From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f71.google.com (mail-pg0-f71.google.com [74.125.83.71]) by kanga.kvack.org (Postfix) with ESMTP id 8E23A6B0266 for ; Mon, 25 Jun 2018 05:15:02 -0400 (EDT) Received: by mail-pg0-f71.google.com with SMTP id r8-v6so4767224pgq.2 for ; Mon, 25 Jun 2018 02:15:02 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o61-v6si13613541pld.109.2018.06.25.02.15.01 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 25 Jun 2018 02:15:01 -0700 (PDT) Date: Mon, 25 Jun 2018 11:14:55 +0200 From: Michal Hocko 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> Sender: owner-linux-mm@kvack.org List-ID: 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 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