This is a note to let you know that I've just added the patch titled
memcg: fix endless loop caused by mem_cgroup_iter
to the 3.13-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
memcg-fix-endless-loop-caused-by-mem_cgroup_iter.patch
and it can be found in the queue-3.13 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From ecc736fc3c71c411a9d201d8588c9e7e049e5d8c Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxx>
Date: Thu, 23 Jan 2014 15:53:35 -0800
Subject: memcg: fix endless loop caused by mem_cgroup_iter
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>
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 | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1139,7 +1139,15 @@ mem_cgroup_iter_load(struct mem_cgroup_r
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;
@@ -1148,9 +1156,11 @@ mem_cgroup_iter_load(struct mem_cgroup_r
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
@@ -1225,7 +1235,8 @@ struct mem_cgroup *mem_cgroup_iter(struc
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++;
Patches currently in stable-queue which might be from mhocko@xxxxxxx are
queue-3.13/mm-page-writeback.c-do-not-count-anon-pages-as-dirtyable-memory.patch
queue-3.13/memcg-fix-css-reference-leak-and-endless-loop-in-mem_cgroup_iter.patch
queue-3.13/memcg-fix-endless-loop-caused-by-mem_cgroup_iter.patch
queue-3.13/mm-memcg-iteration-skip-memcgs-not-yet-fully-initialized.patch
queue-3.13/mm-page-writeback.c-fix-dirty_balance_reserve-subtraction-from-dirtyable-memory.patch
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