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Subject[PATCH stable 3.10] mm: memcontrol: factor out reclaim iterator loading and updating
Date
From: Johannes Weiner <hannes@cmpxchg.org>

commit 519ebea3bf6df45439e79c54bda1d9e29fe13a64 upstream

It turned out that this is actually a bug fix which prevents a double
css_put on last_visited memcg which results in kernel BUG at kernel/cgroup.c:893!
See http://lkml.kernel.org/r/20170726130742.5976-1-wenwei.tww@gmail.com
for more details.

mem_cgroup_iter() is too hard to follow. Factor out the lockless reclaim
iterator loading and updating so it's easier to follow the big picture.

Also document the iterator invalidation mechanism a bit more extensively.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[wt: Backported to linux-3.10.y: adjusted context]
Signed-off-by: Wenwei Tao <wenwei.tww@alibaba-inc.com>
---
mm/memcontrol.c | 97 ++++++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 68 insertions(+), 29 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 437ae2c..fcde430 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1149,6 +1149,68 @@ skip_node:
return NULL;
}

+static void mem_cgroup_iter_invalidate(struct mem_cgroup *root)
+{
+ /*
+ * When a group in the hierarchy below root is destroyed, the
+ * hierarchy iterator can no longer be trusted since it might
+ * have pointed to the destroyed group. Invalidate it.
+ */
+ atomic_inc(&root->dead_count);
+}
+
+static struct mem_cgroup *
+mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
+ struct mem_cgroup *root,
+ int *sequence)
+{
+ struct mem_cgroup *position = NULL;
+ /*
+ * A cgroup destruction happens in two stages: offlining and
+ * release. They are separated by a RCU grace period.
+ *
+ * If the iterator is valid, we may still race with an
+ * offlining. The RCU lock ensures the object won't be
+ * released, tryget will fail if we lost the race.
+ */
+ *sequence = atomic_read(&root->dead_count);
+ if (iter->last_dead_count == *sequence) {
+ smp_rmb();
+ position = iter->last_visited;
+
+ /*
+ * 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;
+}
+
+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)
+{
+ /* 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
+ * loaded successfully instead of rereading it here so that we
+ * don't lose destruction events in between. We could have
+ * raced with the destruction of @new_position after all.
+ */
+ iter->last_visited = new_position;
+ smp_wmb();
+ iter->last_dead_count = sequence;
+}
+
/**
* mem_cgroup_iter - iterate over memory cgroup hierarchy
* @root: hierarchy root
@@ -1172,7 +1234,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
{
struct mem_cgroup *memcg = NULL;
struct mem_cgroup *last_visited = NULL;
- unsigned long uninitialized_var(dead_count);

if (mem_cgroup_disabled())
return NULL;
@@ -1192,6 +1253,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
rcu_read_lock();
while (!memcg) {
struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
+ int uninitialized_var(seq);

if (reclaim) {
int nid = zone_to_nid(reclaim->zone);
@@ -1205,37 +1267,14 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
goto out_unlock;
}

- /*
- * If the dead_count mismatches, a destruction
- * has happened or is happening concurrently.
- * If the dead_count matches, a destruction
- * might still happen concurrently, but since
- * we checked under RCU, that destruction
- * won't free the object until we release the
- * RCU reader lock. Thus, the dead_count
- * check verifies the pointer is still valid,
- * css_tryget() verifies the cgroup pointed to
- * is alive.
- */
- dead_count = atomic_read(&root->dead_count);
- if (dead_count == iter->last_dead_count) {
- smp_rmb();
- last_visited = iter->last_visited;
- if (last_visited && last_visited != root &&
- !css_tryget(&last_visited->css))
- last_visited = NULL;
- }
+ last_visited = mem_cgroup_iter_load(iter, root, &seq);
}

memcg = __mem_cgroup_iter_next(root, last_visited);

if (reclaim) {
- if (last_visited && last_visited != root)
- css_put(&last_visited->css);
-
- iter->last_visited = memcg;
- smp_wmb();
- iter->last_dead_count = dead_count;
+ mem_cgroup_iter_update(iter, last_visited, memcg, root,
+ seq);

if (!memcg)
iter->generation++;
@@ -6346,14 +6385,14 @@ static void mem_cgroup_invalidate_reclaim_iterators(struct mem_cgroup *memcg)
struct mem_cgroup *parent = memcg;

while ((parent = parent_mem_cgroup(parent)))
- atomic_inc(&parent->dead_count);
+ mem_cgroup_iter_invalidate(parent);

/*
* if the root memcg is not hierarchical we have to check it
* explicitely.
*/
if (!root_mem_cgroup->use_hierarchy)
- atomic_inc(&root_mem_cgroup->dead_count);
+ mem_cgroup_iter_invalidate(root_mem_cgroup);
}

static void mem_cgroup_css_offline(struct cgroup *cont)
--
1.8.3.1
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