[PATCH] mm: convert scan_control.priority int => byte

[Greg Thelen]

Reclaim priorities range from 0..12(DEF_PRIORITY).
scan_control.priority is a 4 byte int, which is overkill.

Since commit 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64
stack overflows are not an issue.  But it's inefficient to use 4 bytes
for priority.

Use s8 (signed byte) rather than u8 to allow for loops like:
        do {
                ...
        } while (--sc.priority >= 0);

This reduces sizeof(struct scan_control) from 96 => 88 bytes (x86_64),
which saves some stack.

scan_control.priority field order is changed to occupy otherwise unused
padding.

Signed-off-by: Greg Thelen <gthe...@google.com>
---
 mm/vmscan.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9b697323a88c..541c334bd176 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -83,9 +83,6 @@ 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;
 
@@ -111,6 +108,9 @@ struct scan_control {
        /* One of the zones is ready for compaction */
        unsigned int compaction_ready:1;
 
+       /* Scan (total_size >> priority) pages at once */
+       s8 priority;
+
        /* Incremented by the number of inactive pages that were scanned */
        unsigned long nr_scanned;
 
-- 
2.17.0.921.gf22659ad46-goog