question about foreign mapped page

question about foreign mapped page

[schumi.han]

Hi, All!
I met a bug when destroyed the VMX. page_remove_rmap() BUG on
page->_mapcount when QEMU exited. I try to solve this problem but I'm
not familiar with MM code. So I wanna ask you guys several questions and 
hope you can give some clues:
1)If we map a foreign page(DomainU or VMX) to Domain0 through
xc_map_foreign_range, the pfn(in Domain0's memory space)which points to
the mapped mfn should not correlate with any   page structure.  Is  it
right?  I suppose it to be true. The question is when the kernel calls
unmap_page_range, it should try to call vm_normal_page on the related
pte to get the page structure. Which flag will lead vm_normal_page to
return the NULL pointer?
2)IMHO, I expect only the local mapped page, which belongs to the same
domain could be passed to page_remove_rmap and kick out the bug I met!
direct_rmap_pfn_range looks like a suscipious criminal.Is it possible?
But QEMU never used direct_rmap_pfn_range to map local page. So, I'm
really confused!
Thanks for your help!