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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, 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 8AB7EC433B4 for ; Wed, 14 Apr 2021 10:15:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 67C50613B1 for ; Wed, 14 Apr 2021 10:15:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232989AbhDNKQH (ORCPT ); Wed, 14 Apr 2021 06:16:07 -0400 Received: from mx2.suse.de ([195.135.220.15]:57884 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232032AbhDNKQE (ORCPT ); Wed, 14 Apr 2021 06:16:04 -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=1618395342; 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=e6wKF//P88U/M+zDwsZ3xJTgPUSQgjspEx6v7dGglkw=; b=XixLXnku8keACsrXdywnzaLMPxitUWW1MLzpGkLuHry2mV4ONud24MzYd5uB9jGT5g4crA PLg3P0NDpplSNVmjWSGBluS2VEspTZBk8K8hs5ue6yPgvuJJBehN6DpNUPg0MMq1ZDZRd9 WWWPyYawUhyJQ5vKR5Ep41hOxAMG+X0= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 3DA79AF4F; Wed, 14 Apr 2021 10:15:42 +0000 (UTC) Date: Wed, 14 Apr 2021 12:15:40 +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 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. Anyway Acked-by: Michal Hocko Thanks! -- Michal Hocko SUSE Labs