lkml.org 
[lkml]   [2014]   [Jan]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH -mm 1/2] memcg: fix endless loop caused by mem_cgroup_iter
Date
Hugh has reported an endless loop when the hardlimit reclaim sees the
same group all the time. This might happen when the reclaim races with
the memcg removal.

shrink_zone
[rmdir root]
mem_cgroup_iter(root, NULL, reclaim)
// prev = NULL
rcu_read_lock()
mem_cgroup_iter_load
last_visited = iter->last_visited // gets root || NULL
css_tryget(last_visited) // failed
last_visited = NULL [1]
memcg = root = __mem_cgroup_iter_next(root, NULL)
mem_cgroup_iter_update
iter->last_visited = root;
reclaim->generation = iter->generation

mem_cgroup_iter(root, root, reclaim)
// prev = root
rcu_read_lock
mem_cgroup_iter_load
last_visited = iter->last_visited // gets root
css_tryget(last_visited) // failed
[1]

The issue seemed to be introduced by 5f5781619718 (memcg: relax memcg
iter caching) which has replaced unconditional css_get/css_put by
css_tryget/css_put for the cached iterator.

This patch fixes the issue by skipping css_tryget on the root of the
tree walk in mem_cgroup_iter_load and symmetrically doesn't release it
in mem_cgroup_iter_update.

Stable: stable@vger.kernel.org # 3.10+
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
mm/memcontrol.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f016d26adfd3..45786dc129dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1117,7 +1117,15 @@ mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
if (iter->last_dead_count == *sequence) {
smp_rmb();
position = iter->last_visited;
- if (position && !css_tryget(&position->css))
+
+ /*
+ * We cannot take a reference to root because we might race
+ * with root removal and returning NULL would end up in
+ * an endless loop on the iterator user level when root
+ * would be returned all the time.
+ */
+ if (position && position != root &&
+ !css_tryget(&position->css))
position = NULL;
}
return position;
@@ -1126,9 +1134,11 @@ mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter,
struct mem_cgroup *last_visited,
struct mem_cgroup *new_position,
+ struct mem_cgroup *root,
int sequence)
{
- if (last_visited)
+ /* root reference counting symmetric to mem_cgroup_iter_load */
+ if (last_visited && last_visited != root)
css_put(&last_visited->css);
/*
* We store the sequence count from the time @last_visited was
@@ -1203,7 +1213,8 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
memcg = __mem_cgroup_iter_next(root, last_visited);

if (reclaim) {
- mem_cgroup_iter_update(iter, last_visited, memcg, seq);
+ mem_cgroup_iter_update(iter, last_visited, memcg, root,
+ seq);

if (!memcg)
iter->generation++;
--
1.8.5.2


\
 
 \ /
  Last update: 2014-01-21 12:21    [W:0.123 / U:0.136 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site