mm: fix null pointer dereference in wait_iff_congested()

Zlatko Calusic · torvalds · commit ecccd1248d6e · 2012-12-28T08:42:39.000-08:00
An unintended consequence of commit 4ae0a48 ("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>
diff --git a/mm/vmscan.c b/mm/vmscan.c
@@ -2775,7 +2775,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 		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);
 		}
 

Original file line number	Diff line number	Diff line change
`@@ -2775,7 +2775,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,`
`2775`	`2775`	`if (total_scanned && (sc.priority < DEF_PRIORITY - 2)) {`
`2776`	`2776`	`if (has_under_min_watermark_zone)`
`2777`	`2777`	`count_vm_event(KSWAPD_SKIP_CONGESTION_WAIT);`
`2778`		`- else`
	`2778`	`+ else if (unbalanced_zone)`
`2779`	`2779`	`wait_iff_congested(unbalanced_zone, BLK_RW_ASYNC, HZ/10);`
`2780`	`2780`	`}`
`2781`	`2781`