From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756817AbZCLOMx (ORCPT ); Thu, 12 Mar 2009 10:12:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755452AbZCLOMn (ORCPT ); Thu, 12 Mar 2009 10:12:43 -0400 Received: from waste.org ([66.93.16.53]:37730 "EHLO waste.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754551AbZCLOMm (ORCPT ); Thu, 12 Mar 2009 10:12:42 -0400 Subject: Re: [PATCH] fix/improve generic page table walker From: Matt Mackall To: Martin Schwidefsky Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Gerald Schaefer , akpm@linux-foundation.org, Hugh Dickins , Nick Piggin In-Reply-To: <20090312093335.6dd67251@skybase> References: <20090311144951.58c6ab60@skybase> <1236792263.3205.45.camel@calx> <20090312093335.6dd67251@skybase> Content-Type: text/plain Date: Thu, 12 Mar 2009 09:10:14 -0500 Message-Id: <1236867014.3213.16.camel@calx> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [Nick and Hugh, maybe you can shed some light on this for me] On Thu, 2009-03-12 at 09:33 +0100, Martin Schwidefsky wrote: > On Wed, 11 Mar 2009 12:24:23 -0500 > Matt Mackall wrote: > > > On Wed, 2009-03-11 at 14:49 +0100, Martin Schwidefsky wrote: > > > From: Martin Schwidefsky > > > > > > On s390 the /proc/pid/pagemap interface is currently broken. This is > > > caused by the unconditional loop over all pgd/pud entries as specified > > > by the address range passed to walk_page_range. The tricky bit here > > > is that the pgd++ in the outer loop may only be done if the page table > > > really has 4 levels. For the pud++ in the second loop the page table needs > > > to have at least 3 levels. With the dynamic page tables on s390 we can have > > > page tables with 2, 3 or 4 levels. Which means that the pgd and/or the > > > pud pointer can get out-of-bounds causing all kinds of mayhem. > > > > Not sure why this should be a problem without delving into the S390 > > code. After all, x86 has 2, 3, or 4 levels as well (at compile time) in > > a way that's transparent to the walker. > > Its hard to understand without looking at the s390 details. The main > difference between x86 and s390 in that respect is that on s390 the > number of page table levels is determined at runtime on a per process > basis. A compat process uses 2 levels, a 64 bit process starts with 3 > levels and can "upgrade" to 4 levels if something gets mapped above > 4TB. Which means that a *pgd can point to a region-second (2**53 bytes), > a region-third (2**42 bytes) or a segment table (2**31 bytes), a *pud > can point to a region-third or a segment table. The page table > primitives know about this semantic, in particular pud_offset and > pmd_offset check the type of the page table pointed to by *pgd and *pud > and do nothing with the pointer if it is a lower level page table. > The only operation I can not "patch" is the pgd++/pud++ operation. So in short, sometimes a pgd_t isn't really a pgd_t at all. It's another object with different semantics that generic code can trip over. Can I get you to explain why this is necessary or even preferable to doing it the generic way where pgd_t has a fixed software meaning regardless of how many hardware levels are in play? -- http://selenic.com : development and support for Mercurial and Linux From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with ESMTP id 0722A6B004F for ; Thu, 12 Mar 2009 10:12:10 -0400 (EDT) Subject: Re: [PATCH] fix/improve generic page table walker From: Matt Mackall In-Reply-To: <20090312093335.6dd67251@skybase> References: <20090311144951.58c6ab60@skybase> <1236792263.3205.45.camel@calx> <20090312093335.6dd67251@skybase> Content-Type: text/plain Date: Thu, 12 Mar 2009 09:10:14 -0500 Message-Id: <1236867014.3213.16.camel@calx> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Martin Schwidefsky Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Gerald Schaefer , akpm@linux-foundation.org, Hugh Dickins , Nick Piggin List-ID: [Nick and Hugh, maybe you can shed some light on this for me] On Thu, 2009-03-12 at 09:33 +0100, Martin Schwidefsky wrote: > On Wed, 11 Mar 2009 12:24:23 -0500 > Matt Mackall wrote: > > > On Wed, 2009-03-11 at 14:49 +0100, Martin Schwidefsky wrote: > > > From: Martin Schwidefsky > > > > > > On s390 the /proc/pid/pagemap interface is currently broken. This is > > > caused by the unconditional loop over all pgd/pud entries as specified > > > by the address range passed to walk_page_range. The tricky bit here > > > is that the pgd++ in the outer loop may only be done if the page table > > > really has 4 levels. For the pud++ in the second loop the page table needs > > > to have at least 3 levels. With the dynamic page tables on s390 we can have > > > page tables with 2, 3 or 4 levels. Which means that the pgd and/or the > > > pud pointer can get out-of-bounds causing all kinds of mayhem. > > > > Not sure why this should be a problem without delving into the S390 > > code. After all, x86 has 2, 3, or 4 levels as well (at compile time) in > > a way that's transparent to the walker. > > Its hard to understand without looking at the s390 details. The main > difference between x86 and s390 in that respect is that on s390 the > number of page table levels is determined at runtime on a per process > basis. A compat process uses 2 levels, a 64 bit process starts with 3 > levels and can "upgrade" to 4 levels if something gets mapped above > 4TB. Which means that a *pgd can point to a region-second (2**53 bytes), > a region-third (2**42 bytes) or a segment table (2**31 bytes), a *pud > can point to a region-third or a segment table. The page table > primitives know about this semantic, in particular pud_offset and > pmd_offset check the type of the page table pointed to by *pgd and *pud > and do nothing with the pointer if it is a lower level page table. > The only operation I can not "patch" is the pgd++/pud++ operation. So in short, sometimes a pgd_t isn't really a pgd_t at all. It's another object with different semantics that generic code can trip over. Can I get you to explain why this is necessary or even preferable to doing it the generic way where pgd_t has a fixed software meaning regardless of how many hardware levels are in play? -- http://selenic.com : development and support for Mercurial and Linux -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org