1 files changed, 22 insertions, 10 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index aa8363ef381c..404a13a09efa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -488,6 +488,23 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
         return (memcg == root_mem_cgroup);
 }
 
+static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
+{
+        /*
+         * The ID of the root cgroup is 0, but memcg treat 0 as an
+         * invalid ID, so we return (cgroup_id + 1).
+         */
+        return memcg->css.cgroup->id + 1;
+}
+
+static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
+{
+        struct cgroup_subsys_state *css;
+
+        css = css_from_id(id - 1, &mem_cgroup_subsys);
+        return mem_cgroup_from_css(css);
+}
+
 /* Writing them here to avoid exposing memcg's inner layout */
 #if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
 
@@ -2709,15 +2726,10 @@ static void __mem_cgroup_cancel_local_charge(struct mem_cgroup *memcg,
  */
 static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
 {
-        struct cgroup_subsys_state *css;
-
         /* ID 0 is unused ID */
         if (!id)
                 return NULL;
-        css = css_lookup(&mem_cgroup_subsys, id);
-        if (!css)
-                return NULL;
-        return mem_cgroup_from_css(css);
+        return mem_cgroup_from_id(id);
 }
 
 struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
@@ -4232,7 +4244,7 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
          * css_get() was called in uncharge().
          */
         if (do_swap_account && swapout && memcg)
-                swap_cgroup_record(ent, css_id(&memcg->css));
+                swap_cgroup_record(ent, mem_cgroup_id(memcg));
 }
 #endif
 
@@ -4284,8 +4296,8 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
 {
         unsigned short old_id, new_id;
 
-        old_id = css_id(&from->css);
-        new_id = css_id(&to->css);
+        old_id = mem_cgroup_id(from);
+        new_id = mem_cgroup_id(to);
 
         if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
                 mem_cgroup_swap_statistics(from, false);
@@ -6325,7 +6337,7 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
         }
         /* There is a swap entry and a page doesn't exist or isn't charged */
         if (ent.val && !ret &&
-                        css_id(&mc.from->css) == lookup_swap_cgroup_id(ent)) {
+            mem_cgroup_id(mc.from) == lookup_swap_cgroup_id(ent)) {
                 ret = MC_TARGET_SWAP;
                 if (target)
                         target->ent = ent;