memcg: further simplify mem_cgroup_iter

mem_cgroup_iter basically does two things currently.  It takes care of
the house keeping (reference counting, raclaim cookie) and it iterates
through a hierarchy tree (by using cgroup generic tree walk).  The code
would be much more easier to follow if we move the iteration outside of
the function (to __mem_cgrou_iter_next) so the distinction is more
clear.  This patch doesn't introduce any functional changes.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Ying Han <yinghan@google.com>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 614d0d9..2bdac3e 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1073,6 +1073,51 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
        return memcg;
 }
 
+/*
+ * Returns a next (in a pre-order walk) alive memcg (with elevated css
+ * ref. count) or NULL if the whole root's subtree has been visited.
+ *
+ * helper function to be used by mem_cgroup_iter
+ */
+static struct mem_cgroup *__mem_cgroup_iter_next(struct mem_cgroup *root,
+               struct mem_cgroup *last_visited)
+{
+       struct cgroup *prev_cgroup, *next_cgroup;
+
+       /*
+        * Root is not visited by cgroup iterators so it needs an
+        * explicit visit.
+        */
+       if (!last_visited)
+               return root;
+
+       prev_cgroup = (last_visited == root) ? NULL
+               : last_visited->css.cgroup;
+skip_node:
+       next_cgroup = cgroup_next_descendant_pre(
+                       prev_cgroup, root->css.cgroup);
+
+       /*
+        * Even if we found a group we have to make sure it is
+        * alive. css && !memcg means that the groups should be
+        * skipped and we should continue the tree walk.
+        * last_visited css is safe to use because it is
+        * protected by css_get and the tree walk is rcu safe.
+        */
+       if (next_cgroup) {
+               struct mem_cgroup *mem = mem_cgroup_from_cont(
+                               next_cgroup);
+               if (css_tryget(&mem->css))
+                       return mem;
+               else {
+                       prev_cgroup = next_cgroup;
+                       goto skip_node;
+               }
+       }
+
+       return NULL;
+}
+
 /**
  * mem_cgroup_iter - iterate over memory cgroup hierarchy
  * @root: hierarchy root
@@ -1153,39 +1198,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
                        }
                }
 
-               /*
-                * Root is not visited by cgroup iterators so it needs an
-                * explicit visit.
-                */
-               if (!last_visited) {
-                       memcg = root;
-               } else {
-                       struct cgroup *prev_cgroup, *next_cgroup;
-
-                       prev_cgroup = (last_visited == root) ? NULL
-                               : last_visited->css.cgroup;
-skip_node:
-                       next_cgroup = cgroup_next_descendant_pre(
-                                       prev_cgroup, root->css.cgroup);
-
-                       /*
-                        * Even if we found a group we have to make sure it is
-                        * alive. css && !memcg means that the groups should be
-                        * skipped and we should continue the tree walk.
-                        * last_visited css is safe to use because it is
-                        * protected by css_get and the tree walk is rcu safe.
-                        */
-                       if (next_cgroup) {
-                               struct mem_cgroup *mem = mem_cgroup_from_cont(
-                                               next_cgroup);
-                               if (css_tryget(&mem->css))
-                                       memcg = mem;
-                               else {
-                                       prev_cgroup = next_cgroup;
-                                       goto skip_node;
-                               }
-                       }
-               }
+               memcg = __mem_cgroup_iter_next(root, last_visited);
 
                if (reclaim) {
                        if (last_visited)