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 91D4ACCA480 for ; Mon, 20 Jun 2022 12:13:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242190AbiFTMNg (ORCPT ); Mon, 20 Jun 2022 08:13:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58304 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242384AbiFTMNM (ORCPT ); Mon, 20 Jun 2022 08:13:12 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8FC6419F96 for ; Mon, 20 Jun 2022 05:12:36 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.57]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4LRT4H20CHzhYZh; Mon, 20 Jun 2022 20:10:23 +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 20:12:27 +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> From: Miaohe Lin Message-ID: Date: Mon, 20 Jun 2022 20:12:27 +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: <87r13jrdst.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: dggems701-chm.china.huawei.com (10.3.19.178) 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 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? > > 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] > > . >