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Mon, 10 May 2021 15:04:02 -0700 (PDT) MIME-Version: 1.0 References: <20210425075410.19255-1-songmuchun@bytedance.com> In-Reply-To: <20210425075410.19255-1-songmuchun@bytedance.com> From: Shakeel Butt Date: Mon, 10 May 2021 15:03:52 -0700 Message-ID: Subject: Re: [PATCH v2] mm: memcontrol: fix root_mem_cgroup charging To: Muchun Song Cc: Roman Gushchin , Johannes Weiner , Michal Hocko , Andrew Morton , Vladimir Davydov , LKML , Linux MM , Xiongchun duan , fam.zheng@bytedance.com, zhengqi.arch@bytedance.com Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=LXVG67xB; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf22.hostedemail.com: domain of shakeelb@google.com designates 209.85.167.43 as permitted sender) smtp.mailfrom=shakeelb@google.com X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: AC2EBC0001EA X-Stat-Signature: ibfzuassyfcc4so7omtrzwmki56b1q9h Received-SPF: none (google.com>: No applicable sender policy available) receiver=imf22; identity=mailfrom; envelope-from=""; helo=mail-lf1-f43.google.com; client-ip=209.85.167.43 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620684236-591266 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 Sun, Apr 25, 2021 at 12:57 AM Muchun Song wrote: > > The below scenario can cause the page counters of the root_mem_cgroup > to be out of balance. > > CPU0: CPU1: > > objcg = get_obj_cgroup_from_current() > obj_cgroup_charge_pages(objcg) > memcg_reparent_objcgs() > // reparent to root_mem_cgroup > WRITE_ONCE(iter->memcg, parent) > // memcg == root_mem_cgroup > memcg = get_mem_cgroup_from_objcg(objcg) > // do not charge to the root_mem_cgroup > try_charge(memcg) > > obj_cgroup_uncharge_pages(objcg) > memcg = get_mem_cgroup_from_objcg(objcg) > // uncharge from the root_mem_cgroup > refill_stock(memcg) > drain_stock(memcg) > page_counter_uncharge(&memcg->memory) > > get_obj_cgroup_from_current() never returns a root_mem_cgroup's objcg, > so we never explicitly charge the root_mem_cgroup. And it's not > going to change. It's all about a race when we got an obj_cgroup > pointing at some non-root memcg, but before we were able to charge it, > the cgroup was gone, objcg was reparented to the root and so we're > skipping the charging. Then we store the objcg pointer and later use > to uncharge the root_mem_cgroup. > > This can cause the page counter to be less than the actual value. > Although we do not display the value (mem_cgroup_usage) so there > shouldn't be any actual problem, but there is a WARN_ON_ONCE in > the page_counter_cancel(). Who knows if it will trigger? So it > is better to fix it. > > Signed-off-by: Muchun Song Reviewed-by: Shakeel Butt