Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752520Ab3HUPmm (ORCPT ); Wed, 21 Aug 2013 11:42:42 -0400 Received: from mail-la0-f45.google.com ([209.85.215.45]:60058 "EHLO mail-la0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751717Ab3HUPml (ORCPT ); Wed, 21 Aug 2013 11:42:41 -0400 Date: Wed, 21 Aug 2013 19:42:38 +0400 From: Cyrill Gorcunov To: Jan Beulich Cc: David Vrabel , Andy Lutomirski , Andrew Morton , Linus Torvalds , Xen-devel@lists.xen.org, 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 Message-ID: <20130821154238.GV18673@moon> References: <5214C524.1050900@citrix.com> <20130821141223.GS18673@moon> <5214F09002000078000ED5C3@nat28.tlf.novell.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5214F09002000078000ED5C3@nat28.tlf.novell.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2358 Lines: 62 On Wed, Aug 21, 2013 at 03:53:36PM +0100, Jan Beulich wrote: > >>> 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)? Only to non-present ptes, as far as I know. do_swap_page ... pte = mk_pte(page, vma->vm_page_prot); /* new pte from vm_page_prot generated */ ... set_pte_at(mm, address, page_table, pte); /* and assigned to old place */ with soft dirty in swap it is somehow more weirdy pte = mk_pte(page, vma->vm_page_prot); ... if (pte_swp_soft_dirty(orig_pte)) pte = pte_mksoft_dirty(pte); set_pte_at(mm, address, page_table, pte); orig_pte has pse bit set if page has been soft dirty when it reached swap. -- 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/