Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935263AbdIYMwM (ORCPT ); Mon, 25 Sep 2017 08:52:12 -0400 Received: from mx2.suse.de ([195.135.220.15]:47334 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S933681AbdIYMwK (ORCPT ); Mon, 25 Sep 2017 08:52:10 -0400 Date: Mon, 25 Sep 2017 14:52:07 +0200 From: Michal Hocko To: Florian Weimer Cc: Mike Kravetz , mtk.manpages@gmail.com, linux-man@vger.kernel.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Andrea Arcangeli , "Kirill A . Shutemov" , Vlastimil Babka , Anshuman Khandual , linux-mm@kvack.org Subject: Re: [patch] mremap.2: Add description of old_size == 0 functionality Message-ID: <20170925125207.4tu24sbpnihljknu@dhcp22.suse.cz> References: <20170915213745.6821-1-mike.kravetz@oracle.com> <20170925123508.pzjbe7wgwagnr5li@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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: 1432 Lines: 31 On Mon 25-09-17 14:40:42, Florian Weimer wrote: > On 09/25/2017 02:35 PM, Michal Hocko wrote: > > What would be the usecase. I mean why don't you simply create a new > > mapping by a plain mmap when you have no guarantee about the same > > content? > > I plan to use it for creating an unbounded number of callback thunks at run > time, from a single set of pages in libc.so, in case we need this > functionality. > > The idea is to duplicate existing position-independent machine code in > libc.so, prefixed by a data mapping which controls its behavior. Each > data/code combination would only give us a fixed number of thunks, so we'd > need to create a new mapping to increase the total number. > > Instead, we could re-map the code from the executable in disk, but not if > chroot has been called or glibc has been updated on disk. Creating an alias > mapping does not have these problems. > > Another application (but that's for anonymous memory) would be to duplicate > class metadata in a Java-style VM, so that you can use bits in the class > pointer in each Java object (which is similar to the vtable pointer in C++) > for the garbage collector, without having to mask it when accessing the > class metadata in regular (mutator) code. So, how are you going to deal with the CoW and the implementation which basically means that the newm mmap content is not the same as the original one? -- Michal Hocko SUSE Labs