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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 AD4C1C04EBD for ; Tue, 16 Oct 2018 13:19:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7D017208B3 for ; Tue, 16 Oct 2018 13:19:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7D017208B3 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=virtuozzo.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727141AbeJPVKT (ORCPT ); Tue, 16 Oct 2018 17:10:19 -0400 Received: from relay.sw.ru ([185.231.240.75]:40416 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726760AbeJPVKS (ORCPT ); Tue, 16 Oct 2018 17:10:18 -0400 Received: from [172.16.25.169] by relay.sw.ru with esmtp (Exim 4.90_1) (envelope-from ) id 1gCPGA-00018u-7T; Tue, 16 Oct 2018 16:19:42 +0300 Subject: Re: [PATCH] mm: Convert mem_cgroup_id::ref to refcount_t type To: Andrea Parri Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <153910718919.7006.13400779039257185427.stgit@localhost.localdomain> <20181016124939.GA13278@andrea> From: Kirill Tkhai Message-ID: Date: Tue, 16 Oct 2018 16:19:40 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20181016124939.GA13278@andrea> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, Andrea, On 16.10.2018 15:49, Andrea Parri wrote: > Hi Kirill, > > On Tue, Oct 09, 2018 at 08:46:56PM +0300, Kirill Tkhai wrote: >> This will allow to use generic refcount_t interfaces >> to check counters overflow instead of currently existing >> VM_BUG_ON(). The only difference after the patch is >> VM_BUG_ON() may cause BUG(), while refcount_t fires >> with WARN(). > > refcount_{sub_and_test,inc_not_zero}() are documented to provide > "slightly" more relaxed ordering than their atomic_* counterpart, > c.f., > > Documentation/core-api/refcount-vs-atomic.rst > lib/refcount.c (inline comments) > > IIUC, this semantic change won't cause problems here (but please > double-check? ;D ). I just don't see a place, where we may think about using a modification of struct mem_cgroup::id::ref as a memory barrier to order something, and all this looks safe for me. Kirill >> But this seems not to be significant here, >> since such the problems are usually caught by syzbot >> with panic-on-warn enabled. >> >> Signed-off-by: Kirill Tkhai >> --- >> include/linux/memcontrol.h | 2 +- >> mm/memcontrol.c | 10 ++++------ >> 2 files changed, 5 insertions(+), 7 deletions(-) >> >> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h >> index 4399cc3f00e4..7ab2120155a4 100644 >> --- a/include/linux/memcontrol.h >> +++ b/include/linux/memcontrol.h >> @@ -78,7 +78,7 @@ struct mem_cgroup_reclaim_cookie { >> >> struct mem_cgroup_id { >> int id; >> - atomic_t ref; >> + refcount_t ref; >> }; >> >> /* >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >> index 7bebe2ddec05..aa728d5b3d72 100644 >> --- a/mm/memcontrol.c >> +++ b/mm/memcontrol.c >> @@ -4299,14 +4299,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg) >> >> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n) >> { >> - VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0); >> - atomic_add(n, &memcg->id.ref); >> + refcount_add(n, &memcg->id.ref); >> } >> >> static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n) >> { >> - VM_BUG_ON(atomic_read(&memcg->id.ref) < n); >> - if (atomic_sub_and_test(n, &memcg->id.ref)) { >> + if (refcount_sub_and_test(n, &memcg->id.ref)) { >> mem_cgroup_id_remove(memcg); >> >> /* Memcg ID pins CSS */ >> @@ -4523,7 +4521,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css) >> } >> >> /* Online state pins memcg ID, memcg ID pins CSS */ >> - atomic_set(&memcg->id.ref, 1); >> + refcount_set(&memcg->id.ref, 1); >> css_get(css); >> return 0; >> } >> @@ -6357,7 +6355,7 @@ subsys_initcall(mem_cgroup_init); >> #ifdef CONFIG_MEMCG_SWAP >> static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg) >> { >> - while (!atomic_inc_not_zero(&memcg->id.ref)) { >> + while (!refcount_inc_not_zero(&memcg->id.ref)) { >> /* >> * The root cgroup cannot be destroyed, so it's refcount must >> * always be >= 1. >>