 * kswapd needs to be woken up, and to avoid the cost of setting up
+	 * alloc_flags precisely. So we do that now.
+	 */
+	alloc_flags = gfp_to_alloc_flags(gfp_mask);
+
 retry_cpuset:
 	compaction_retries = 0;
 	no_progress_loops = 0;
@@ -3607,14 +3614,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (!ac->preferred_zoneref->zone)
 		goto nopage;
 
-
-	/*
-	 * The fast path uses conservative alloc_flags to succeed only until
-	 * kswapd needs to be woken up, and to avoid the cost of setting up
-	 * alloc_flags precisely. So we do that now.
-	 */
-	alloc_flags = gfp_to_alloc_flags(gfp_mask);
-
 	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
 		wake_all_kswapds(order, ac);
 
-- 
2.9.0