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[202.108.3.162]) by mx.google.com with SMTP id v136si36887519pfc.9.2019.08.01.20.33.45 for ; Thu, 01 Aug 2019 20:33:46 -0700 (PDT) Received-SPF: pass (google.com: domain of hdanton@sina.com designates 202.108.3.162 as permitted sender) client-ip=202.108.3.162; Authentication-Results: mx.google.com; spf=pass (google.com: domain of hdanton@sina.com designates 202.108.3.162 as permitted sender) smtp.mailfrom=hdanton@sina.com Received: from unknown (HELO localhost.localdomain)([124.64.0.239]) by sina.com with ESMTP id 5D43AF16000075DB; Fri, 2 Aug 2019 11:33:44 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com X-SMAIL-MID: 68876130410446 From: Hillf Danton To: Roman Gushchin Cc: Andrew Morton , linux-mm@kvack.org, Michal Hocko , Johannes Weiner , linux-kernel@vger.kernel.org, kernel-team@fb.com, Michal Hocko Subject: Re: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock() Date: Fri, 2 Aug 2019 11:33:33 +0800 Message-Id: <20190801233513.137917-1-guro@fb.com> X-Mailer: git-send-email 2.17.1 X-FB-Internal: Safe MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Archived-At: List-Archive: List-Post: Content-Transfer-Encoding: 8bit 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: Message-ID: <20190802033333.OO_sZQO9dpFnZ1vumT_ij3msqGe1vh_tKY8duC4mJOQ@z> On Thu, 1 Aug 2019 16:35:13 -0700 Roman Gushchin wrote: > > Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_charge") > introduced css_tryget()/css_put() calls in drain_all_stock(), > which are supposed to protect the target memory cgroup from being > released during the mem_cgroup_is_descendant() call. > > However, it's not completely safe. In theory, memcg can go away > between reading stock->cached pointer and calling css_tryget(). Good catch! > > So, let's read the stock->cached pointer and evaluate the memory > cgroup inside a rcu read section, and get rid of > css_tryget()/css_put() calls. You need to either adjust the boundry of the rcu-protected section, or retain the call pairs, as the memcg cache is dereferenced again in drain_stock(). > > Signed-off-by: Roman Gushchin > Cc: Michal Hocko > --- > mm/memcontrol.c | 17 +++++++++-------- > 1 file changed, 9 insertions(+), 8 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 5c7b9facb0eb..d856b64426b7 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2235,21 +2235,22 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) > for_each_online_cpu(cpu) { > struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu); > struct mem_cgroup *memcg; > + bool flush = false; > > + rcu_read_lock(); > memcg = stock->cached; > - if (!memcg || !stock->nr_pages || !css_tryget(&memcg->css)) > - continue; > - if (!mem_cgroup_is_descendant(memcg, root_memcg)) { > - css_put(&memcg->css); > - continue; > - } > - if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) { > + if (memcg && stock->nr_pages && > + mem_cgroup_is_descendant(memcg, root_memcg)) > + flush = true; > + rcu_read_unlock(); > + > + if (flush && > + !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) { > if (cpu == curcpu) > drain_local_stock(&stock->work); > else > schedule_work_on(cpu, &stock->work); > } > - css_put(&memcg->css); > } > put_cpu(); > mutex_unlock(&percpu_charge_mutex); > -- > 2.21.0 >