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=-13.7 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,URIBL_BLOCKED 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 A5CC8C433DB for ; Fri, 5 Feb 2021 09:15:11 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 1A66B64F38 for ; Fri, 5 Feb 2021 09:15:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1A66B64F38 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 86EFA6B0005; Fri, 5 Feb 2021 04:15:10 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 81E116B006C; Fri, 5 Feb 2021 04:15:10 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 735266B0075; Fri, 5 Feb 2021 04:15:10 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0159.hostedemail.com [216.40.44.159]) by kanga.kvack.org (Postfix) with ESMTP id 5E6976B0005 for ; Fri, 5 Feb 2021 04:15:10 -0500 (EST) Received: from smtpin12.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 1FA35181AEF1E for ; Fri, 5 Feb 2021 09:15:10 +0000 (UTC) X-FDA: 77783655180.12.dust15_1c0c9ab275e3 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin12.hostedemail.com (Postfix) with ESMTP id 0243918026221 for ; Fri, 5 Feb 2021 09:15:09 +0000 (UTC) X-HE-Tag: dust15_1c0c9ab275e3 X-Filterd-Recvd-Size: 5170 Received: from mail-pj1-f48.google.com (mail-pj1-f48.google.com [209.85.216.48]) by imf36.hostedemail.com (Postfix) with ESMTP for ; Fri, 5 Feb 2021 09:15:09 +0000 (UTC) Received: by mail-pj1-f48.google.com with SMTP id q72so3283473pjq.2 for ; Fri, 05 Feb 2021 01:15:08 -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=53pDsjk4dsfRxbD/kmYDiUVFcJx5hI2GOqEILwRUysY=; b=lI6xNbyP7s/yr16Sg6aC5FlliprvUbrxjSsyUrxMaPEP1KqYYQyRHtd9K0UqINg26g QmTgj8Spi3UKo86zeJGV83yXawxMbYM9CVFSrujTaGLXq/82Y0Ayv6njyfjK2fGS486s aK97+eddmJcU79YSI2w/3NYY8Cz5dFwUkAM0cps0WLK3XdhrzxC9i+UYWwr1A8prvV8X RP1T+YqbYhVSnYMEpWju7n9oT1AD5a3+cjb6SGUlmT1rX1pqmxW2MlNQEMuS1c1aXhPC acMS102YFmfhPAkrdrIGjnj//Ku6/enRZpVxSIW/x5r9lGBvP3E6a4a3x/IPrwui2Jbe D88w== 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=53pDsjk4dsfRxbD/kmYDiUVFcJx5hI2GOqEILwRUysY=; b=B8gQYC7p1BkSg0RAQ9xurFzkENW70e8r6+yHjEXGoopuQTLNFnB2U07Ix6m59i1eVU oDyNyLL3gR5l7I8StQ6O1DBK55vgr8QLC5zmlyBNiRldt7qL6NUZsmrH5lqWtii21CsV CGgtsnGX+Pt0JmETnGMFZ1wal44LD2iCybt0ffTa4+uYq0mh1gF0bb/xnvC2pTXaCqGm uSnofn+axL8m0veTUG82WKoIeuXvFlIzD78rVx0jWTYA+xkEXtrbJswPUSybQwva/QuC 1DVs+u4opyFryg+OM9mi4+RYhx+h9ivD1RNkDrijhktGtMO30FRoHlNlvDtOB226S7lE AwRg== X-Gm-Message-State: AOAM531JeC2YkyTYpiBIDJ918p/o3FE8IEZsz8wWN61rnyo+dws54lKu /lUOWdjrR8SpRB6xj8/rUlyIj1CPD/VwosT2oAOVxQ== X-Google-Smtp-Source: ABdhPJxMGYIycGAi3Uj9lQkUIgQRunnV305CBd9rmYFIKTOmtfjshvUBHb8MsZSHh3bzL9Wh6v8UWUN4E/B3LOdY1Co= X-Received: by 2002:a17:902:b7c7:b029:e1:6cbe:db19 with SMTP id v7-20020a170902b7c7b02900e16cbedb19mr3444327plz.20.1612516507874; Fri, 05 Feb 2021 01:15:07 -0800 (PST) MIME-Version: 1.0 References: <20210205062719.74431-1-songmuchun@bytedance.com> In-Reply-To: From: Muchun Song Date: Fri, 5 Feb 2021 17:14:30 +0800 Message-ID: Subject: Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page To: Michal Hocko Cc: Johannes Weiner , Vladimir Davydov , Andrew Morton , Cgroups , Linux Memory Management List , LKML Content-Type: text/plain; charset="UTF-8" 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 Fri, Feb 5, 2021 at 4:36 PM Michal Hocko wrote: > > On Fri 05-02-21 14:27:19, Muchun Song wrote: > > The get_mem_cgroup_from_page() is called under page lock, so the page > > memcg cannot be changed under us. > > Where is the page lock enforced? Because it is called from alloc_page_buffers(). This path is under page lock. > > > Also, css_get is enough because page > > has a reference to the memcg. > > tryget used to be there to guard against offlined memcg but we have > concluded this is impossible in this path. tryget stayed there to catch > some unexpected cases IIRC. Yeah, it can catch some unexpected cases. But why is this path special so that we need a tryget? > > > If we really want to make the get_mem_cgroup_from_page() suitable for > > arbitrary page, we should use page_memcg_rcu() instead of page_memcg() > > and call it after rcu_read_lock(). > > What is the primary motivation to change this code? is the overhead of > tryget/RCU something that needs optimizing? Actually, the rcu_read_lock() is not necessary here. So it is better to remove it (indeed reduce some code). Thanks. > > > Signed-off-by: Muchun Song > > --- > > mm/memcontrol.c | 9 ++++----- > > 1 file changed, 4 insertions(+), 5 deletions(-) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index 87f01bc05d1f..6c7f1ea3955e 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -1063,16 +1063,15 @@ EXPORT_SYMBOL(get_mem_cgroup_from_mm); > > */ > > struct mem_cgroup *get_mem_cgroup_from_page(struct page *page) > > { > > - struct mem_cgroup *memcg = page_memcg(page); > > + struct mem_cgroup *memcg; > > > > if (mem_cgroup_disabled()) > > return NULL; > > > > - rcu_read_lock(); > > /* Page should not get uncharged and freed memcg under us. */ > > - if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css))) > > - memcg = root_mem_cgroup; > > - rcu_read_unlock(); > > + memcg = page_memcg(page) ? : root_mem_cgroup; > > + css_get(&memcg->css); > > + > > return memcg; > > } > > EXPORT_SYMBOL(get_mem_cgroup_from_page); > > -- > > 2.11.0 > > -- > Michal Hocko > SUSE Labs