[Patch v2] mm/vmscan: fix unsequenced modification and access warning

From: Nick Desaulniers <nick.desaulniers@gmail.com>
Cc: akpm@linux-foundation.org, hannes@cmpxchg.org,
	mgorman@techsingularity.net, mhocko@suse.com, vbabka@suse.cz,
	minchan@kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	Nick Desaulniers <nick.desaulniers@gmail.com>
Subject: [Patch v2] mm/vmscan: fix unsequenced modification and access warning
Date: Wed, 10 May 2017 01:27:34 -0700	[thread overview]
Message-ID: <20170510082734.2055-1-nick.desaulniers@gmail.com> (raw)
In-Reply-To: <20170510071511.GA31466@dhcp22.suse.cz>

Clang flags this file with the -Wunsequenced error that GCC does not
have.

unsequenced modification and access to 'gfp_mask'

It seems that gfp_mask is both read and written without a sequence point
in between, which is undefined behavior.

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
---
Changes in v2:
- don't assign back to gfp_mask, reuse sc.gfp_mask
- initialize reclaim_idx directly, without classzone_idx

 mm/vmscan.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4e7ed65842af..d32c42d17935 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2958,7 +2958,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 	unsigned long nr_reclaimed;
 	struct scan_control sc = {
 		.nr_to_reclaim = SWAP_CLUSTER_MAX,
-		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
+		.gfp_mask = current_gfp_context(gfp_mask),
 		.reclaim_idx = gfp_zone(gfp_mask),
 		.order = order,
 		.nodemask = nodemask,
@@ -2973,12 +2973,12 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 	 * 1 is returned so that the page allocator does not OOM kill at this
 	 * point.
 	 */
-	if (throttle_direct_reclaim(gfp_mask, zonelist, nodemask))
+	if (throttle_direct_reclaim(sc.gfp_mask, zonelist, nodemask))
 		return 1;
 
 	trace_mm_vmscan_direct_reclaim_begin(order,
 				sc.may_writepage,
-				gfp_mask,
+				sc.gfp_mask,
 				sc.reclaim_idx);
 
 	nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
@@ -3763,16 +3763,15 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	const unsigned long nr_pages = 1 << order;
 	struct task_struct *p = current;
 	struct reclaim_state reclaim_state;
-	int classzone_idx = gfp_zone(gfp_mask);
 	struct scan_control sc = {
 		.nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
-		.gfp_mask = (gfp_mask = current_gfp_context(gfp_mask)),
+		.gfp_mask = current_gfp_context(gfp_mask),
 		.order = order,
 		.priority = NODE_RECLAIM_PRIORITY,
 		.may_writepage = !!(node_reclaim_mode & RECLAIM_WRITE),
 		.may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
 		.may_swap = 1,
-		.reclaim_idx = classzone_idx,
+		.reclaim_idx = gfp_zone(gfp_mask),
 	};
 
 	cond_resched();
@@ -3782,7 +3781,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	 * and RECLAIM_UNMAP.
 	 */
 	p->flags |= PF_MEMALLOC | PF_SWAPWRITE;
-	lockdep_set_current_reclaim_state(gfp_mask);
+	lockdep_set_current_reclaim_state(sc.gfp_mask);
 	reclaim_state.reclaimed_slab = 0;
 	p->reclaim_state = &reclaim_state;
 
-- 
2.11.0

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