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>

1 files changed, 46 insertions, 33 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 614d0d9c0deb..2bdac3ececd0 100644
--- 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)