From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751246AbbIZGU3 (ORCPT ); Sat, 26 Sep 2015 02:20:29 -0400 Received: from mail-wi0-f173.google.com ([209.85.212.173]:33467 "EHLO mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750936AbbIZGU2 (ORCPT ); Sat, 26 Sep 2015 02:20:28 -0400 Date: Sat, 26 Sep 2015 08:20:23 +0200 From: Ingo Molnar To: Dave Hansen Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Linus Torvalds , Andrew Morton , Peter Zijlstra , Thomas Gleixner Subject: Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key faults Message-ID: <20150926062023.GB27841@gmail.com> References: <20150916174903.E112E464@viggo.jf.intel.com> <20150916174906.51062FBC@viggo.jf.intel.com> <20150924092320.GA26876@gmail.com> <20150924093026.GA29699@gmail.com> <560435B4.1010603@sr71.net> <20150925071119.GB15753@gmail.com> <5605D660.8000009@sr71.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5605D660.8000009@sr71.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Dave Hansen wrote: > On 09/25/2015 12:11 AM, Ingo Molnar wrote: > >>> > > Btw., how does pkey support interact with hugepages? > >> > > >> > Surprisingly little. I've made sure that everything works with huge pages and > >> > that the (huge) PTEs and VMAs get set up correctly, but I'm not sure I had to > >> > touch the huge page code at all. I have test code to ensure that it works the > >> > same as with small pages, but everything worked pretty naturally. > > Yeah, so the reason I'm asking about expectations is that this code: > > > > + follow_ret = follow_pte(tsk->mm, address, &ptep, &ptl); > > + if (!follow_ret) { > > + /* > > + * On a successful follow, make sure to > > + * drop the lock. > > + */ > > + pte = *ptep; > > + pte_unmap_unlock(ptep, ptl); > > + ret = pte_pkey(pte); > > > > is visibly hugepage-unsafe: if a vma is hugepage mapped, there are no ptes, only > > pmds - and the protection key index lives in the pmd. We don't seem to recover > > that information properly. > > You got me on this one. I assumed that follow_pte() handled huge pages. > It does not. > > But, the code still worked. Since follow_pte() fails for all huge > pages, it just falls back to pulling the protection key out of the VMA, > which _does_ work for huge pages. That might be true for explicit hugetlb vmas, but what about transparent hugepages that can show up in regular vmas? > I've actually removed the PTE walking and I just now use the VMA directly. I > don't see a ton of additional value from walking the page tables when we can get > what we need from the VMA. That's actually good, because it's also cheap, especially if we can get rid of the extra find_vma(). and we (thankfully) have no non-linear vmas to worry about anymore. Thanks, Ingo