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 E9E5CC433E0 for ; Mon, 15 Feb 2021 09:40:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B0E5564DB1 for ; Mon, 15 Feb 2021 09:40:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230134AbhBOJko (ORCPT ); Mon, 15 Feb 2021 04:40:44 -0500 Received: from mx2.suse.de ([195.135.220.15]:33728 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230100AbhBOJkT (ORCPT ); Mon, 15 Feb 2021 04:40:19 -0500 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=1613381971; 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=AfxkVXqAHAm17EFwPtPwrVPHnFGJsC0+h+W+8Wzcudk=; b=QpHe7YRtB8RNMZWfhyKrDekJEH7y6gOUpQo21orpdf1GlGcYRJBY+zW+QzA+lirhVWw++K yOBrSPMoA3XkI+nhEojgostgEWblMww2qAUgFjp7I/jp588fiPOdtY07CLFsUCYeMLvlfa ggFU0zuPhY/fGiRMc63SIu0fWrZAOKs= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id DC7DFAC32; Mon, 15 Feb 2021 09:39:30 +0000 (UTC) Date: Mon, 15 Feb 2021 10:39:30 +0100 From: Michal Hocko To: Muchun Song Cc: hannes@cmpxchg.org, vdavydov.dev@gmail.com, akpm@linux-foundation.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero Message-ID: References: <20210212170159.32153-1-songmuchun@bytedance.com> <20210212170159.32153-3-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210212170159.32153-3-songmuchun@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 13-02-21 01:01:58, Muchun Song wrote: > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > so idr_find() is pointless and wastes CPU cycles. Is this possible at all to happen? If not why should we add a test for _all_ invocations? > > Signed-off-by: Muchun Song > --- > mm/memcontrol.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index a3f26522765a..68ed4b297c13 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > { > WARN_ON_ONCE(!rcu_read_lock_held()); > + /* The memcg ID cannot be zero. */ > + if (id == 0) > + return NULL; > return idr_find(&mem_cgroup_idr, id); > } > > -- > 2.11.0 -- Michal Hocko SUSE Labs