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




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