[PATCH RFC 02/10] mm: Handle pmd entries in follow_pfn()

[Joao Martins]

When follow_pfn hits a pmd_huge() it won't return a valid PFN
given it's usage of follow_pte(). Fix that up to pass a @pmdpp
and thus allow callers to get the pmd pointer. If we encounter
such a huge page, we calculate the pfn offset to the PMD
accordingly.

This allows KVM to handle 2M hugepage pfns on VM_PFNMAP vmas.

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 mm/memory.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index cfc3668bddeb..db99684d2cb3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4366,6 +4366,7 @@ EXPORT_SYMBOL(follow_pte_pmd);
 int follow_pfn(struct vm_area_struct *vma, unsigned long address,
 	unsigned long *pfn)
 {
+	pmd_t *pmdpp = NULL;
 	int ret = -EINVAL;
 	spinlock_t *ptl;
 	pte_t *ptep;
@@ -4373,10 +4374,14 @@ int follow_pfn(struct vm_area_struct *vma, unsigned long address,
 	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
 		return ret;
 
-	ret = follow_pte(vma->vm_mm, address, &ptep, &ptl);
+	ret = follow_pte_pmd(vma->vm_mm, address, NULL,
+			     &ptep, &pmdpp, &ptl);
 	if (ret)
 		return ret;
-	*pfn = pte_pfn(*ptep);
+	if (pmdpp)
+		*pfn = pmd_pfn(*pmdpp) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
+	else
+		*pfn = pte_pfn(*ptep);
 	pte_unmap_unlock(ptep, ptl);
 	return 0;
 }
-- 
2.17.1