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

[David Hildenbrand <david@xxxxxxxxxx>]

> > > 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 :) )

> 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.
--

Thanks,

David / dhildenb


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