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Re: [Xen-devel] [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes
- To: Dan Williams <dan.j.williams@xxxxxxxxx>
- From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
- Date: Tue, 5 Nov 2019 15:13:16 -0800
- Cc: linux-hyperv@xxxxxxxxxxxxxxx, Michal Hocko <mhocko@xxxxxxxx>, Radim Krčmář <rkrcmar@xxxxxxxxxx>, KVM list <kvm@xxxxxxxxxxxxxxx>, David Hildenbrand <david@xxxxxxxxxx>, KarimAllah Ahmed <karahmed@xxxxxxxxx>, Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Alexander Duyck <alexander.duyck@xxxxxxxxx>, Michal Hocko <mhocko@xxxxxxxxxx>, Paul Mackerras <paulus@xxxxxxxxxx>, Linux MM <linux-mm@xxxxxxxxx>, Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>, Paul Mackerras <paulus@xxxxxxxxx>, Michael Ellerman <mpe@xxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Wanpeng Li <wanpengli@xxxxxxxxxxx>, Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>, "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, devel@xxxxxxxxxxxxxxxxxxxx, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>, "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>, Joerg Roedel <joro@xxxxxxxxxx>, X86 ML <x86@xxxxxxxxxx>, YueHaibing <yuehaibing@xxxxxxxxxx>, "Matthew Wilcox \(Oracle\)" <willy@xxxxxxxxxxxxx>, Mike Rapoport <rppt@xxxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Vlastimil Babka <vbabka@xxxxxxx>, Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>, "Isaac J. Manjarres" <isaacm@xxxxxxxxxxxxxx>, Matt Sickler <Matt.Sickler@xxxxxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Anshuman Khandual <anshuman.khandual@xxxxxxx>, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>, Sasha Levin <sashal@xxxxxxxxxx>, kvm-ppc@xxxxxxxxxxxxxxx, Qian Cai <cai@xxxxxx>, Alex Williamson <alex.williamson@xxxxxxxxxx>, Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Nicholas Piggin <npiggin@xxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>, Allison Randal <allison@xxxxxxxxxxx>, Jim Mattson <jmattson@xxxxxxxxxx>, Christophe Leroy <christophe.leroy@xxxxxx>, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>, Adam Borowski <kilobyte@xxxxxxxxxx>, Cornelia Huck <cohuck@xxxxxxxxxx>, Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Johannes Weiner <hannes@xxxxxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, linuxppc-dev <linuxppc-dev@xxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 05 Nov 2019 23:13:37 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Nov 05, 2019 at 03:02:40PM -0800, Dan Williams wrote:
> On Tue, Nov 5, 2019 at 12:31 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > 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.
> >
> > The x86 KVM MMU code is one of the ugliest code I know (sorry, but it
> > had to be said :/ ). Luckily, this should be independent of the
> > PG_reserved thingy AFAIKs.
>
> Both transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte()
> are honoring kvm_is_reserved_pfn(), so again I'm missing where the
> page count gets mismanaged and leads to the reported hang.
When mapping pages into the guest, KVM gets the page via gup(), which
increments the page count for ZONE_DEVICE pages. But KVM puts the page
using kvm_release_pfn_clean(), which skips put_page() if PageReserved()
and so never puts its reference to ZONE_DEVICE pages.
My transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte()
comments were for a post-patch/series scenario wheren PageReserved() is
no longer true for ZONE_DEVICE pages.
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