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[195.135.220.15]) by mx.google.com with ESMTPS id n21si25596871eju.16.2019.08.05.04.11.38 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 05 Aug 2019 04:11:38 -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 735B0AEF3; Mon, 5 Aug 2019 11:11:38 +0000 (UTC) Date: Mon, 5 Aug 2019 13:11:35 +0200 From: Michal Hocko To: Roman Gushchin Cc: Andrew Morton , linux-mm@kvack.org, Johannes Weiner , linux-kernel@vger.kernel.org, kernel-team@fb.com, Hillf Danton Subject: Re: [PATCH v2] mm: memcontrol: switch to rcu protection in drain_all_stock() Message-ID: <20190805111135.GE7597@dhcp22.suse.cz> References: <20190802192241.3253165-1-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190802192241.3253165-1-guro@fb.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 Fri 02-08-19 12:22:41, 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(). > > This can happen if drain_all_stock() races with drain_local_stock() > performed on the remote cpu as a result of a work, scheduled > by the previous invocation of drain_all_stock(). Maybe I am still missing something but I do not see how 72f0184c8a00 changed the existing race. get_online_cpus doesn't prevent the same race right? If this is the case then it would be great to clarify that. I know that you are mostly after clarifying that css_tryget is insufficient but the above sounds like 72f0184c8a00 has introduced a regression. > The race is a bit theoretical and there are few chances to trigger > it, but the current code looks a bit confusing, so it makes sense > to fix it anyway. The code looks like as if css_tryget() and > css_put() are used to protect stocks drainage. It's not necessary > because stocked pages are holding references to the cached cgroup. > And it obviously won't work for works, scheduled on other cpus. > > 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. > > v2: added some explanations to the commit message, no code changes > > Signed-off-by: Roman Gushchin > Cc: Michal Hocko > Cc: Hillf Danton Other than that. Acked-by: 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 -- Michal Hocko SUSE Labs