Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753858Ab3HVHrY (ORCPT ); Thu, 22 Aug 2013 03:47:24 -0400 Received: from nat28.tlf.novell.com ([130.57.49.28]:43906 "EHLO nat28.tlf.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753462Ab3HVHrX convert rfc822-to-8bit (ORCPT ); Thu, 22 Aug 2013 03:47:23 -0400 Message-Id: <5215DE2702000078000ED8AE@nat28.tlf.novell.com> X-Mailer: Novell GroupWise Internet Agent 12.0.2 Date: Thu, 22 Aug 2013 08:47:19 +0100 From: "Jan Beulich" To: "Linus Torvalds" Cc: "Andy Lutomirski" , "David Vrabel" , "Cyrill Gorcunov" , "Andrew Morton" , , "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> <5214F09002000078000ED5C3@nat28.tlf.novell.com> <20130821154238.GV18673@moon> <521500E102000078000ED65C@nat28.tlf.novell.com> <20130821161946.GW18673@moon> <5214F128.1000901@citrix.com> <20130821172547.GY18673@moon> <20130821181733.GC3814@moon> <4fec3e5b-695c-438b-ad6d-55ca50becc4c@email.android.com> <20130821190307.GB18673@moon> In-Reply-To: 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: 1932 Lines: 44 >>> On 22.08.13 at 01:04, Linus Torvalds wrote: > On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov wrote: >> >> I personally don't see bug here because >> >> - this swapped page soft dirty bit is set for non-present entries only, >> never for present ones, just at moment we form swap pte entry >> >> - i don't find any code which would test for this bit directly without >> is_swap_pte call > > Ok, having gone through the places that use swp_*soft_dirty(), I have > to agree. Afaik, it's only ever used on a swap-entry that has (by > definition) the P bit clear. So with or without Xen, I don't see how > it can make any difference. > > David/Konrad - did you actually see any issues, or was this just from > (mis)reading the code? It was actually me (mis)reading the code - as pointed out to Cyrill already, setting _PAGE_PAT in a pte_t without even a comment saying that this can only ever be done with a non-present entry made me expect problems on Xen, because it's clear that to date bare metal Linux doesn't care about the state of _PAGE_PAT in present entries due to the way the PAT MSR gets set (and hence quite likely no-one would have noticed the supposed problem while testing). So a comment either alongside the definition of _PAGE_SWP_SOFT_DIRTY or directly in pte_swp_{mk,clear_}soft_dirty() would have been the minimal thing I'd have expected for this sort of re-use of bits. Ideally even a VM_BUG_ON(pte_present()) or similar. And perhaps pte_swp_soft_dirty() should be either looking at the present bit too or similarly asserting that it's clear... In any event - I'm sorry for the red herring. 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/