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 9E023C5DF60 for ; Tue, 5 Nov 2019 20:28:18 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 4242A21929 for ; Tue, 5 Nov 2019 20:28:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4242A21929 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4771Sw1zJSzF1dt for ; Wed, 6 Nov 2019 07:28:16 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=intel.com (client-ip=134.134.136.31; helo=mga06.intel.com; envelope-from=sean.j.christopherson@intel.com; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=intel.com Received: from mga06.intel.com (mga06.intel.com [134.134.136.31]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 476vWb3R1zzF4yH for ; Wed, 6 Nov 2019 03:00:04 +1100 (AEDT) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 05 Nov 2019 08:00:01 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.68,271,1569308400"; d="scan'208";a="205019568" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by orsmga003.jf.intel.com with ESMTP; 05 Nov 2019 08:00:00 -0800 Date: Tue, 5 Nov 2019 08:00:00 -0800 From: Sean Christopherson To: David Hildenbrand Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes Message-ID: <20191105160000.GC8128@linux.intel.com> References: <20191024120938.11237-1-david@redhat.com> <20191024120938.11237-4-david@redhat.com> <01adb4cb-6092-638c-0bab-e61322be7cf5@redhat.com> <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Mailman-Approved-At: Wed, 06 Nov 2019 07:24:41 +1100 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-hyperv@vger.kernel.org, Michal Hocko , Radim =?utf-8?B?S3LEjW3DocWZ?= , KVM list , Pavel Tatashin , KarimAllah Ahmed , Dave Hansen , Alexander Duyck , Michal Hocko , Linux MM , Paul Mackerras , "H. Peter Anvin" , Wanpeng Li , Alexander Duyck , "K. Y. Srinivasan" , Boris Ostrovsky , Kees Cook , devel@driverdev.osuosl.org, Stefano Stabellini , Stephen Hemminger , "Aneesh Kumar K.V" , Joerg Roedel , X86 ML , YueHaibing , "Matthew Wilcox \(Oracle\)" , Mike Rapoport , Peter Zijlstra , Ingo Molnar , Vlastimil Babka , Anthony Yznaga , Oscar Salvador , "Isaac J. Manjarres" , Matt Sickler , Juergen Gross , Anshuman Khandual , Haiyang Zhang , Sasha Levin , kvm-ppc@vger.kernel.org, Qian Cai , Alex Williamson , Mike Rapoport , Borislav Petkov , Nicholas Piggin , Andy Lutomirski , xen-devel , Dan Williams , Vitaly Kuznetsov , Allison Randal , Jim Mattson , Mel Gorman , Adam Borowski , Cornelia Huck , Pavel Tatashin , Linux Kernel Mailing List , Thomas Gleixner , Johannes Weiner , Paolo Bonzini , Andrew Morton , linuxppc-dev Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Nov 05, 2019 at 11:02:46AM +0100, David Hildenbrand wrote: > On 05.11.19 10:49, David Hildenbrand wrote: > >On 05.11.19 10:17, David Hildenbrand wrote: > >>On 05.11.19 05:38, Dan Williams wrote: > >>>On Thu, Oct 24, 2019 at 5:11 AM David Hildenbrand wrote: > >>>> > >>>>Right now, ZONE_DEVICE memory is always set PG_reserved. We want to > >>>>change that. > >>>> > >>>>KVM has this weird use case that you can map anything from /dev/mem > >>>>into the guest. pfn_valid() is not a reliable check whether the memmap > >>>>was initialized and can be touched. pfn_to_online_page() makes sure > >>>>that we have an initialized memmap (and don't have ZONE_DEVICE memory). > >>>> > >>>>Rewrite kvm_is_reserved_pfn() to make sure the function produces the > >>>>same result once we stop setting ZONE_DEVICE pages PG_reserved. > >>>> > >>>>Cc: Paolo Bonzini > >>>>Cc: "Radim Krčmář" > >>>>Cc: Michal Hocko > >>>>Cc: Dan Williams > >>>>Cc: KarimAllah Ahmed > >>>>Signed-off-by: David