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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 571DDC433ED for ; Wed, 14 Apr 2021 09:24:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 33501613C8 for ; Wed, 14 Apr 2021 09:24:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350413AbhDNJZP (ORCPT ); Wed, 14 Apr 2021 05:25:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:35028 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1350373AbhDNJY5 (ORCPT ); Wed, 14 Apr 2021 05:24:57 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1618392275; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=eU2ARRKP2HKvM+HAl1bRPX/7KV3rG7+C5uUtQ33atXM=; b=IGgGnIQtjcrIzFVumv1YMozZ6hq3GY17VTmUjGWyzKv4uT67rU/hjhLEcBTyrE9Zhyx0jR kEPfF1i9Euyvldus/fo4V4iS0STdYp/aVRUQgQ8AM+AaUOwUjyb+wz6d3TQ33Js2M28Q82 g3v7PveyPuDVZ9u3lhscBZJ21+IPvwA= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 8BC63ACC4; Wed, 14 Apr 2021 09:24:35 +0000 (UTC) Date: Wed, 14 Apr 2021 11:24:34 +0200 From: Michal Hocko To: Muchun Song Cc: guro@fb.com, hannes@cmpxchg.org, akpm@linux-foundation.org, shakeelb@google.com, vdavydov.dev@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, duanxiongchun@bytedance.com, fam.zheng@bytedance.com Subject: Re: [PATCH 2/7] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm Message-ID: References: <20210413065153.63431-1-songmuchun@bytedance.com> <20210413065153.63431-3-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210413065153.63431-3-songmuchun@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 13-04-21 14:51:48, Muchun Song wrote: > When mm is NULL, we do not need to hold rcu lock and call css_tryget for > the root memcg. And we also do not need to check !mm in every loop of > while. So bail out early when !mm. mem_cgroup_charge and other callers unconditionally drop the reference so how come this does not underflow reference count? > Signed-off-by: Muchun Song > Acked-by: Johannes Weiner > Reviewed-by: Shakeel Butt > --- > mm/memcontrol.c | 21 ++++++++++----------- > 1 file changed, 10 insertions(+), 11 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index f229de925aa5..9cbfff59b171 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -901,20 +901,19 @@ 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 etc. > + */ > + if (unlikely(!mm)) > + return root_mem_cgroup; > + > rcu_read_lock(); > do { > - /* > - * Page cache insertions can happen without an > - * actual mm context, e.g. during disk probing > - * on boot, loopback IO, acct() writes etc. > - */ > - if (unlikely(!mm)) > + memcg = mem_cgroup_from_task(rcu_dereference(mm->owner)); > + if (unlikely(!memcg)) > memcg = root_mem_cgroup; > - else { > - memcg = mem_cgroup_from_task(rcu_dereference(mm->owner)); > - if (unlikely(!memcg)) > - memcg = root_mem_cgroup; > - } > } while (!css_tryget(&memcg->css)); > rcu_read_unlock(); > return memcg; > -- > 2.11.0 -- Michal Hocko SUSE Labs