From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f42.google.com (mail-pa0-f42.google.com [209.85.220.42]) by kanga.kvack.org (Postfix) with ESMTP id 9EAC96B00B6 for ; Thu, 26 Jun 2014 20:19:57 -0400 (EDT) Received: by mail-pa0-f42.google.com with SMTP id lj1so3854852pab.1 for ; Thu, 26 Jun 2014 17:19:57 -0700 (PDT) Received: from mga02.intel.com (mga02.intel.com. [134.134.136.20]) by mx.google.com with ESMTP id ez5si11897283pbc.174.2014.06.26.17.19.56 for ; Thu, 26 Jun 2014 17:19:56 -0700 (PDT) Message-ID: <53ACB8A7.9050002@intel.com> Date: Thu, 26 Jun 2014 17:19:51 -0700 From: Dave Hansen MIME-Version: 1.0 Subject: Re: [PATCH v6 02/10] x86, mpx: add MPX specific mmap interface 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> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: "Ren, Qiaowei" , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , X86 ML , "linux-kernel@vger.kernel.org" , Linux MM 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? :) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org