[PATCH] mm: fix null pointer dereference in wait_iff_congested()

[Zlatko Calusic]

From: Zlatko Calusic <zlatko.calusic@xxxxxxxx>

The unintended consequence of commit 4ae0a48b is that
wait_iff_congested() can now be called with NULL struct zone*
producing kernel oops like this:

BUG: unable to handle kernel NULL pointer dereference
IP: [<ffffffff811542d9>] wait_iff_congested+0x59/0x140

This trivial patch fixes it.

Reported-by: Zhouping Liu <zliu@xxxxxxxxxx>
Reported-and-tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Zlatko Calusic <zlatko.calusic@xxxxxxxx>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 02bcfa3..e55ce55 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2782,7 +2782,7 @@ loop_again:
 		if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {
 			if (has_under_min_watermark_zone)
 				count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);
-			else
+			else if (unbalanced_zone)
 				wait_iff_congested(unbalanced_zone, BLK_RW_ASYNC, HZ/10);
 		}
 
-- 
1.8.1.rc3

-- 
Zlatko
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