On Sat, Mar 20, 2021 at 12:38:14AM +0800, Muchun Song wrote:

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 <redacted>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Good catch! Did you trigger the WARN_ON() in
percpu_ref_kill_and_confirm() during testing?