Use smaller scan_control fields for order, priority, and reclaim_idx.
Convert fields from int => s8. All easily fit within a byte:
* allocation order range: 0..MAX_ORDER(64?)
* priority range: 0..12(DEF_PRIORITY)
* reclaim_idx range: 0..6(__MAX_NR_ZONES)
Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
stack overflows are not an issue. But it's inefficient to use ints.
Use s8 (signed byte) rather than u8 to allow for loops like:
do {
...
} while (--sc.priority >= 0);
Add BUILD_BUG_ON to verify that s8 is capable of storing max values.
This reduces sizeof(struct scan_control):
* 96 => 80 bytes (x86_64)
* 68 => 56 bytes (i386)
scan_control structure field order is changed to utilize padding.
After this patch there is 1 bit of scan_control padding.
Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
---
mm/vmscan.c | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9b697323a88c..42731faea306 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -65,12 +65,6 @@ struct scan_control {
/* How many pages shrink_list() should reclaim */
unsigned long nr_to_reclaim;
- /* This context's GFP mask */
- gfp_t gfp_mask;
-
- /* Allocation order */
- int order;
-
/*
* Nodemask of nodes allowed by the caller. If NULL, all nodes
* are scanned.
@@ -83,12 +77,6 @@ struct scan_control {
*/
struct mem_cgroup *target_mem_cgroup;
- /* Scan (total_size >> priority) pages at once */
- int priority;
-
- /* The highest zone to isolate pages for reclaim from */
- enum zone_type reclaim_idx;
-
/* Writepage batching in laptop mode; RECLAIM_WRITE */
unsigned int may_writepage:1;
@@ -111,6 +99,18 @@ struct scan_control {
/* One of the zones is ready for compaction */
unsigned int compaction_ready:1;
+ /* Allocation order */
+ s8 order;
+
+ /* Scan (total_size >> priority) pages at once */
+ s8 priority;
+
+ /* The highest zone to isolate pages for reclaim from */
+ s8 reclaim_idx;
+
+ /* This context's GFP mask */
+ gfp_t gfp_mask;
+
/* Incremented by the number of inactive pages that were scanned */
unsigned long nr_scanned;
@@ -3047,6 +3047,14 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
.may_swap = 1,
};
+ /*
+ * scan_control uses s8 fields for order, priority, and reclaim_idx.
+ * Confirm they are large enough for max values.
+ */
+ BUILD_BUG_ON(MAX_ORDER > S8_MAX);
+ BUILD_BUG_ON(DEF_PRIORITY > S8_MAX);
+ BUILD_BUG_ON(MAX_NR_ZONES > S8_MAX);
+
/*
* Do not enter reclaim if fatal signal was delivered while throttled.
* 1 is returned so that the page allocator does not OOM kill at this
--
2.17.0.921.gf22659ad46-goog
