From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757284AbXJCVbh (ORCPT ); Wed, 3 Oct 2007 17:31:37 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752981AbXJCVba (ORCPT ); Wed, 3 Oct 2007 17:31:30 -0400 Received: from waste.org ([66.93.16.53]:34082 "EHLO waste.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752658AbXJCVb3 (ORCPT ); Wed, 3 Oct 2007 17:31:29 -0400 Date: Wed, 3 Oct 2007 16:31:12 -0500 From: Matt Mackall To: Hugh Dickins Cc: Nick Piggin , linux-kernel Subject: Re: pgd_none_or_clear_bad strangeness? Message-ID: <20071003213112.GV19691@waste.org> References: <20071002222003.GL19691@waste.org> <20071003112527.GA10437@wotan.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 03, 2007 at 07:18:23PM +0100, Hugh Dickins wrote: > On Wed, 3 Oct 2007, Nick Piggin wrote: > > On Tue, Oct 02, 2007 at 05:20:03PM -0500, Matt Mackall wrote: > > > In lib/pagewalk.c, I've been using the various forms of > > > {pgd,pud,pmd}_none_or_clear_bad while walking page tables as that > > > seemed the canonical way to do things. Lately (eg with -rc7-mm1), > > > these have been triggering messages like "bad pgd 0x01e3" and causing > > > nasty double faults. It appears this is actually triggered at the pmd > > > level (mm/memory.c:116), though it appears to produce the wrong > > > message. > > I guess the "wrong message" is an artifact of pud/pmd folding; > but I get too confused by the different levels myself to want to > think more about it - I'll just assume it's "right" somehow ;) > > > > > > > Has something changed here? I'm pretty sure this used to work! Is this > > I don't know of anything changing here, sorry. > > > > not a kosher thing to do? Does it make any sense I'd repeatedly run > > > into a bad pmd in the middle of bash's page table right after boot? > > > The simple _none variant seems to work, but I worry that it's papering > > > over a real problem. > > > > No, I think that should be the right thing to do for userspace pages. > > You're not walking into a hugetlb area or a kernel mapping are you? > > (the bad pgd: line could be important... 0x01e3 would be a linear kernel > > mapping I think?). > > I should have spent more time reading Nick's reply and less time trying > to work it out for myself! Yes, that's the conclusion I came to, for > some reason you're now going beyond the user vmas and walking into the > linear kernel mapping, which has _PAGE_GLOBAL and _PAGE_PSE bits set. Indeed, that's precisely what's happening. I'm walking one page past the end of userspace. And the reason is I changed my walker from using for loops to do/while loops at Nick's insistance, so start==end no longer gets noticed immediately. This also explains why the bug doesn't manifest in lguest: no PSE mappings. Thanks, guys! -- Mathematics is the supreme nostalgia of our time.