3.10-stable review patch. If anyone has any objections, please let me know.
------------------
From: Michal Hocko <mhocko@xxxxxxx>
commit ecc736fc3c71c411a9d201d8588c9e7e049e5d8c upstream.
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 commit 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.
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
Tested-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.10+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1220,7 +1220,7 @@ struct mem_cgroup *mem_cgroup_iter(struc
if (dead_count == iter->last_dead_count) {
smp_rmb();
last_visited = iter->last_visited;
- if (last_visited &&
+ if (last_visited && last_visited != root &&
!css_tryget(&last_visited->css))
last_visited = NULL;
}
@@ -1229,7 +1229,7 @@ struct mem_cgroup *mem_cgroup_iter(struc
memcg = __mem_cgroup_iter_next(root, last_visited);
if (reclaim) {
- if (last_visited)
+ if (last_visited && last_visited != root)
css_put(&last_visited->css);
iter->last_visited = memcg;
--
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