From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752643AbcGAEqR (ORCPT ); Fri, 1 Jul 2016 00:46:17 -0400 Received: from www.sr71.net ([198.145.64.142]:39119 "EHLO blackbird.sr71.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750769AbcGAEqP (ORCPT ); Fri, 1 Jul 2016 00:46:15 -0400 Subject: Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs To: Linus Torvalds References: <20160701001209.7DA24D1C@viggo.jf.intel.com> <20160701001218.3D316260@viggo.jf.intel.com> Cc: Linux Kernel Mailing List , the arch/x86 maintainers , linux-mm , Andrew Morton , Borislav Petkov , Andi Kleen , Michal Hocko , Dave Hansen From: Dave Hansen Message-ID: <5775F418.2000803@sr71.net> Date: Thu, 30 Jun 2016 21:39:52 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/30/2016 07:55 PM, Linus Torvalds wrote: > On Thu, Jun 30, 2016 at 5:12 PM, Dave Hansen wrote: >> From: Dave Hansen >> The Intel(R) Xeon Phi(TM) Processor x200 Family (codename: Knights >> Landing) has an erratum where a processor thread setting the Accessed >> or Dirty bits may not do so atomically against its checks for the >> Present bit. This may cause a thread (which is about to page fault) >> to set A and/or D, even though the Present bit had already been >> atomically cleared. > > So I don't think your approach is wrong, but I suspect this is > overkill, and what we should instead just do is to not use the A/D > bits at all in the swap representation. We actually don't even use Dirty today. It's (implicitly) used to determine pte_none(), but it ends up being masked out for the swp_offset/type() calculations entirely, much to my surprise. I think what you suggest will work if we don't consider A/D in pte_none(). I think there are a bunch of code path where assume that !pte_present() && !pte_none() means swap. > The swap-entry representation was a bit tight on 32-bit page table > entries, but in 64-bit ones, I think we have tons of bits, don't we? > So we could decide just to not use those two bits on x86. Yeah, we've definitely got space. I'll go poke around and make sure that this works everywhere. I agree that throwing 32-bit non-PAE under the bus is definitely worth it here. Nobody will care about that in a million years.