Hildenbrand > >>>>--- > >>>> virt/kvm/kvm_main.c | 10 ++++++++-- > >>>> 1 file changed, 8 insertions(+), 2 deletions(-) > >>>> > >>>>diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > >>>>index e9eb666eb6e8..9d18cc67d124 100644 > >>>>--- a/virt/kvm/kvm_main.c > >>>>+++ b/virt/kvm/kvm_main.c > >>>>@@ -151,9 +151,15 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm, > >>>> > >>>> bool kvm_is_reserved_pfn(kvm_pfn_t pfn) > >>>> { > >>>>- if (pfn_valid(pfn)) > >>>>- return PageReserved(pfn_to_page(pfn)); > >>>>+ struct page *page = pfn_to_online_page(pfn); > >>>> > >>>>+ /* > >>>>+ * We treat any pages that are not online (not managed by the buddy) > >>>>+ * as reserved - this includes ZONE_DEVICE pages and pages without > >>>>+ * a memmap (e.g., mapped via /dev/mem). > >>>>+ */ > >>>>+ if (page) > >>>>+ return PageReserved(page); > >>>> return true; > >>>> } > >>> > >>>So after this all the pfn_valid() usage in kvm_main.c is replaced with > >>>pfn_to_online_page()? Looks correct to me. > >>> > >>>However, I'm worried that kvm is taking reference on ZONE_DEVICE pages > >>>through some other path resulting in this: > >>> > >>> https://lore.kernel.org/linux-nvdimm/20190919154708.GA24650@angband.pl/ > >>> > >>>I'll see if this patch set modulates or maintains that failure mode. > >>> > >> > >>I'd assume that the behavior is unchanged. Ithink we get a reference to > >>these ZONE_DEVICE pages via __get_user_pages_fast() and friends in > >>hva_to_pfn_fast() and friends in virt/kvm/kvm_main.c > >> > > > >I think I know what's going wrong: > > > >Pages that are pinned via gfn_to_pfn() and friends take a references, > >however are often released via > >kvm_release_pfn_clean()/kvm_release_pfn_dirty()/kvm_release_page_clean()... > > > > > >E.g., in arch/x86/kvm/x86.c:reexecute_instruction() > > > >... > >pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa)); > >... > >kvm_release_pfn_clean(pfn); > > > > > > > >void kvm_release_pfn_clean(kvm_pfn_t pfn) > >{ > > if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn)) > > put_page(pfn_to_page(pfn)); > >} > > > >This function makes perfect sense as the counterpart for kvm_get_pfn(): > > > >void kvm_get_pfn(kvm_pfn_t pfn) > >{ > > if (!kvm_is_reserved_pfn(pfn)) > > get_page(pfn_to_page(pfn)); > >} > > > > > >As all ZONE_DEVICE pages are currently reserved, pages pinned via > >gfn_to_pfn() and friends will often not see a put_page() AFAIKS. Assuming gup() takes a reference for ZONE_DEVICE pages, yes, this is a KVM bug. > >Now, my patch does not change that, the result of > >kvm_is_reserved_pfn(pfn) will be unchanged. A proper fix for that would > >probably be > > > >a) To drop the reference to ZONE_DEVICE pages in gfn_to_pfn() and > >friends, after you successfully pinned the pages. (not sure if that's > >the right thing to do but you're the expert) > > > >b) To not use kvm_release_pfn_clean() and friends on pages that were > >definitely pinned. This is already KVM's intent, i.e. the purpose of the PageReserved() check is simply to avoid putting a non-existent reference. The problem is that KVM assumes pages with PG_reserved set are never pinned, which AFAICT was true when the code was first added. > (talking to myself, sorry) > > Thinking again, dropping this patch from this series could effectively also > fix that issue. E.g., kvm_release_pfn_clean() and friends would always do a > put_page() if "pfn_valid() and !PageReserved()", so after patch 9 also on > ZONDE_DEVICE pages. Yeah, this appears to be the correct fix. > But it would have side effects that might not be desired. E.g.,: > > 