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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 6CC9DC433B4 for ; Thu, 15 Apr 2021 07:10:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 436A9611CD for ; Thu, 15 Apr 2021 07:10:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231248AbhDOHKY (ORCPT ); Thu, 15 Apr 2021 03:10:24 -0400 Received: from mx2.suse.de ([195.135.220.15]:45180 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230056AbhDOHKX (ORCPT ); Thu, 15 Apr 2021 03:10:23 -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=1618470599; 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=O34oorYRMDyrmgMKgoTmWvClmcyXSz4Fkb1f6zWfq3Q=; b=fvy9hkrg6pvauC7J6sX3c0DpPTulBZKkWEgtvmoT/CWRahIA80V+vk61CToRBkWrSxg+46 YsVzsVRg2zJ4rRPg4zkn/OL3KZVVD3mSH6kbzu+rJFEb32te56AZc1+xIYoZfGtfC0Tqh8 K9NkH9o6wR8ypiYnL2jqIG6il7PqIAY= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B257DB035; Thu, 15 Apr 2021 07:09:59 +0000 (UTC) Date: Thu, 15 Apr 2021 09:09:59 +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: [External] 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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 15-04-21 11:13:16, Muchun Song wrote: > On Wed, Apr 14, 2021 at 6:15 PM Michal Hocko wrote: > > > > On Wed 14-04-21 18:04:35, Muchun Song wrote: > > > On Wed, Apr 14, 2021 at 5:24 PM Michal Hocko wrote: > > > > > > > > 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? > > > > > > For the root memcg, the CSS_NO_REF flag is set, so css_get > > > and css_put do not get or put reference. > > > > Ohh, right you are. I must have forgot about that special case. I am > > pretty sure I (and likely few more) will stumble over that in the future > > again. A small comment explaining that the reference can be safely > > ignore would be helpful. > > OK. Will do. I would go with the following if that helps /* * No need to css_get on root memcg as the reference counting is * disabled on the root level in the cgroup core. See CSS_NO_REF */ Thanks -- Michal Hocko SUSE Labs