Re: [PATCH] Handle vma regions with no backing page

[Avi Kivity <avi@qumranet.com>]

Ben-Ami Yassour wrote:
> Anthony Liguori <aliguori@us.ibm.com> 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?
>

Looks like we need to reintroduce a refcount bit in the pte, and check 
the page using the VMA.

Nick Piggin's lockless pagecache patches, which have the same issue, 
also introduce a pte_special bit.  We could follow a similar route.

http://www.mail-archive.com/linux-arch@vger.kernel.org/msg04789.html

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.