From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8CACBC433DB for ; Wed, 6 Jan 2021 09:56:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3979723104 for ; Wed, 6 Jan 2021 09:56:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726776AbhAFJzr (ORCPT ); Wed, 6 Jan 2021 04:55:47 -0500 Received: from relay.sw.ru ([185.231.240.75]:48020 "EHLO relay3.sw.ru" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726298AbhAFJzp (ORCPT ); Wed, 6 Jan 2021 04:55:45 -0500 Received: from [192.168.15.143] by relay3.sw.ru with esmtp (Exim 4.94) (envelope-from ) id 1kx5Vr-00FcgL-7c; Wed, 06 Jan 2021 12:53:56 +0300 Subject: Re: [v3 PATCH 03/11] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation To: Yang Shi , guro@fb.com, shakeelb@google.com, david@fromorbit.com, hannes@cmpxchg.org, mhocko@suse.com, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210105225817.1036378-1-shy828301@gmail.com> <20210105225817.1036378-4-shy828301@gmail.com> From: Kirill Tkhai Message-ID: <56d26993-1577-3747-2d89-1275d92f7a15@virtuozzo.com> Date: Wed, 6 Jan 2021 12:54:05 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210105225817.1036378-4-shy828301@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06.01.2021 01:58, Yang Shi wrote: > Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem > exclusively, the read side can be protected by holding read lock, so it sounds > superfluous to have a dedicated mutex. This should not exacerbate the contention > to shrinker_rwsem since just one read side critical section is added. > > Signed-off-by: Yang Shi > --- > mm/vmscan.c | 16 ++++++---------- > 1 file changed, 6 insertions(+), 10 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 9db7b4d6d0ae..ddb9f972f856 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem); > #ifdef CONFIG_MEMCG > > static int memcg_shrinker_map_size; > -static DEFINE_MUTEX(memcg_shrinker_map_mutex); > > static void memcg_free_shrinker_map_rcu(struct rcu_head *head) > { > @@ -200,8 +199,6 @@ static int memcg_expand_one_shrinker_map(struct mem_cgroup *memcg, > struct memcg_shrinker_map *new, *old; > int nid; > > - lockdep_assert_held(&memcg_shrinker_map_mutex); > - > for_each_node(nid) { > old = rcu_dereference_protected( > mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true); > @@ -250,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg) > if (mem_cgroup_is_root(memcg)) > return 0; > > - mutex_lock(&memcg_shrinker_map_mutex); > + down_read(&shrinker_rwsem); > size = memcg_shrinker_map_size; > for_each_node(nid) { > map = kvzalloc(sizeof(*map) + size, GFP_KERNEL); > @@ -261,7 +258,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg) > } > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map); Here we do STORE operation, and since we want the assignment is visible for shrink_slab_memcg() under down_read(), we have to use down_write() in memcg_alloc_shrinker_maps(). > } > - mutex_unlock(&memcg_shrinker_map_mutex); > + up_read(&shrinker_rwsem); > > return ret; > } > @@ -276,9 +273,8 @@ static int memcg_expand_shrinker_maps(int new_id) > if (size <= old_size) > return 0; > > - mutex_lock(&memcg_shrinker_map_mutex); > if (!root_mem_cgroup) > - goto unlock; > + goto out; > > memcg = mem_cgroup_iter(NULL, NULL, NULL); > do { > @@ -287,13 +283,13 @@ static int memcg_expand_shrinker_maps(int new_id) > ret = memcg_expand_one_shrinker_map(memcg, size, old_size); > if (ret) { > mem_cgroup_iter_break(NULL, memcg); > - goto unlock; > + goto out; > } > } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); > -unlock: > +out: > if (!ret) > memcg_shrinker_map_size = size; > - mutex_unlock(&memcg_shrinker_map_mutex); > + > return ret; > } > >