From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 87AECC5DF63 for ; Wed, 6 Nov 2019 20:26:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 60F74217D7 for ; Wed, 6 Nov 2019 20:26:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732496AbfKFU02 (ORCPT ); Wed, 6 Nov 2019 15:26:28 -0500 Received: from mga02.intel.com ([134.134.136.20]:55890 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727721AbfKFU02 (ORCPT ); Wed, 6 Nov 2019 15:26:28 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by orsmga101.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 06 Nov 2019 12:26:27 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.68,275,1569308400"; d="scan'208";a="353587051" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by orsmga004.jf.intel.com with ESMTP; 06 Nov 2019 12:26:27 -0800 Date: Wed, 6 Nov 2019 12:26:27 -0800 From: Sean Christopherson To: Dan Williams Cc: Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , KVM list , Linux Kernel Mailing List , Adam Borowski , David Hildenbrand Subject: Re: [PATCH 1/2] KVM: MMU: Do not treat ZONE_DEVICE pages as being reserved Message-ID: <20191106202627.GF16249@linux.intel.com> References: <20191106170727.14457-1-sean.j.christopherson@intel.com> <20191106170727.14457-2-sean.j.christopherson@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 06, 2019 at 10:04:22AM -0800, Dan Williams wrote: > On Wed, Nov 6, 2019 at 9:07 AM Sean Christopherson > wrote: > > > > Explicitly exempt ZONE_DEVICE pages from kvm_is_reserved_pfn() and > > instead manually handle ZONE_DEVICE on a case-by-case basis. For things > > like page refcounts, KVM needs to treat ZONE_DEVICE pages like normal > > pages, e.g. put pages grabbed via gup(). But KVM needs special handling > > in other flows where ZONE_DEVICE pages lack the underlying machinery, > > e.g. when setting accessed/dirty bits and shifting refcounts for > > transparent huge pages. > > > > This fixes a hang reported by Adam Borowski[*] in dev_pagemap_cleanup() > > when running a KVM guest backed with /dev/dax memory, as KVM straight up > > doesn't put any references to ZONE_DEVICE pages acquired by gup(). > > > > [*] http://lkml.kernel.org/r/20190919115547.GA17963@angband.pl > > > > Reported-by: Adam Borowski > > Debugged-by: David Hildenbrand > > Cc: Dan Williams > > Cc: stable@vger.kernel.org > > Signed-off-by: Sean Christopherson > > --- > > arch/x86/kvm/mmu.c | 8 ++++---- > > include/linux/kvm_host.h | 1 + > > virt/kvm/kvm_main.c | 19 +++++++++++++++---- > > 3 files changed, 20 insertions(+), 8 deletions(-) > > > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c > > index 24c23c66b226..bf82b1f2e834 100644 > > --- a/arch/x86/kvm/mmu.c > > +++ b/arch/x86/kvm/mmu.c > > @@ -3306,7 +3306,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu, > > * here. > > */ > > if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) && > > - level == PT_PAGE_TABLE_LEVEL && > > + !kvm_is_zone_device_pfn(pfn) && level == PT_PAGE_TABLE_LEVEL && > > PageTransCompoundMap(pfn_to_page(pfn)) && > > !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) { > > unsigned long mask; > > @@ -5914,9 +5914,9 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm, > > * the guest, and the guest page table is using 4K page size > > * mapping if the indirect sp has level = 1. > > */ > > - if (sp->role.direct && > > - !kvm_is_reserved_pfn(pfn) && > > - PageTransCompoundMap(pfn_to_page(pfn))) { > > + if (sp->role.direct && !kvm_is_reserved_pfn(pfn) && > > + !kvm_is_zone_device_pfn(pfn) && > > + PageTransCompoundMap(pfn_to_page(pfn))) { > > pte_list_remove(rmap_head, sptep); > > > > if (kvm_available_flush_tlb_with_range()) > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h > > index a817e446c9aa..4ad1cd7d2d4d 100644 > > --- a/include/linux/kvm_host.h > > +++ b/include/linux/kvm_host.h > > @@ -966,6 +966,7 @@ int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu); > > void kvm_vcpu_kick(struct kvm_vcpu *vcpu); > > > > bool kvm_is_reserved_pfn(kvm_pfn_t pfn); > > +bool kvm_is_zone_device_pfn(kvm_pfn_t pfn); > > > > struct kvm_irq_ack_notifier { > > struct hlist_node link; > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index b8534c6b8cf6..0a781b1fb8f0 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -151,12 +151,23 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm, > > > > bool kvm_is_reserved_pfn(kvm_pfn_t pfn) > > { > > + /* > > + * ZONE_DEVICE pages currently set PG_reserved, but from a refcounting > > + * perspective they are "normal" pages, albeit with slightly different > > + * usage rules. > > + */ > > if (pfn_valid(pfn)) > > - return PageReserved(pfn_to_page(pfn)); > > + return PageReserved(pfn_to_page(pfn)) && > > + !is_zone_device_page(pfn_to_page(pfn)); > > This is racy unless you can be certain that the pfn and resulting page > has already been pinned by get_user_pages(). This is why I told David > to steer clear of adding more is_zone_device_page() usage, it's > difficult to audit. Without an existing pin the metadata to determine > whether a page is ZONE_DEVICE or not could be in the process of being > torn down. Ouch, that's brutal. Makes perfect sense why you'd want to avoid is_zone_device_page(). > Ideally KVM would pass around a struct { struct page *page, > unsigned long pfn } tuple so it would not have to do this "recall > context" dance on every pfn operation. Implementing something of that nature is doable, but it's going to be a significant overhaul as it will require updating every architecture. So yeah, let's go with David's suggestion of fixing the immediate bug with your quick and dirty change and punting a more complete rework to future cleanup. There are still multiple KVM flows that don't correctly handle ZONE_DEVICE, especially outside of x86, but I don't think there's a quick fix for those issues.