error when getting page from page table in KVM ept violation handler

From: Ruby Zhang
Date: Sat Jan 04 2014 - 15:40:22 EST

I'm trying to get a page structure at KVM ept violation handler (in
tdp_page_fault). My code is invoked when tdp_page_fault calls
__direct_map (where the page fault should have been handled, is it?).
But I get a "unable to handle kerenel paging request" error
occasionally when I try to access the second double word of the page
structure. Here's my code:

Inside __direct_map

for_each_shadow_entry(vcpu, (u64)gfn << PAGE_SHIFT, iterator) {
if (iterator.level == level) {
mmu_set_spte(vcpu, iterator.sptep, ACC_ALL,
write, &emulate, level, gfn, pfn, prefault, map_writeable);
direct_pte_prefetch(vcpu, iterator.sptep);
// beginning of my code:
if (level == 1) {
pgd_t *pgdp;
pud_t *pudp;
pmd_t *pmdp;
pte_t *ptep;
unsigned long hva = gfn_to_hva(vcpu->kvm, gfn);
struct page *page = NULL;
pgdp = pgd_offset(current->mm, hva);
if (!pgd_present(*pgdp))
goto out;
pudp = pud_offset(pgdp, hva);
if (!pud_present(*pudp))
goto out;
pmdp = pmd_offset(pudp, hva);
if (!pmd_preset(*pmdp))
goto out;
ptep = pte_offset_map(pmdp, hva);
if (!pte_present(*ptep) || pte_none(*ptep))
goto out;
if (!pfn_valid(pte_pfn(*ptep)))
goto out;
page = pte_page(*ptep);
if (!page)
goto out;

// this prink gives the unable to handle paging request" error:
printk(KERN_CRIT "page %p count %d mapping %p\n",
page, atomic_read(&page->_count), page->mapping);


// end of my code


Is there anything I did wrong here to get the page? The error only
happens occasionally.

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