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. 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.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
