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=ham 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 68A6BC433E0 for ; Mon, 15 Feb 2021 09:39:34 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C1A8064E05 for ; Mon, 15 Feb 2021 09:39:33 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C1A8064E05 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 09C898D0060; Mon, 15 Feb 2021 04:39:33 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 075448D00E6; Mon, 15 Feb 2021 04:39:33 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id ED67A8D0060; Mon, 15 Feb 2021 04:39:32 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0001.hostedemail.com [216.40.44.1]) by kanga.kvack.org (Postfix) with ESMTP id D64878D0060 for ; Mon, 15 Feb 2021 04:39:32 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 9BCE0F029 for ; Mon, 15 Feb 2021 09:39:32 +0000 (UTC) X-FDA: 77820004584.30.idea17_1402ff22763a Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id 81661181923DD for ; Mon, 15 Feb 2021 09:39:32 +0000 (UTC) X-HE-Tag: idea17_1402ff22763a X-Filterd-Recvd-Size: 2470 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf32.hostedemail.com (Postfix) with ESMTP for ; Mon, 15 Feb 2021 09:39:31 +0000 (UTC) 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> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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