[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.mart...@oracle.com>
---
 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
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