mm/memcg: iteration skip memcgs not yet fully initialized

commit d8ad30559715ce97afb7d1a93a12fd90e8fff312 upstream.

It is surprising that the mem_cgroup iterator can return memcgs which
have not yet been fully initialized.  By accident (or trial and error?)
this appears not to present an actual problem; but it may be better to
prevent such surprises, by skipping memcgs not yet online.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e0e9792..b55eb75 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1081,10 +1081,8 @@ skip_node:
         * protected by css_get and the tree walk is rcu safe.
         */
        if (next_css) {
-               struct mem_cgroup *mem = mem_cgroup_from_css(next_css);
-
-               if (css_tryget(&mem->css))
-                       return mem;
+               if ((next_css->flags & CSS_ONLINE) && css_tryget(next_css))
+                       return mem_cgroup_from_css(next_css);
                else {
                        prev_css = next_css;
                        goto skip_node;