1. kvm_pfn_to_page() would also return ZONE_DEVICE pages (might even be the > right thing to do). This should be ok, at least on x86. There are only three users of kvm_pfn_to_page(). Two of those are on allocations that are controlled by KVM and are guaranteed to be vanilla MAP_ANONYMOUS. The third is on guest memory when running a nested guest, and in that case supporting ZONE_DEVICE memory is desirable, i.e. KVM should play nice with a guest that is backed by ZONE_DEVICE memory. > 2. kvm_set_pfn_dirty() would also set ZONE_DEVICE pages dirty (might be > okay) This is ok from a KVM perspective. The scarier code (for me) is transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte(), as I don't at all understand the interaction between THP and _PAGE_DEVMAP. 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, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 E45E6C5DF60 for ; Tue, 5 Nov 2019 16:01:50 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id BDB7220663 for ; Tue, 5 Nov 2019 16:01:50 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BDB7220663 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1iS1Go-00032C-0N; Tue, 05 Nov 2019 16:01:26 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1iS1FU-0002m3-SR for xen-devel@lists.xenproject.org; Tue, 05 Nov 2019 16:00:04 +0000 X-Inumbo-ID: 533c8b00-ffe5-11e9-a19b-12813bfff9fa Received: from mga09.intel.com (unknown [134.134.136.24]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 533c8b00-ffe5-11e9-a19b-12813bfff9fa; Tue, 05 Nov 2019 16:00:02 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga102.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 05 Nov 2019 08:00:01 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.68,271,1569308400"; d="scan'208";a="205019568" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by orsmga003.jf.intel.com with ESMTP; 05 Nov 2019 08:00:00 -0800 Date: Tue, 5 Nov 2019 08:00:00 -0800 From: Sean Christopherson To: David Hildenbrand Message-ID: <20191105160000.GC8128@linux.intel.com> References: <20191024120938.11237-1-david@redhat.com> <20191024120938.11237-4-david@redhat.com> <01adb4cb-6092-638c-0bab-e61322be7cf5@redhat.com> <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Mailman-Approved-At: Tue, 05 Nov 2019 16:01:24 +0000 Subject: Re: [Xen-devel] [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: linux-hyperv@vger.kernel.org, Michal Hocko , Radim =?utf-8?B?S3LEjW3DocWZ?= , KVM list , Pavel Tatashin , KarimAllah Ahmed , Benjamin Herrenschmidt , Dave Hansen , Alexander Duyck , Michal Hocko , Paul Mackerras , Linux MM , Paul Mackerras , Michael Ellerman , "H. Peter Anvin" , Wanpeng Li , Alexander Duyck , "K. Y. Srinivasan" , Boris Ostrovsky , Kees Cook , devel@driverdev.osuosl.org, Stefano Stabellini , Stephen Hemminger , "Aneesh Kumar K.V" , Joerg Roedel , X86 ML , YueHaibing , "Matthew Wilcox \(Oracle\)" , Mike Rapoport , Peter Zijlstra , Ingo Molnar , Vlastimil Babka , Anthony Yznaga , Oscar Salvador , "Isaac J. Manjarres" , Matt Sickler , Juergen Gross , Anshuman Khandual , Haiyang Zhang , Sasha Levin , kvm-ppc@vger.kernel.org, Qian Cai , Alex Williamson , Mike Rapoport , Borislav Petkov , Nicholas Piggin , Andy Lutomirski , xen-devel , Dan Williams , Vitaly Kuznetsov , Allison Randal , Jim Mattson , Christophe Leroy , Mel Gorman , Adam Borowski , Cornelia Huck , Pavel Tatashin , Linux Kernel Mailing List , Thomas Gleixner , Johannes Weiner , Paolo Bonzini , Andrew Morton , linuxppc-dev Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" T24gVHVlLCBOb3YgMDUsIDIwMTkgYXQgMTE6MDI6NDZBTSArMDEwMCwgRGF2aWQgSGlsZGVuYnJh