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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-hyperv@vger.kernel.org, "Michal Hocko" <mhocko@suse.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"KVM list" <kvm@vger.kernel.org>,
	"David Hildenbrand" <david@redhat.com>,
	"KarimAllah Ahmed" <karahmed@amazon.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Alexander Duyck" <alexander.duyck@gmail.com>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Pavel Tatashin" <pavel.tatashin@microsoft.com>,
	"Paul Mackerras" <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Alexander Duyck" <alexander.h.duyck@linux.intel.com>,
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	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"Sasha Levin" <sashal@kernel.org>,
	kvm-ppc@vger.kernel.org, "Qian Cai" <cai@lca.pw>,
	"Alex Williamson" <alex.williamson@redhat.com>,
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	xen-devel <xen-devel@lists.xenproject.org>,
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	"Allison Randal" <allison@lohutok.net>,
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	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes
Date: Tue, 5 Nov 2019 16:03:15 -0800
Message-ID: <20191106000315.GI23297@linux.intel.com> (raw)
In-Reply-To: <CAPcyv4i7tnjyghYhSjK8fxUu8Qkdc2RuD9kUwJcKEMDzOf51ng@mail.gmail.com>

On Tue, Nov 05, 2019 at 03:43:29PM -0800, Dan Williams wrote:
> On Tue, Nov 5, 2019 at 3:30 PM Dan Williams <dan.j.williams@intel.com> wrote:
> >
> > On Tue, Nov 5, 2019 at 3:13 PM Sean Christopherson
> > <sean.j.christopherson@intel.com> wrote:
> > >
> > > 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@redhat.com> 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.
> >
> > Oh, yeah, that's busted.
> 
> Ugh, it's extra busted because every other gup user in the kernel
> tracks the pages resulting from gup and puts them (put_page()) when
> they are done. KVM wants to forget about whether it did a gup to get
> the page and optionally trigger put_page() based purely on the pfn.
> Outside of VFIO device assignment that needs pages pinned for DMA, why
> does KVM itself need to pin pages? If pages are pinned over a return
> to userspace that needs to be a FOLL_LONGTERM gup.

Short answer, KVM pins the page to ensure correctness with respect to the
primary MMU invalidating the associated host virtual address, e.g. when
the page is being migrated or unmapped from host userspace.

The main use of gup() is to handle guest page faults and map pages into
the guest, i.e. into KVM's secondary MMU.  KVM uses gup() to both get the
PFN and to temporarily pin the page.  The pin is held just long enough to
guaranteed that any invalidation via the mmu_notifier will be stalled
until after KVM finishes installing the page into the secondary MMU, i.e.
the pin is short-term and not held across a return to userspace or entry
into the guest.  When a subsequent mmu_notifier invalidation occurs, KVM
pulls the PFN from the secondary MMU and uses that to update accessed
and dirty bits in the host.

There are a few other KVM flows that eventually call into gup(), but those
are "traditional" short-term pins and use put_page() directly.

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-24 12:09 [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 01/10] mm/memory_hotplug: Don't allow to online/offline memory blocks with holes David Hildenbrand
2019-11-05  1:30   ` Dan Williams
2019-11-05  9:31     ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 02/10] KVM: x86/mmu: Prepare kvm_is_mmio_pfn() for PG_reserved changes David Hildenbrand
2019-11-05  1:37   ` Dan Williams
2019-11-05 11:09     ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() " David Hildenbrand
2019-11-05  4:38   ` Dan Williams
2019-11-05  9:17     ` David Hildenbrand
2019-11-05  9:49       ` David Hildenbrand
2019-11-05 10:02         ` David Hildenbrand
2019-11-05 16:00           ` Sean Christopherson
2019-11-05 20:30             ` David Hildenbrand
2019-11-05 22:22               ` Sean Christopherson
2019-11-05 23:02               ` Dan Williams
2019-11-05 23:13                 ` Sean Christopherson
2019-11-05 23:30                   ` Dan Williams
2019-11-05 23:42                     ` Sean Christopherson
2019-11-05 23:43                     ` Dan Williams
2019-11-06  0:03                       ` Sean Christopherson [this message]
2019-11-06  0:08                         ` Dan Williams
2019-11-06  6:56                           ` David Hildenbrand
2019-11-06 16:09                             ` Sean Christopherson
2019-10-24 12:09 ` [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() " David Hildenbrand
2019-11-07 15:40   ` Dan Williams
2019-11-07 18:22     ` David Hildenbrand
2019-11-07 22:07       ` David Hildenbrand
2019-11-08  5:09         ` Dan Williams
2019-11-08  7:14           ` David Hildenbrand
2019-11-08 10:21             ` David Hildenbrand
2019-11-08 18:29               ` Dan Williams
2019-11-08 23:01                 ` David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 05/10] powerpc/book3s: Prepare kvmppc_book3s_instantiate_page() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 06/10] powerpc/64s: Prepare hash_page_do_lazy_icache() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 07/10] powerpc/mm: Prepare maybe_pte_to_page() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 08/10] x86/mm: Prepare __ioremap_check_ram() " David Hildenbrand
2019-10-24 12:09 ` [PATCH v1 09/10] mm/memory_hotplug: Don't mark pages PG_reserved when initializing the memmap David Hildenbrand
2019-11-04 22:44   ` Boris Ostrovsky
2019-11-05 10:18     ` David Hildenbrand
2019-11-05 16:06       ` Boris Ostrovsky
2019-10-24 12:09 ` [PATCH v1 10/10] mm/usercopy.c: Update comment in check_page_span() regarding ZONE_DEVICE David Hildenbrand
2019-11-01 19:24 ` [PATCH v1 00/10] mm: Don't mark hotplugged pages PG_reserved (including ZONE_DEVICE) David Hildenbrand

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