On Tue, 2008-06-03 at 13:39 +0200, Andrea Arcangeli wrote:
> On Tue, Jun 03, 2008 at 02:17:55PM +0300, Ben-Ami Yassour wrote:
> > Anthony Liguori <[EMAIL PROTECTED]> wrote on 04/29/2008 05:32:09 PM:
> >
> >> Subject
> >>
> >> [PATCH] Handle vma regions with no backing page
> >>
> >> This patch allows VMA's that contain no backing page to be used for guest
> >> memory. This is a drop-in replacement for Ben-Ami's first page in his
> >> direct
> >> mmio series. Here, we continue to allow mmio pages to be represented in
> >> the
> >> rmap.
> >
> >> struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
> >> {
> >> - return pfn_to_page(gfn_to_pfn(kvm, gfn));
> >> + pfn_t pfn;
> >> +
> >> + pfn = gfn_to_pfn(kvm, gfn);
> >> + if (pfn_valid(pfn))
> >> + return pfn_to_page(pfn);
> >> +
> >> + return NULL;
> >> }
> >
> > We noticed that pfn_valid does not always works as expected by this patch
> > to indicate that a pfn has a backing page.
> > We have seen a case where CONFIG_NUMA was not set and then where pfn_valid
> > returned 1 for an mmio pfn.
> > We then changed the config file with CONFIG_NUMA set and it worked fine as
> > expected (since a different implementation of pfn_valid was used).
> >
> > How should we overcome this issue?
>
> There's a page_is_ram() too, but that's the e820 map check and it
> means it's RAM not that there's a page backing store. Certainly if
> it's not ram we should go ahead with just the pfn but it'd be a
> workaround.
>
> I really think it'd be better off to fix pfn_valid to work for NUMA.
It does work for NUMA, it does not work without the NUMA option.
> I
> can't see how pfn_valid can be ok to return true when there's no
> backing page... Probably pfn_valid was used for debugging todate, but
> if you check vm_normal_page you'll see that it is not used just for
> debugging and it seems VM_MIXEDMAP will break as much as KVM.
>
> I can't see how VM_MIXEDMAP can be sane doing pfn_to_page(pfn) and
> pretending this is a normal page, when there's no 'struct page'
> backing the pfn.
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