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,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 409A2C433ED for ; Wed, 21 Apr 2021 07:34:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0807D61438 for ; Wed, 21 Apr 2021 07:34:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236735AbhDUHe6 (ORCPT ); Wed, 21 Apr 2021 03:34:58 -0400 Received: from mx2.suse.de ([195.135.220.15]:54786 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234038AbhDUHe5 (ORCPT ); Wed, 21 Apr 2021 03:34:57 -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=1618990464; 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=Zly4Y5bY/7sVzFR1D7MJtPS/ysK81xWuNjbRZ79r6+w=; b=NXomZfSNfkrXR5oqhS+ficumz14UX3ZMVY54MUXmSxWVkw6HzkfjnsMhQPr4C6xqrT3fGM bYw5SZeMzJlFCSGqZaI91Z7nOz3ZrJ+/lt5IJMzh0Ud1pzwWKefbhpHU++PB+AaiPdbike bGaEDNgZ8rWiokCMH3sEgOZf1/2VNOg= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id F2BB4B127; Wed, 21 Apr 2021 07:34:23 +0000 (UTC) Date: Wed, 21 Apr 2021 09:34:23 +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: [PATCH] mm: memcontrol: fix root_mem_cgroup charging Message-ID: References: <20210421062644.68331-1-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210421062644.68331-1-songmuchun@bytedance.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 21-04-21 14:26:44, Muchun Song wrote: > The below scenario can cause the page counters of the root_mem_cgroup > to be out of balance. > > CPU0: CPU1: > > objcg = get_obj_cgroup_from_current() > obj_cgroup_charge_pages(objcg) > memcg_reparent_objcgs() > // reparent to root_mem_cgroup > WRITE_ONCE(iter->memcg, parent) > // memcg == root_mem_cgroup > memcg = get_mem_cgroup_from_objcg(objcg) > // do not charge to the root_mem_cgroup > try_charge(memcg) > > obj_cgroup_uncharge_pages(objcg) > memcg = get_mem_cgroup_from_objcg(objcg) > // uncharge from the root_mem_cgroup > page_counter_uncharge(&memcg->memory) > > This can cause the page counter to be less than the actual value, > Although we do not display the value (mem_cgroup_usage) so there > shouldn't be any actual problem, but there is a WARN_ON_ONCE in > the page_counter_cancel(). Who knows if it will trigger? So it > is better to fix it. The changelog doesn't explain the fix and why you have chosen to charge kmem objects to root memcg and left all other try_charge users intact. The reason is likely that those are not reparented now but that just adds an inconsistency. Is there any reason you haven't simply matched obj_cgroup_uncharge_pages to check for the root memcg and bail out early? > Signed-off-by: Muchun Song > --- > mm/memcontrol.c | 17 ++++++++++++----- > 1 file changed, 12 insertions(+), 5 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 1e68a9992b01..81b54bd9b9e0 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2686,8 +2686,8 @@ void mem_cgroup_handle_over_high(void) > css_put(&memcg->css); > } > > -static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask, > - unsigned int nr_pages) > +static int __try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask, > + unsigned int nr_pages) > { > unsigned int batch = max(MEMCG_CHARGE_BATCH, nr_pages); > int nr_retries = MAX_RECLAIM_RETRIES; > @@ -2699,8 +2699,6 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask, > bool drained = false; > unsigned long pflags; > > - if (mem_cgroup_is_root(memcg)) > - return 0; > retry: > if (consume_stock(memcg, nr_pages)) > return 0; > @@ -2880,6 +2878,15 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask, > return 0; > } > > +static inline int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask, > + unsigned int nr_pages) > +{ > + if (mem_cgroup_is_root(memcg)) > + return 0; > + > + return __try_charge(memcg, gfp_mask, nr_pages); > +} > + > #if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MMU) > static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages) > { > @@ -3125,7 +3132,7 @@ static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp, > > memcg = get_mem_cgroup_from_objcg(objcg); > > - ret = try_charge(memcg, gfp, nr_pages); > + ret = __try_charge(memcg, gfp, nr_pages); > if (ret) > goto out; > > -- > 2.11.0 -- Michal Hocko SUSE Labs