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Tue, 23 Mar 2021 02:18:44 -0700 (PDT) MIME-Version: 1.0 References: <20210319163821.20704-1-songmuchun@bytedance.com> <20210319163821.20704-2-songmuchun@bytedance.com> In-Reply-To: From: Muchun Song Date: Tue, 23 Mar 2021 17:18:08 +0800 Message-ID: Subject: Re: [External] Re: [PATCH v5 1/7] mm: memcontrol: slab: fix obtain a reference to a freeing memcg To: Johannes Weiner Cc: Roman Gushchin , Michal Hocko , Andrew Morton , Shakeel Butt , Vladimir Davydov , LKML , Linux Memory Management List , Xiongchun duan Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: A9DC6E0011E1 X-Stat-Signature: oj7rchrcu5azdzoukntb7mndzckb4k91 Received-SPF: none (bytedance.com>: No applicable sender policy available) receiver=imf21; identity=mailfrom; envelope-from=""; helo=mail-pf1-f181.google.com; client-ip=209.85.210.181 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616491125-82664 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Mar 22, 2021 at 10:46 PM Johannes Weiner wrote: > > 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 > > Acked-by: Johannes Weiner > > Good catch! Did you trigger the WARN_ON() in > percpu_ref_kill_and_confirm() during testing? No. The race window is very small, it should be difficult to trigger. When I reviewed the code here, I suddenly realized that there might be a problem here. Very coincidental. Thanks.