memcg: rework mem_cgroup_iter to use cgroup iterators

memcg: rework mem_cgroup_iter to use cgroup iterators

mem_cgroup_iter curently relies on css->id when walking down a group
hierarchy tree.  This is really awkward because the tree walk depends on
the groups creation ordering.  The only guarantee is that a parent node is
visited before its children.

Example:

 1) mkdir -p a a/d a/b/c
 2) mkdir -a a/b/c a/d

Will create the same trees but the tree walks will be different:

 1) a, d, b, c
 2) a, b, c, d

574bd9f7 ("cgroup: implement generic child / descendant walk macros") has
introduced generic cgroup tree walkers which provide either pre-order or
post-order tree walk.  This patch converts css->id based iteration to
pre-order tree walk to keep the semantic with the original iterator where
parent is always visited before its subtree.

cgroup_for_each_descendant_pre suggests using post_create and pre_destroy
for proper synchronization with groups addidition resp.  removal.  This
implementation doesn't use those because a new memory cgroup is
initialized sufficiently for iteration in mem_cgroup_css_alloc already and
css reference counting enforces that the group is alive for both the last
seen cgroup and the found one resp.  it signals that the group is dead and
it should be skipped.

If the reclaim cookie is used we need to store the last visited group into
the iterator so we have to be careful that it doesn't disappear in the
mean time.  Elevated reference count on the css keeps it alive even though
the group have been removed (parked waiting for the last dput so that it
can be freed).

Per node-zone-prio iter_lock has been introduced to ensure that css_tryget
and iter->last_visited is set atomically.  Otherwise two racing walkers
could both take a references and only one release it leading to a css leak
(which pins cgroup dentry).

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
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>