[RFC PATCH 2/7] mm: use rcu_ptr instead of rcu_head

[Harry Yoo <harry.yoo@oracle.com>]

When slab objects are freed with kfree_rcu() and not call_rcu(),
using struct rcu_head (16 bytes on 64-bit) is unnecessary and
struct rcu_ptr (8 bytes on 64-bit) is enough. Save one pointer
per slab object by using struct rcu_ptr.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 include/linux/list_lru.h | 2 +-
 include/linux/shrinker.h | 2 +-
 mm/vmalloc.c             | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
index fe739d35a864..c79bccb7dafa 100644
--- a/include/linux/list_lru.h
+++ b/include/linux/list_lru.h
@@ -37,7 +37,7 @@ struct list_lru_one {
 };
 
 struct list_lru_memcg {
-	struct rcu_head		rcu;
+	struct rcu_ptr		rcu;
 	/* array of per cgroup per node lists, indexed by node id */
 	struct list_lru_one	node[];
 };
diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 1a00be90d93a..bad20de2803a 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -19,7 +19,7 @@ struct shrinker_info_unit {
 };
 
 struct shrinker_info {
-	struct rcu_head rcu;
+	struct rcu_ptr rcu;
 	int map_nr_max;
 	struct shrinker_info_unit *unit[];
 };
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 41dd01e8430c..89c781dcab58 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2596,7 +2596,7 @@ struct vmap_block {
 	DECLARE_BITMAP(used_map, VMAP_BBMAP_BITS);
 	unsigned long dirty_min, dirty_max; /*< dirty range */
 	struct list_head free_list;
-	struct rcu_head rcu_head;
+	struct rcu_ptr rcu;
 	struct list_head purge;
 	unsigned int cpu;
 };
@@ -2765,7 +2765,7 @@ static void free_vmap_block(struct vmap_block *vb)
 	spin_unlock(&vn->busy.lock);
 
 	free_vmap_area_noflush(vb->va);
-	kfree_rcu(vb, rcu_head);
+	kfree_rcu(vb, rcu);
 }
 
 static bool purge_fragmented_block(struct vmap_block *vb,
-- 
2.43.0