[PATCH v5 1/7] mm: memcontrol: slab: fix obtain a reference to a freeing memcg

[Muchun Song]

The rcu_read_lock/unlock only can guarantee that the memcg will not be
freed, but it cannot guarantee the success of css_get (which is in the
refill_stock when cached memcg changed) to memcg.

  rcu_read_lock()
  memcg = obj_cgroup_memcg(old)
  __memcg_kmem_uncharge(memcg)
      refill_stock(memcg)
          if (stock->cached != memcg)
              // css_get can change the ref counter from 0 back to 1.
              css_get(&memcg->css)
  rcu_read_unlock()

This fix is very like the commit:

  eefbfa7fd678 ("mm: memcg/slab: fix use after free in obj_cgroup_charge")

Fix this by holding a reference to the memcg which is passed to the
__memcg_kmem_uncharge() before calling __memcg_kmem_uncharge().

Fixes: 3de7d4f25a74 ("mm: memcg/slab: optimize objcg stock draining")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 845eec01ef9d..2cda76ff0629 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3181,9 +3181,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
 		if (nr_pages) {
+			struct mem_cgroup *memcg;
+
 			rcu_read_lock();
-			__memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
+retry:
+			memcg = obj_cgroup_memcg(old);
+			if (unlikely(!css_tryget(&memcg->css)))
+				goto retry;
 			rcu_read_unlock();
+
+			__memcg_kmem_uncharge(memcg, nr_pages);
+			css_put(&memcg->css);
 		}
 
 		/*
-- 
2.11.0