[PATCH] fix sparse warning from include/linux/mmzone.h

[Harvey Harrison <harvey.harrison@gmail.com>]

include/linux/mmzone.h:640:22: warning: potentially expensive pointer subtraction

The code in question was doing a pointer subtraction to find the index of
the zone argument in the node_zones array.  This essentially boils down to:

((unsigned long)zone - (unsigned long)zone->zone_pgdat->node_zones)/sizeof(struct zone)

Which can be expensive if struct zone is not a power of two.  Instead, just
calculate the offsets rather than the index in the array.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
---
This shows up > 30 times in an x86 allyesconfig, would be nice to get
rid of it and makes the code a little more efficient.  I believe this
is correct, unless there is a subtlety I have missed.

 include/linux/mmzone.h |    9 ++++++---
 1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 8d8d197..cb07758 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -637,9 +637,12 @@ static inline int is_normal_idx(enum zone_type idx)
 static inline int is_highmem(struct zone *zone)
 {
 #ifdef CONFIG_HIGHMEM
-	int zone_idx = zone - zone->zone_pgdat->node_zones;
-	return zone_idx == ZONE_HIGHMEM ||
-		(zone_idx == ZONE_MOVABLE && zone_movable_is_highmem());
+	const int highmem_off = ZONE_HIGHMEM * sizeof(*zone);
+	const int movable_off = ZONE_MOVABLE * sizeof(*zone);
+
+	int zone_off = (unsigned long)zone - (unsigned long)zone->zone_pgdat->node_zones;
+	return zone_off == highmem_off ||
+		(zone_off == movable_off && zone_movable_is_highmem());
 #else
 	return 0;
 #endif
-- 
1.5.4.1219.g65b9