[PATCH v2] mm: memcontrol: avoid unused function warning

[Arnd Bergmann]

A bugfix in v4.8-rc2 introduced a harmless warning when CONFIG_MEMCG_SWAP
is disabled but CONFIG_MEMCG is enabled:

mm/memcontrol.c:4085:27: error: 'mem_cgroup_id_get_online' defined but not used [-Werror=unused-function]
 static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)

This moves the function inside of the #ifdef block that hides the
calling function, to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Fixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

This is the alternative to the original patch, as suggested by Michal Hocko.
Andrew, please pick whichever version you like better.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2ff0289ad061..9a6a51a7c416 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4082,24 +4082,6 @@ static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
 	atomic_add(n, &memcg->id.ref);
 }
 
-static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
-{
-	while (!atomic_inc_not_zero(&memcg->id.ref)) {
-		/*
-		 * The root cgroup cannot be destroyed, so it's refcount must
-		 * always be >= 1.
-		 */
-		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
-			VM_BUG_ON(1);
-			break;
-		}
-		memcg = parent_mem_cgroup(memcg);
-		if (!memcg)
-			memcg = root_mem_cgroup;
-	}
-	return memcg;
-}
-
 static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
 {
 	if (atomic_sub_and_test(n, &memcg->id.ref)) {
@@ -5821,6 +5803,24 @@ static int __init mem_cgroup_init(void)
 subsys_initcall(mem_cgroup_init);
 
 #ifdef CONFIG_MEMCG_SWAP
+static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
+{
+	while (!atomic_inc_not_zero(&memcg->id.ref)) {
+		/*
+		 * The root cgroup cannot be destroyed, so it's refcount must
+		 * always be >= 1.
+		 */
+		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
+			VM_BUG_ON(1);
+			break;
+		}
+		memcg = parent_mem_cgroup(memcg);
+		if (!memcg)
+			memcg = root_mem_cgroup;
+	}
+	return memcg;
+}
+
 /**
  * mem_cgroup_swapout - transfer a memsw charge to swap
  * @page: page whose memsw charge to transfer
-- 
2.9.0