memcg: free mem_cgroup by RCU to fix oops

After fixing the GPF in mem_cgroup_lru_del_list(), three times one
machine running a similar load (moving and removing memcgs while
swapping) has oopsed in mem_cgroup_zone_nr_lru_pages(), when retrieving
memcg zone numbers for get_scan_count() for shrink_mem_cgroup_zone():
this is where a struct mem_cgroup is first accessed after being chosen
by mem_cgroup_iter().

Just what protects a struct mem_cgroup from being freed, in between
mem_cgroup_iter()'s css_get_next() and its css_tryget()? css_tryget()
fails once css->refcnt is zero with CSS_REMOVED set in flags, yes: but
what if that memory is freed and reused for something else, which sets
"refcnt" non-zero? Hmm, and scope for an indefinite freeze if refcnt is
left at zero but flags are cleared.

It's tempting to move the css_tryget() into css_get_next(), to make it
really "get" the css, but I don't think that actually solves anything:
the same difficulty in moving from css_id found to stable css remains.

But we already have rcu_read_lock() around the two, so it's easily fixed
if __mem_cgroup_free() just uses kfree_rcu() to free mem_cgroup.

However, a big struct mem_cgroup is allocated with vzalloc() instead of
kzalloc(), and we're not allowed to vfree() at interrupt time: there
doesn't appear to be a general vfree_rcu() to help with this, so roll
our own using schedule_work().  The compiler decently removes
vfree_work() and vfree_rcu() when the config doesn't need them.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d0e57a3cda1899c156274d53e49e11be42957f87..58a08fc7414aaf4de59b57a924994dfa43452468 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -230,10 +230,30 @@ struct mem_cgroup {
         * the counter to account for memory usage
         */
        struct res_counter res;
-       /*
-        * the counter to account for mem+swap usage.
-        */
-       struct res_counter memsw;
+
+       union {
+               /*
+                * the counter to account for mem+swap usage.
+                */
+               struct res_counter memsw;
+
+               /*
+                * rcu_freeing is used only when freeing struct mem_cgroup,
+                * so put it into a union to avoid wasting more memory.
+                * It must be disjoint from the css field.  It could be
+                * in a union with the res field, but res plays a much
+                * larger part in mem_cgroup life than memsw, and might
+                * be of interest, even at time of free, when debugging.
+                * So share rcu_head with the less interesting memsw.
+                */
+               struct rcu_head rcu_freeing;
+               /*
+                * But when using vfree(), that cannot be done at
+                * interrupt time, so we must then queue the work.
+                */
+               struct work_struct work_freeing;
+       };
+
        /*
         * Per cgroup active and inactive list, similar to the
         * per zone LRU lists.
@@ -4779,6 +4799,27 @@ out_free:
        return NULL;
 }
 
+/*
+ * Helpers for freeing a vzalloc()ed mem_cgroup by RCU,
+ * but in process context.  The work_freeing structure is overlaid
+ * on the rcu_freeing structure, which itself is overlaid on memsw.
+ */
+static void vfree_work(struct work_struct *work)
+{
+       struct mem_cgroup *memcg;
+
+       memcg = container_of(work, struct mem_cgroup, work_freeing);
+       vfree(memcg);
+}
+static void vfree_rcu(struct rcu_head *rcu_head)
+{
+       struct mem_cgroup *memcg;
+
+       memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
+       INIT_WORK(&memcg->work_freeing, vfree_work);
+       schedule_work(&memcg->work_freeing);
+}
+
 /*
  * At destroying mem_cgroup, references from swap_cgroup can remain.
  * (scanning all at force_empty is too costly...)
@@ -4802,9 +4843,9 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 
        free_percpu(memcg->stat);
        if (sizeof(struct mem_cgroup) < PAGE_SIZE)
-               kfree(memcg);
+               kfree_rcu(memcg, rcu_freeing);
        else
-               vfree(memcg);
+               call_rcu(&memcg->rcu_freeing, vfree_rcu);
 }
 
 static void mem_cgroup_get(struct mem_cgroup *memcg)