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

[Johannes Weiner]

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 <songmuchun@xxxxxxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

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