Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751950AbaF0A1F (ORCPT ); Thu, 26 Jun 2014 20:27:05 -0400 Received: from mail-la0-f44.google.com ([209.85.215.44]:50205 "EHLO mail-la0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751127AbaF0A1D (ORCPT ); Thu, 26 Jun 2014 20:27:03 -0400 MIME-Version: 1.0 In-Reply-To: <53ACB8A7.9050002@intel.com> References: <1403084656-27284-1-git-send-email-qiaowei.ren@intel.com> <1403084656-27284-3-git-send-email-qiaowei.ren@intel.com> <53A884B2.5070702@mit.edu> <53A88806.1060908@intel.com> <53A88DE4.8050107@intel.com> <9E0BE1322F2F2246BD820DA9FC397ADE016AF41C@shsmsx102.ccr.corp.intel.com> <9E0BE1322F2F2246BD820DA9FC397ADE016B26AB@shsmsx102.ccr.corp.intel.com> <53AB42E1.4090102@intel.com> <53ACA5B3.3010702@intel.com> <53ACB8A7.9050002@intel.com> From: Andy Lutomirski Date: Thu, 26 Jun 2014 17:26:40 -0700 Message-ID: Subject: Re: [PATCH v6 02/10] x86, mpx: add MPX specific mmap interface To: Dave Hansen Cc: "Ren, Qiaowei" , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , X86 ML , "linux-kernel@vger.kernel.org" , Linux MM Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 26, 2014 at 5:19 PM, Dave Hansen wrote: > On 06/26/2014 04:15 PM, Andy Lutomirski wrote: >> So here's my mental image of how I might do this if I were doing it >> entirely in userspace: I'd create a file or memfd for the bound tables >> and another for the bound directory. These files would be *huge*: the >> bound directory file would be 2GB and the bounds table file would be >> 2^48 bytes or whatever it is. (Maybe even bigger?) >> >> Then I'd just map pieces of those files wherever they'd need to be, >> and I'd make the mappings sparse. I suspect that you don't actually >> want a vma for each piece of bound table that gets mapped -- the space >> of vmas could end up incredibly sparse. So I'd at least map (in the >> vma sense, not the pte sense) and entire bound table at a time. And >> I'd probably just map the bound directory in one big piece. >> >> Then I'd populate it in the fault handler. >> >> This is almost what the code is doing, I think, modulo the files. >> >> This has one killer problem: these mappings need to be private (cowed >> on fork). So memfd is no good. > > This essentially uses the page cache's radix tree as a parallel data > structure in order to keep a vaddr->mpx_vma map. That's not a bad idea, > but it is a parallel data structure that does not handle copy-on-write > very well. > > I'm pretty sure we need the semantics that anonymous memory provides. > >> There's got to be an easyish way to >> modify the mm code to allow anonymous maps with vm_ops. Maybe a new >> mmap_region parameter or something? Maybe even a special anon_vma, >> but I don't really understand how those work. > > Yeah, we very well might end up having to go down that path. > >> Also, egads: what happens when a bound table entry is associated with >> a MAP_SHARED page? > > Bounds table entries are for pointers. Do we keep pointers inside of > MAP_SHARED-mapped things? :) Sure, if it's MAP_SHARED | MAP_ANONYMOUS. For example: struct thing { struct thing *next; }; struct thing *storage = mmap(..., MAP_SHARED | MAP_ANONYMOUS, ...); storage[0].next = &storage[1]; fork(); I'm not suggesting that this needs to *work* in the first incarnation of this :) --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/