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=-13.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,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 223D6C433E0 for ; Mon, 15 Feb 2021 10:10:24 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 72D6C64DF0 for ; Mon, 15 Feb 2021 10:10:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 72D6C64DF0 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=bytedance.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id CCD1A8D00E9; Mon, 15 Feb 2021 05:10:22 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id C7E418D00E6; Mon, 15 Feb 2021 05:10:22 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B942F8D00E9; Mon, 15 Feb 2021 05:10:22 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0096.hostedemail.com [216.40.44.96]) by kanga.kvack.org (Postfix) with ESMTP id A33EA8D00E6 for ; Mon, 15 Feb 2021 05:10:22 -0500 (EST) Received: from smtpin11.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 6FCC13633 for ; Mon, 15 Feb 2021 10:10:22 +0000 (UTC) X-FDA: 77820082284.11.75AA66C Received: from mail-pj1-f43.google.com (mail-pj1-f43.google.com [209.85.216.43]) by imf15.hostedemail.com (Postfix) with ESMTP id 2A0F6A0009CD for ; Mon, 15 Feb 2021 10:10:21 +0000 (UTC) Received: by mail-pj1-f43.google.com with SMTP id d2so3609158pjs.4 for ; Mon, 15 Feb 2021 02:10:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=uUkfBcazTUCSySsbiIpdf7O4sDkmamjhiSz7LSXU/q4=; b=H1TVCYlZbW83imFghMNQrgZVHaopx3G7upqnpaCf3o+DIiA7AB2ZPsVxte6UcaFTcX trlyNHKP2lqVPJhwEVc3NJADj9CHjpL1Xsch5BrecHXGTg8DGaWMACtBHW/D5d1h/ab3 hwkpAA34c7CyoHgsja4N8BYXtgbS09kr+Nc8tDkomKLEaxh72pl/5HKUbwf50m7wg6Ha sjnAGUX5OPvd86woy4ls62/I9TR7JnfTFAjB1ODQ1RlG8YFW6I6rSMiumlVg+E9FXuIe 7aVNrDtbVmpMzOQIQ6JMr6tCP/1m4VOC0eP9+AK/GzDTOHk52YNLBFYCEmXR4L5cFGoh rTlA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=uUkfBcazTUCSySsbiIpdf7O4sDkmamjhiSz7LSXU/q4=; b=Lh8u3M2WFyDplQNUIIc1jbo+gT1Yx8q0dsnEPZ2k190sgI8r4JLl+pFMzNQ05WNkk5 D6U9oZ6lJcyxlt5xnC8ri0rxskrwXgtb4cBTMWAyXlfgQSz0G3ESO9MmqAp7GtP82qYr jrDWMojtWZt9JoONnNS40w5rCQ1i5HAIFpZyCQtwTmhnLHsutW8mz7PnJxbjklb7R7zV QOV0CPDzXVs+wCeRpPyhhQXZOwGEdDHdeUaXw3uJevZH772W0cwEri650+W5ocMvHRe1 s5I0HCfJM0GNPYM5HfM1G84xTLigoJHNiyzcrjtjgnCtN8RvaNbMJOWsO+wMa162TDaQ 69og== X-Gm-Message-State: AOAM532RPjvby86tPAxcJtn9THRWi89qql5M9E6nG+FvwpZEASt2XmP5 E2j0PsbBvGNkZnKPczT+EAyew8odEjkAQ+6YOBIZrg== X-Google-Smtp-Source: ABdhPJzmO6TWU1MkQeMZEYcgMqkOlI9BRJZd2Fv5/20y4X9tFe6U500m6dr22NKX22P40SGqOg/5qp2H0O2Uixeox+I= X-Received: by 2002:a17:902:9341:b029:e1:7b4e:57a8 with SMTP id g1-20020a1709029341b02900e17b4e57a8mr14556708plp.34.1613383820620; Mon, 15 Feb 2021 02:10:20 -0800 (PST) MIME-Version: 1.0 References: <20210212170159.32153-1-songmuchun@bytedance.com> <20210212170159.32153-3-songmuchun@bytedance.com> In-Reply-To: From: Muchun Song Date: Mon, 15 Feb 2021 18:09:44 +0800 Message-ID: Subject: Re: [External] Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero To: Michal Hocko Cc: Johannes Weiner , Vladimir Davydov , Andrew Morton , Cgroups , Linux Memory Management List , LKML Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 2A0F6A0009CD X-Stat-Signature: 8tr8znixhx71pjpzr7dz8tdsdxuanket Received-SPF: none (bytedance.com>: No applicable sender policy available) receiver=imf15; identity=mailfrom; envelope-from=""; helo=mail-pj1-f43.google.com; client-ip=209.85.216.43 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1613383821-12752 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 Mon, Feb 15, 2021 at 5:39 PM Michal Hocko wrote: > > 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? Yeah, this indeed can happen. If we allocate a new swap cache page and charge it via mem_cgroup_charge, then the page will uncharge the swap counter via mem_cgroup_uncharge_swap. When the swap entry is indeed freed, we will call mem_cgroup_uncharge_swap again, In this routine, we can pass zero to mem_cgroup_from_id. Right? > > > > > 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