mm: fix null pointer dereference in wait_iff_congested()

An unintended consequence of commit 4ae0a48b5efc ("mm: modify
pgdat_balanced() so that it also handles order-0") 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@redhat.com>
Reported-and-tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 23291b9ae871..16b42af393ac 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2775,7 +2775,7 @@ 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);
                 }