bmQgd3JvdGU6Cj4gT24gMDUuMTEuMTkgMTA6NDksIERhdmlkIEhpbGRlbmJyYW5kIHdyb3RlOgo+ ID5PbiAwNS4xMS4xOSAxMDoxNywgRGF2aWQgSGlsZGVuYnJhbmQgd3JvdGU6Cj4gPj5PbiAwNS4x MS4xOSAwNTozOCwgRGFuIFdpbGxpYW1zIHdyb3RlOgo+ID4+Pk9uIFRodSwgT2N0IDI0LCAyMDE5 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2019 08:00:00 -0800 From: Sean Christopherson To: David Hildenbrand Cc: Dan Williams , Linux Kernel Mailing List , Linux MM , Michal Hocko , Andrew Morton , kvm-ppc@vger.kernel.org, linuxppc-dev , KVM list , linux-hyperv@vger.kernel.org, devel@driverdev.osuosl.org, xen-devel , X86 ML , Alexander Duyck , Alexander Duyck , Alex Williamson , Allison Randal , Andy Lutomirski , "Aneesh Kumar K.V" , Anshuman Khandual , Anthony Yznaga , Benjamin Herrenschmidt , Borislav Petkov , Boris Ostrovsky , Christophe Leroy , Cornelia Huck , Dave Hansen , Haiyang Zhang , "H. Peter Anvin" , Ingo Molnar , "Isaac J. Manjarres" , Jim Mattson , Joerg Roedel , Johannes Weiner , Juergen Gross , KarimAllah Ahmed , Kees Cook , "K. Y. Srinivasan" , "Matthew Wilcox (Oracle)" , Matt Sickler , Mel Gorman , Michael Ellerman , Michal Hocko , Mike Rapoport , Mike Rapoport , Nicholas Piggin , Oscar Salvador , Paolo Bonzini , Paul Mackerras , Paul Mackerras , Pavel Tatashin , Pavel Tatashin , Peter Zijlstra , Qian Cai , Radim =?utf-8?B?S3LEjW3DocWZ?= , Sasha Levin , Stefano Stabellini , Stephen Hemminger , Thomas Gleixner , Vitaly Kuznetsov , Vlastimil Babka , Wanpeng Li , YueHaibing , Adam Borowski Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes Message-ID: <20191105160000.GC8128@linux.intel.com> References: <20191024120938.11237-1-david@redhat.com> <20191024120938.11237-4-david@redhat.com> <01adb4cb-6092-638c-0bab-e61322be7cf5@redhat.com> <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Nov 05, 2019 at 11:02:46AM +0100, David Hildenbrand wrote: > On 05.11.19 10:49, David Hildenbrand wrote: > >On 05.11.19 10:17, David Hildenbrand wrote: > >>On 05.11.19 05:38, Dan Williams wrote: > >>>On Thu, Oct 24, 2019 at 5:11 AM David Hildenbrand = wrote: > >>>> > >>>>Right now, ZONE_DEVICE memory is always set PG_reserved. We want to > >>>>change that. > >>>> > >>>>KVM has this weird use case that you can map anything from /dev/mem > >>>>into the guest. pfn_valid() is not a reliable check whether the mem= map > >>>>was initialized and can be touched. pfn_to_online_page() makes sure > >>>>that we have an initialized memmap (and don't have ZONE_DEVICE memo= ry). > >>>> > >>>>Rewrite kvm_is_reserved_pfn() to make sure the function produces th= e > >>>>same result once we stop setting ZONE_DEVICE pages PG_reserved. > >>>> > >>>>Cc: Paolo Bonzini > >>>>Cc: "Radim Kr=C4=8Dm=C3=A1=C5=99" > >>>>Cc: Michal Hocko > >>>>Cc: Dan Williams > >>>>Cc: KarimAllah Ahmed > >>>>Signed-off-by: David Hildenbrand > >>>>--- > >>>> virt/kvm/kvm_main.c | 10 ++++++++-- > >>>> 1 file changed, 8 insertions(+), 2 deletions(-) > >>>> > >>>>diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > >>>>index e9eb666eb6e8..9d18cc67d124 100644 > >>>>--- a/virt/kvm/kvm_main.c > >>>>+++ b/virt/kvm/kvm_main.c > >>>>@@ -151,9 +151,15 @@ __weak int kvm_arch_mmu_notifier_invalidate_ra= nge(struct kvm *kvm, > >>>> > >>>> bool kvm_is_reserved_pfn(kvm_pfn_t pfn) > >>>> { > >>>>- if (pfn_valid(pfn)) > >>>>- return PageReserved(pfn_to_page(pfn)); > >>>>+ struct page *page =3D pfn_to_online_page(pfn); > >>>> > >>>>+ /* > >>>>+ * We treat any pages that are not online (not managed by t= he buddy) > >>>>+ * as reserved - this includes ZONE_DEVICE pages and pages = without > >>>>+ * a memmap (e.g., mapped via /dev/mem). > >>>>+ */ > >>>>+ if (page) > >>>>+ return PageReserved(page); > >>>> return true; > >>>> } > >>> > >>>So after this all the pfn_valid() usage in kvm_main.c is replaced wi= th > >>>pfn_to_online_page()? Looks correct to me. > >>> > >>>However, I'm worried that kvm is taking reference on ZONE_DEVICE pag= es > >>>through some other path resulting in this: > >>> > >>> https://lore.kernel.org/linux-nvdimm/20190919154708.GA24650@a= ngband.pl/ > >>> > >>>I'll see if this patch set modulates or maintains that failure mode. > >>> > >> > >>I'd assume that the behavior is unchanged. Ithink we get a reference = to > >>these ZONE_DEVICE pages via __get_user_pages_fast() and friends in > >>hva_to_pfn_fast() and friends in virt/kvm/kvm_main.c > >> > > > >I think I know what's going wrong: > > > >Pages that are pinned via gfn_to_pfn() and friends take a references, > >however are often released via > >kvm_release_pfn_clean()/kvm_release_pfn_dirty()/kvm_release_page_clean= ()... > > > > > >E.g., in arch/x86/kvm/x86.c:reexecute_instruction() > > > >... > >pfn =3D gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa)); > >... > >kvm_release_pfn_clean(pfn); > > > > > > > >void kvm_release_pfn_clean(kvm_pfn_t pfn) > >{ > > if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn)) > > put_page(pfn_to_page(pfn)); > >} > > > >This function makes perfect sense as the counterpart for kvm_get_pfn()= : > > > >void kvm_get_pfn(kvm_pfn_t pfn) > >{ > > if (!kvm_is_reserved_pfn(pfn)) > > get_page(pfn_to_page(pfn)); > >} > > > > > >As all ZONE_DEVICE pages are currently reserved, pages pinned via > >gfn_to_pfn() and friends will often not see a put_page() AFAIKS. Assuming gup() takes a reference for ZONE_DEVICE pages, yes, this is a KVM bug. > >Now, my patch does not change that, the result of > >kvm_is_reserved_pfn(pfn) will be unchanged. A proper fix for that woul= d > >probably be > > > >a) To drop the reference to ZONE_DEVICE pages in gfn_to_pfn() and > >friends, after you successfully pinned the pages. (not sure if that's > >the right thing to do but you're the expert) > > > >b) To not use kvm_release_pfn_clean() and friends on pages that were > >definitely pinned. This is already KVM's intent, i.e. the purpose of the PageReserved() chec= k is simply to avoid putting a non-existent reference. The problem is that KVM assumes pages with PG_reserved set are never pinned, which AFAICT was true when the code was first added. > (talking to myself, sorry) >=20 > Thinking again, dropping this patch from this series could effectively = also > fix that issue. E.g., kvm_release_pfn_clean() and friends would always = do a > put_page() if "pfn_valid() and !PageReserved()", so after patch 9 also = on > ZONDE_DEVICE pages. Yeah, this appears to be the correct fix. > But it would have side effects that might not be desired. E.g.,: >=20 > 1. kvm_pfn_to_page() would also return ZONE_DEVICE pages (might even be= the > right thing to do). This should be ok, at least on x86. There are only three users of kvm_pfn_to_page(). Two of those are on allocations that are controlled b= y KVM and are guaranteed to be vanilla MAP_ANONYMOUS. The third is on gues= t memory when running a nested guest, and in that case supporting ZONE_DEVI= CE memory is desirable, i.e. KVM should play nice with a guest that is backe= d by ZONE_DEVICE memory. > 2. kvm_set_pfn_dirty() would also set ZONE_DEVICE pages dirty (might be > okay) This is ok from a KVM perspective. The scarier code (for me) is transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte(), as I don't at all understand the interaction between THP and _PAGE_DEVMAP.