Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755368AbXJVTHv (ORCPT ); Mon, 22 Oct 2007 15:07:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752599AbXJVTHo (ORCPT ); Mon, 22 Oct 2007 15:07:44 -0400 Received: from one.firstfloor.org ([213.235.205.2]:40939 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752369AbXJVTHn (ORCPT ); Mon, 22 Oct 2007 15:07:43 -0400 Date: Mon, 22 Oct 2007 21:07:40 +0200 From: Andi Kleen To: Jeremy Fitzhardinge Cc: Andi Kleen , dean gaudet , Nick Piggin , Xen-devel , Morten@suse.de, David Chinner , Linux Kernel Mailing List , =?iso-8859-1?Q?B=F8geskov?= , xfs@oss.sgi.com, xfs-masters@oss.sgi.com, Mark Williamson Subject: Re: Interaction between Xen and XFS: stray RW mappings Message-ID: <20071022190740.GA1695@one.firstfloor.org> References: <470FA7C3.90404@goop.org> <20071014225618.GN23367404@sgi.com> <4712A254.4090604@goop.org> <200710151415.07248.nickpiggin@yahoo.com.au> <471C1A61.1010001@goop.org> <471CEEB4.9040807@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <471CEEB4.9040807@goop.org> User-Agent: Mutt/1.4.2.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2026 Lines: 50 On Mon, Oct 22, 2007 at 11:40:52AM -0700, Jeremy Fitzhardinge wrote: > Andi Kleen wrote: > > Jeremy Fitzhardinge writes: > > > >> Yes, that's precisely the problem. xfs does delay the unmap, leaving > >> stray mappings, which upsets Xen. > >> > > > > Again it not just upsets Xen, keeping mappings to freed pages is wrong generally > > and violates the x86 (and likely others like PPC) architecture because it can > > cause illegal caching attribute aliases. > > > > The patch that went into the tree was really not correct -- this > > bogus optimization should have been unconditionally removed > > or if you really wanted an ifdef made dependent on !CONFIG_XEN && > > !CONFIG_AGP (and likely && !CONFIG_DRM && !CONFIG_anything else that > > uses uncached mappings in memory). > > > > You just worked around the obvious failure and leave the non obvious > > rare corruptions in, which isn't a good strategy. > > Well, at least it becomes a known issue and/or placeholder for when Nick It's hidden now so it causes any obvious failures any more. Just subtle ones which is much worse. But why not just disable it? It's not critical functionality, just a optimization that unfortunately turned out to be incorrect. It could be made correct short term by not freeing the pages until vunmap for example. > does his grand unified vmap manager. I guess a clean workaround would > be to add a CONFIG_XFS_LAZY_UNMAP, and do it at the Kconfig level... > I'll cook up a patch. This option could only be safely set in architectures that don't care about caching attribute aliases or never remap kernel pages uncached. That's not x86, powerpc, ia64 at a minimum. I think the only good short term fix is to turn your ifdef CONFIG_XEN into a #if 0 -Andi - 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/