From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752620Ab2GSJmL (ORCPT ); Thu, 19 Jul 2012 05:42:11 -0400 Received: from e28smtp09.in.ibm.com ([122.248.162.9]:42205 "EHLO e28smtp09.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751822Ab2GSJmJ (ORCPT ); Thu, 19 Jul 2012 05:42:09 -0400 From: "Aneesh Kumar K.V" To: Li Zefan Cc: Andrew Morton , linux-mm@kvack.org, kamezawa.hiroyu@jp.fujitsu.com, mhocko@suse.cz, linux-kernel@vger.kernel.org Subject: Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback In-Reply-To: <5007B034.4030909@huawei.com> References: <1342589649-15066-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <20120718142628.76bf78b3.akpm@linux-foundation.org> <87hat4794l.fsf@skywalker.in.ibm.com> <5007B034.4030909@huawei.com> User-Agent: Notmuch/0.13.2+63~g548a9bf (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu) Date: Thu, 19 Jul 2012 15:11:44 +0530 Message-ID: <87wr20f5pj.fsf@skywalker.in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii x-cbid: 12071909-2674-0000-0000-0000054DB8F5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Li Zefan writes: > on 2012/7/19 10:55, Aneesh Kumar K.V wrote: > >> Andrew Morton writes: >> >>> On Wed, 18 Jul 2012 11:04:09 +0530 >>> "Aneesh Kumar K.V" wrote: >>> >>>> From: "Aneesh Kumar K.V" >>>> >>>> Since we cannot fail in hugetlb_cgroup_move_parent, we don't really >>>> need to check whether cgroup have any change left after that. Also skip >>>> those hstates for which we don't have any charge in this cgroup. >>>> >>>> ... >>>> >>>> + for_each_hstate(h) { >>>> + /* >>>> + * if we don't have any charge, skip this hstate >>>> + */ >>>> + idx = hstate_index(h); >>>> + if (res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE) == 0) >>>> + continue; >>>> + spin_lock(&hugetlb_lock); >>>> + list_for_each_entry(page, &h->hugepage_activelist, lru) >>>> + hugetlb_cgroup_move_parent(idx, cgroup, page); >>>> + spin_unlock(&hugetlb_lock); >>>> + VM_BUG_ON(res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE)); >>>> + } >>>> out: >>>> return ret; >>>> } >>> >>> This looks fishy. >>> >>> We test RES_USAGE before taking hugetlb_lock. What prevents some other >>> thread from increasing RES_USAGE after that test? >>> >>> After walking the list we test RES_USAGE after dropping hugetlb_lock. >>> What prevents another thread from incrementing RES_USAGE before that >>> test, triggering the BUG? >> >> IIUC core cgroup will prevent a new task getting added to the cgroup >> when we are in pre_destroy. Since we already check that the cgroup doesn't >> have any task, the RES_USAGE cannot increase in pre_destroy. >> > > > You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve > the lock after that, so a task can be attached to the cgroup in this interval. > But that means rmdir can be racy right ? What happens if the task got added, allocated few pages and then moved out ? We still would have task count 0 but few pages, which we missed to to move to parent cgroup. -aneesh