Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes

From: Dan Williams <dan.j.williams@intel.com>
To: David Hildenbrand <david@redhat.com>
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Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes
Date: Tue, 5 Nov 2019 15:02:40 -0800	[thread overview]
Message-ID: <CAPcyv4htPCeui80fOOno+7AFo3V-=VEiWkAv8j+-Kkad+UnFGQ@mail.gmail.com> (raw)
In-Reply-To: <ed89cd61-7c45-8c9c-ffeb-f27b1872bd7a@redhat.com>

On Tue, Nov 5, 2019 at 12:31 PM David Hildenbrand <david@redhat.com> wrote:
>
> >>> 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.
>
> Yes, it does take a reference AFAIKs. E.g.,
>
> mm/gup.c:gup_pte_range():
> ...
>                 if (pte_devmap(pte)) {
>                         if (unlikely(flags & FOLL_LONGTERM))
>                                 goto pte_unmap;
>
>                         pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
>                         if (unlikely(!pgmap)) {
>                                 undo_dev_pagemap(nr, nr_start, pages);
>                                 goto pte_unmap;
>                         }
>                 } else if (pte_special(pte))
>                         goto pte_unmap;
>
>                 VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>                 page = pte_page(pte);
>
>                 head = try_get_compound_head(page, 1);
>
> try_get_compound_head() will increment the reference count.
>
> >
> >>> 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.
>
> What about
>
> void kvm_get_pfn(kvm_pfn_t pfn)
> {
>         if (!kvm_is_reserved_pfn(pfn))
>                 get_page(pfn_to_page(pfn));
> }
>
> Is a pure get_page() sufficient in case of ZONE_DEVICE?
> (asking because of the live references obtained via
> get_dev_pagemap(pte_pfn(pte), pgmap) in mm/gup.c:gup_pte_range()
> somewhat confuse me :) )

It is not sufficient. PTE_DEVMAP is there to tell the gup path "be
careful, this pfn has device-lifetime, make sure the device is pinned
and not actively in the process of dying before pinning any pages
associated with this device".

However, if kvm_get_pfn() is honoring kvm_is_reserved_pfn() that
returns true for ZONE_DEVICE, I'm missing how it is messing up the
reference counts.

> > 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.