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 DE5BDC433EF for ; Tue, 21 Jun 2022 08:21:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229826AbiFUIVH (ORCPT ); Tue, 21 Jun 2022 04:21:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39722 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348745AbiFUIUt (ORCPT ); Tue, 21 Jun 2022 04:20:49 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 41FC1201A9 for ; Tue, 21 Jun 2022 01:20:48 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4LRztz0GP0zkWL0; Tue, 21 Jun 2022 16:19:07 +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; Tue, 21 Jun 2022 16:20:17 +0800 Subject: Re: [PATCH v2 1/3] mm/swapfile: make security_vm_enough_memory_mm() work as expected To: "Huang, Ying" CC: , , , References: <20220608144031.829-1-linmiaohe@huawei.com> <20220608144031.829-2-linmiaohe@huawei.com> <87r13jrdst.fsf@yhuang6-desk2.ccr.corp.intel.com> <87letqpzm1.fsf@yhuang6-desk2.ccr.corp.intel.com> <463fe0cd-504a-f887-0201-691bacd9e69d@huawei.com> <87pmj2ea3g.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Miaohe Lin Message-ID: Date: Tue, 21 Jun 2022 16:20:17 +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: <87pmj2ea3g.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="windows-1252" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) 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 2022/6/21 15:42, Huang, Ying wrote: > Miaohe Lin writes: > >> On 2022/6/21 9:35, Huang, Ying wrote: >>> Miaohe Lin writes: >>> >>>> On 2022/6/20 15:31, Huang, Ying wrote: >>>>> Miaohe Lin writes: >>>>> >>>>>> security_vm_enough_memory_mm() checks whether a process has enough memory >>>>>> to allocate a new virtual mapping. And total_swap_pages is considered as >>>>>> available memory while swapoff tries to make sure there's enough memory >>>>>> that can hold the swapped out memory. But total_swap_pages contains the >>>>>> swap space that is being swapoff. So security_vm_enough_memory_mm() will >>>>>> success even if there's no memory to hold the swapped out memory because >>>>>> total_swap_pages always greater than or equal to p->pages. >>>>> >>>>> Per my understanding, swapoff will not allocate virtual mapping by >>>>> itself. But after swapoff, the overcommit limit could be exceeded. >>>>> security_vm_enough_memory_mm() is used to check that. For example, in a >>>>> system with 4GB memory and 8GB swap, and 10GB is in use, >>>>> >>>>> CommitLimit: 4+8 = 12GB >>>>> Committed_AS: 10GB >>>>> >>>>> security_vm_enough_memory_mm() in swapoff() will fail because >>>>> 10+8 = 18 > 12. This is expected because after swapoff, the overcommit >>>>> limit will be exceeded. >>>>> >>>>> If 3GB is in use, >>>>> >>>>> CommitLimit: 4+8 = 12GB >>>>> Committed_AS: 3GB >>>>> >>>>> security_vm_enough_memory_mm() in swapoff() will succeed because >>>>> 3+8 = 11 < 12. This is expected because after swapoff, the overcommit >>>>> limit will not be exceeded. >>>> >>>> In OVERCOMMIT_NEVER scene, I think you're right. >>>> >>>>> >>>>> So, what's the real problem of the original implementation? Can you >>>>> show it with an example as above? >>>> >>>> In OVERCOMMIT_GUESS scene, in a system with 4GB memory and 8GB swap, and 10GB is in use, >>>> pages below is 8GB, totalram_pages() + total_swap_pages is 12GB, so swapoff() will succeed >>>> instead of expected failure because 8 < 12. The overcommit limit is always *ignored* in the >>>> below case. >>>> >>>> if (sysctl_overcommit_memory == OVERCOMMIT_GUESS) { >>>> if (pages > totalram_pages() + total_swap_pages) >>>> goto error; >>>> return 0; >>>> } >>>> >>>> Or am I miss something? >>> >>> Per my understanding, with OVERCOMMIT_GUESS, the number of in-use pages >>> isn't checked at all. The only restriction is that the size of the >>> virtual mapping created should be less than total RAM + total swap >> >> Do you mean the only restriction is that the size of the virtual mapping >> *created every time* should be less than total RAM + total swap pages but >> *total virtual mapping* is not limited in OVERCOMMIT_GUESS scene? If so, >> the current behavior should be sane and I will drop this patch. > > Yes. This is my understanding. I see. Thank you. > > Best Regards, > Huang, Ying > >> Thanks! >> >>> pages. Because swapoff() will not create virtual mapping, so it's >>> expected that security_vm_enough_memory_mm() in swapoff() always >>> succeeds. >>> >>> Best Regards, >>> Huang, Ying >>> >>>> >>>> Thanks! >>>> >>>>> >>>>>> In order to fix it, p->pages should be retracted from total_swap_pages >>>>>> first and then check whether there's enough memory for inuse swap pages. >>>>>> >>>>>> Signed-off-by: Miaohe Lin >>>>> >>>>> [snip] >>>>> >>>>> . >>>>> >>> >>> . >>> > > . >