From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755844AbaEOSjs (ORCPT ); Thu, 15 May 2014 14:39:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:12104 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755392AbaEOSjr (ORCPT ); Thu, 15 May 2014 14:39:47 -0400 Message-ID: <1400179157.21547.147.camel@deneb.redhat.com> Subject: Re: [PATCH] arm64: fix pud_huge() for 2-level pagetables From: Mark Salter To: Steve Capper Cc: Catalin Marinas , Will Deacon , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Date: Thu, 15 May 2014 14:39:17 -0400 In-Reply-To: References: <1400163562-7481-1-git-send-email-msalter@redhat.com> <20140515144424.GA23884@linaro.org> <1400171258.21547.136.camel@deneb.redhat.com> Organization: Red Hat, Inc Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-05-15 at 18:55 +0100, Steve Capper wrote: > On 15 May 2014 17:27, Mark Salter wrote: > > On Thu, 2014-05-15 at 15:44 +0100, Steve Capper wrote: > >> On Thu, May 15, 2014 at 10:19:22AM -0400, Mark Salter wrote: > >> > The following happens when trying to run a kvm guest on a kernel > >> > configured for 64k pages. This doesn't happen with 4k pages: > >> > > >> > BUG: failure at include/linux/mm.h:297/put_page_testzero()! > >> > Kernel panic - not syncing: BUG! > >> > CPU: 2 PID: 4228 Comm: qemu-system-aar Tainted: GF 3.13.0-0.rc7.31.sa2.k32v1.aarch64.debug #1 > >> > Call trace: > >> > [] dump_backtrace+0x0/0x16c > >> > [] show_stack+0x14/0x1c > >> > [] dump_stack+0x84/0xb0 > >> > [] panic+0xf4/0x220 > >> > [] free_reserved_area+0x0/0x110 > >> > [] free_pages+0x50/0x88 > >> > [] kvm_free_stage2_pgd+0x30/0x40 > >> > [] kvm_arch_destroy_vm+0x18/0x44 > >> > [] kvm_put_kvm+0xf0/0x184 > >> > [] kvm_vm_release+0x10/0x1c > >> > [] __fput+0xb0/0x288 > >> > [] ____fput+0xc/0x14 > >> > [] task_work_run+0xa8/0x11c > >> > [] do_notify_resume+0x54/0x58 > >> > > >> > In arch/arm/kvm/mmu.c:unmap_range(), we end up doing an extra put_page() > >> > on the stage2 pgd which leads to the BUG in put_page_testzero(). This > >> > happens because a pud_huge() test in unmap_range() returns true when it > >> > should always be false with 2-level pages tables used by 64k pages. > >> > This patch removes support for huge puds if 2-level pagetables are > >> > being used. > >> > >> Hi Mark, > >> I'm still catching up with myself, sorry (was off sick for a couple > >> of days)... > >> > >> I thought unmap_range was going to be changed? > >> Does the following help things? > >> https://lists.cs.columbia.edu/pipermail/kvmarm/2014-May/009388.html > > > > No, I get the same BUG. Regardless, pud_huge() should always return > > false for 2-level pagetables, right? > > Okay, thanks for giving that a go. > > Yeah I agree for 64K granule it doesn't make sense to have a huge_pud. > The patch looks sound now, but checking for a folded pmd may run into > problems if/when we get to 3-levels and 64K pages in future. > > Perhaps checking for PAGE_SHIFT==12 (or something similar) would be a > bit more robust? > I don't think testing based on granule size is generally correct either. Maybe support for 3-level page tables with 64k granule gets added as an option. That would break the pagesize based test. With a folded pmd, we know there is no pud, so pud_huge() should always be false.