Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752413AbdLEHIp (ORCPT ); Tue, 5 Dec 2017 02:08:45 -0500 Received: from mx2.suse.de ([195.135.220.15]:55390 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752138AbdLEHIl (ORCPT ); Tue, 5 Dec 2017 02:08:41 -0500 Date: Tue, 5 Dec 2017 08:08:38 +0100 From: Michal Hocko To: John Hubbard Cc: Mike Rapoport , Michael Kerrisk , linux-man , linux-api@vger.kernel.org, Michael Ellerman , linux-mm@kvack.org, LKML , linux-arch@vger.kernel.org, Jann Horn , Matthew Wilcox Subject: Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation Message-ID: <20171205070838.u3br5lvshywkwxby@dhcp22.suse.cz> References: <20171204021411.4786-1-jhubbard@nvidia.com> <20171204113113.GA13465@rapoport-lnx> <6777116d-ad9e-48c9-0009-01d10274135e@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6777116d-ad9e-48c9-0009-01d10274135e@nvidia.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2343 Lines: 49 On Mon 04-12-17 18:52:27, John Hubbard wrote: > On 12/04/2017 03:31 AM, Mike Rapoport wrote: > > On Sun, Dec 03, 2017 at 06:14:11PM -0800, john.hubbard@gmail.com wrote: > >> From: John Hubbard > >> > [...] > >> +.IP > >> +Given the above limitations, one of the very few ways to use this option > >> +safely is: mmap() a region, without specifying MAP_FIXED. Then, within that > >> +region, call mmap(MAP_FIXED) to suballocate regions. This avoids both the > >> +portability problem (because the first mmap call lets the kernel pick the > >> +address), and the address space corruption problem (because the region being > >> +overwritten is already owned by the calling thread). > > > > Maybe "address space corruption problem caused by implicit calls to mmap"? > > The region allocated with the first mmap is not exactly owned by the > > thread and a multi-thread application can still corrupt its memory if > > different threads use mmap(MAP_FIXED) for overlapping regions. > > > > My 2 cents. > > > > Hi Mike, > > Yes, thanks for picking through this, and I agree that the above is misleading. > It should definitely not use the word "owned" at all. Re-doing the whole > paragraph in order to make it all fit together nicely, I get this: > > "Given the above limitations, one of the very few ways to use this option > safely is: mmap() an enclosing region, without specifying MAP_FIXED. > Then, within that region, call mmap(MAP_FIXED) to suballocate regions > within the enclosing region. This avoids both the portability problem > (because the first mmap call lets the kernel pick the address), and the > address space corruption problem (because implicit calls to mmap will > not affect the already-mapped enclosing region)." > > ...how's that sound to you? I'll post a v3 soon with this. It sounds to me you are trying to tell way to much while actually being a bit misleading. Even sub-range MAP_FIXED is not multi-thread safe. Really the more corner cases you will try to cover the worse the end result will end up. I would just try to be simple here and mention the address space corruption issues you've had earlier and be done with it. Maybe add a note that some architectures might need a special alignement and fail if it is not the case but nothing really specific. -- Michal Hocko SUSE Labs