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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3324AC433EF for ; Mon, 20 Jun 2022 12:23:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240716AbiFTMXq (ORCPT ); Mon, 20 Jun 2022 08:23:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40954 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237517AbiFTMXn (ORCPT ); Mon, 20 Jun 2022 08:23:43 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 36E4C17594 for ; Mon, 20 Jun 2022 05:23:42 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.55]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4LRTHj6XgkzSgtk; Mon, 20 Jun 2022 20:20:17 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Mon, 20 Jun 2022 20:23:33 +0800 Subject: Re: [PATCH v2 2/3] mm/swapfile: fix possible data races of inuse_pages To: Muchun Song , "Huang, Ying" , CC: , , , References: <20220608144031.829-1-linmiaohe@huawei.com> <20220608144031.829-3-linmiaohe@huawei.com> <87edzjrcq8.fsf@yhuang6-desk2.ccr.corp.intel.com> <13414d6a-9e72-fb6c-f0a8-8b83ba0455de@huawei.com> From: Miaohe Lin Message-ID: <09ffac27-7fe9-0977-cb33-30433e78e662@huawei.com> Date: Mon, 20 Jun 2022 20:23:33 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/6/20 17:23, Muchun Song wrote: > On Mon, Jun 20, 2022 at 05:04:50PM +0800, Miaohe Lin wrote: >> On 2022/6/20 15:54, Huang, Ying wrote: >>> Miaohe Lin writes: >>> >>>> si->inuse_pages could still be accessed concurrently now. The plain reads >>>> outside si->lock critical section, i.e. swap_show and si_swapinfo, which >>>> results in data races. But these should be ok because they're just used >>>> for showing swap info. >>>> >>>> Signed-off-by: Miaohe Lin >>>> Reviewed-by: David Hildenbrand >>>> --- >>>> mm/swapfile.c | 4 ++-- >>>> 1 file changed, 2 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/mm/swapfile.c b/mm/swapfile.c >>>> index d2bead7b8b70..3fa26f6971e9 100644 >>>> --- a/mm/swapfile.c >>>> +++ b/mm/swapfile.c >>>> @@ -2646,7 +2646,7 @@ static int swap_show(struct seq_file *swap, void *v) >>>> } >>>> >>>> bytes = si->pages << (PAGE_SHIFT - 10); >>>> - inuse = si->inuse_pages << (PAGE_SHIFT - 10); >>>> + inuse = READ_ONCE(si->inuse_pages) << (PAGE_SHIFT - 10); >>>> >>>> file = si->swap_file; >>>> len = seq_file_path(swap, file, " \t\n\\"); >>>> @@ -3265,7 +3265,7 @@ void si_swapinfo(struct sysinfo *val) >>>> struct swap_info_struct *si = swap_info[type]; >>>> >>>> if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK)) >>>> - nr_to_be_unused += si->inuse_pages; >>>> + nr_to_be_unused += READ_ONCE(si->inuse_pages); >>>> } >>>> val->freeswap = atomic_long_read(&nr_swap_pages) + nr_to_be_unused; >>>> val->totalswap = total_swap_pages + nr_to_be_unused; >>> >>> READ_ONCE() should be paired with WRITE_ONCE(). So, change the writer >>> side too? >> >> READ_ONCE() is used to fix the complaint of concurrent accessing to si->inuse_pages from KCSAN here. >> The similar commit is 218209487c3d ("mm/swapfile: fix data races in try_to_unuse()"). IMHO, it's fine > > I think the fix 218209487c3d is incomplete. The write side in swap_range_free() should > also be fixed. Otherwise, IIUC, it cannot stop KCSAN complaining. I tend to agree with you. READ_ONCE() should be paired with WRITE_ONCE() theoretically. But WRITTE_ONCE() is ignored while the commit is introduced. Add Qian Cai for helping verify it. It's very kind of @Qian Cai if he could tell us whether WRITTE_ONCE() is ignored deliberately. Thanks all of you. :) > >> to see a not-uptodate value of si->inuse_pages because it's just used for showing swap info. So >> WRITE_ONCE() is not obligatory. Or am I miss something? >> >>> >>> Best Regards, >>> Huang, Ying >> >> Thanks! >> >>> . >>> >> >> > . >