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[195.135.220.15]) by mx.google.com with ESMTPS id w23si7328002ejn.141.2019.06.05.05.55.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 05 Jun 2019 05:55:21 -0700 (PDT) Received-SPF: softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A415BAEE6; Wed, 5 Jun 2019 12:55:20 +0000 (UTC) Date: Wed, 5 Jun 2019 14:55:20 +0200 From: Michal Hocko To: Tejun Heo Cc: Johannes Weiner , Vladimir Davydov , cgroups@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com Subject: Re: [PATCH for-5.2-fixes] memcg: Don't loop on css_tryget_online() failure Message-ID: <20190605125520.GF15685@dhcp22.suse.cz> References: <20190529210617.GP374014@devbig004.ftw2.facebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190529210617.GP374014@devbig004.ftw2.facebook.com> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Wed 29-05-19 14:06:17, Tejun Heo wrote: > A PF_EXITING task may stay associated with an offline css. > get_mem_cgroup_from_mm() may deadlock if mm->owner is in such state. > All similar logics in memcg are falling back to root memcg on > tryget_online failure and get_mem_cgroup_from_mm() can do the same. > > A similar failure existed for task_get_css() and could be triggered > through BSD process accounting racing against memcg offlining. See > 18fa84a2db0e ("cgroup: Use css_tryget() instead of css_tryget_online() > in task_get_css()") for details. > > Signed-off-by: Tejun Heo Do we need to mark this patch for stable or this is too unlikely to happen? Acked-by: Michal Hocko Thanks! > --- > mm/memcontrol.c | 24 ++++++++++-------------- > 1 file changed, 10 insertions(+), 14 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index e50a2db5b4ff..be1fa89db198 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -918,23 +918,19 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm) > > if (mem_cgroup_disabled()) > return NULL; > + /* > + * Page cache insertions can happen without an actual mm context, > + * e.g. during disk probing on boot, loopback IO, acct() writes. > + */ > + if (unlikely(!mm)) > + return root_mem_cgroup; > > rcu_read_lock(); > - do { > - /* > - * Page cache insertions can happen withou an > - * actual mm context, e.g. during disk probing > - * on boot, loopback IO, acct() writes etc. > - */ > - if (unlikely(!mm)) > - memcg = root_mem_cgroup; > - else { > - memcg = mem_cgroup_from_task(rcu_dereference(mm->owner)); > - if (unlikely(!memcg)) > - memcg = root_mem_cgroup; > - } > - } while (!css_tryget_online(&memcg->css)); > + memcg = mem_cgroup_from_task(rcu_dereference(mm->owner)); > + if (!css_tryget_online(&memcg->css)) > + memcg = root_mem_cgroup; > rcu_read_unlock(); > + > return memcg; > } > EXPORT_SYMBOL(get_mem_cgroup_from_mm); -- Michal Hocko SUSE Labs