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=-2.5 required=3.0 tests=MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT 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 38490C43144 for ; Mon, 25 Jun 2018 09:15:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EBFBA23D50 for ; Mon, 25 Jun 2018 09:15:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EBFBA23D50 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org 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 S1755027AbeFYJPC (ORCPT ); Mon, 25 Jun 2018 05:15:02 -0400 Received: from mx2.suse.de ([195.135.220.15]:56455 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755008AbeFYJO6 (ORCPT ); Mon, 25 Jun 2018 05:14:58 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (charybdis-ext-too.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 24B59AB43; Mon, 25 Jun 2018 09:14:57 +0000 (UTC) Date: Mon, 25 Jun 2018 11:14:55 +0200 From: Michal Hocko To: Yang Shi Cc: Nadav Amit , Matthew Wilcox , ldufour@linux.vnet.ibm.com, Andrew Morton , Peter Zijlstra , Ingo Molnar , acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, "open list:MEMORY MANAGEMENT" , linux-kernel@vger.kernel.org Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping Message-ID: <20180625091455.GH28965@dhcp22.suse.cz> References: <1529364856-49589-1-git-send-email-yang.shi@linux.alibaba.com> <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com> <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com> <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com> <20180620071817.GJ13685@dhcp22.suse.cz> <94bdfcf0-68ea-404c-a60f-362f677884b6@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <94bdfcf0-68ea-404c-a60f-362f677884b6@linux.alibaba.com> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 22-06-18 18:01:08, Yang Shi wrote: > Yes, this is true but I guess what Yang Shi meant was that an userspace > > > access racing with munmap is not well defined. You never know whether > > > you get your data, #PTF or SEGV because it depends on timing. The user > > > visible change might be that you lose content and get zero page instead > > > if you hit the race window while we are unmapping which was not possible > > > before. But whouldn't such an access pattern be buggy anyway? You need > > > some form of external synchronization AFAICS. > > > > > > But maybe some userspace depends on "getting right data or get SEGV" > > > semantic. If we have to preserve that then we can come up with a VM_DEAD > > > flag set before we tear it down and force the SEGV on the #PF path. > > > Something similar we already do for MMF_UNSTABLE. > > > > Set VM_DEAD with read mmap_sem held? It should be ok since this is the > > only place to set this flag for this unique special case. > > BTW, it looks the vm flags have used up in 32 bit. If we really need > VM_DEAD, it should be for both 32-bit and 64-bit. Do we really need any special handling for 32b? Who is going to create GB mappings for all this to be worth doing? -- Michal Hocko SUSE Labs