Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751846Ab3HUOxn (ORCPT ); Wed, 21 Aug 2013 10:53:43 -0400 Received: from nat28.tlf.novell.com ([130.57.49.28]:36017 "EHLO nat28.tlf.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751557Ab3HUOxm convert rfc822-to-8bit (ORCPT ); Wed, 21 Aug 2013 10:53:42 -0400 Message-Id: <5214F09002000078000ED5C3@nat28.tlf.novell.com> X-Mailer: Novell GroupWise Internet Agent 12.0.2 Date: Wed, 21 Aug 2013 15:53:36 +0100 From: "Jan Beulich" To: "David Vrabel" , "Cyrill Gorcunov" Cc: "Andy Lutomirski" , "Andrew Morton" , "Linus Torvalds" , , "Boris Ostrovsky" , "Konrad Rzeszutek Wilk" , "Pavel Emelyanov" , "Ingo Molnar" , "linux-kernel@vger.kernel.org" , "H. Peter Anvin" Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use References: <5214C524.1050900@citrix.com> <20130821141223.GS18673@moon> In-Reply-To: <20130821141223.GS18673@moon> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1724 Lines: 42 >>> On 21.08.13 at 16:12, Cyrill Gorcunov wrote: > On Wed, Aug 21, 2013 at 02:48:20PM +0100, David Vrabel wrote: >> All, >> >> 179ef71c (mm: save soft-dirty bits on swapped pages) introduces a new >> PTE bit on x86 _PTE_SWP_SOFT_DIRTY which has the same value as _PTE_PSE >> and _PTE_PAT. >> >> With a Xen PV guest, the use of the _PTE_PAT will result in the page >> having unexpected cachability which will introduce a range of subtle >> performance and correctness issues. Xen programs the entry 4 in the PAT >> table with WC so a page that was previously WB will end up as WC. >> > > David, could you please explain, Xen keeps and analyze _PTE_PAT bit > for ptes which are not present? No, the problem isn't with not-present PTEs (i.e. swap entries), but with present ones - the same bit (7) is being used for both, according to this comment: /* * Tracking soft dirty bit when a page goes to a swap is tricky. * We need a bit which can be stored in pte _and_ not conflict * with swap entry format. On x86 bits 6 and 7 are *not* involved * into swap entry computation, but bit 6 is used for nonlinear * file mapping, so we borrow bit 7 for soft dirty tracking. */ Or are you telling me that the comment is misleading (at least me), and this applies only to not-present PTEs? And even then - where would the value of the original PAT bit be stored while swapped out (or is it impossible - now and forever - for WC pages to get swapped)? Jan -- 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/