thp: fix zap_huge_pmd() for DAX

The original DAX code assumed that pgtable_t was a pointer, which isn't
true on all architectures.  Restructure the code to not rely on that
assumption.

[willy@linux.intel.com: further fixes integrated into this patch]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1456,12 +1456,11 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		 pmd_t *pmd, unsigned long addr)
 {
+	pmd_t orig_pmd;
 	spinlock_t *ptl;
-	int ret = 0;
 
-	if (__pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
-		pgtable_t pgtable;
-		pmd_t orig_pmd;
+	if (__pmd_trans_huge_lock(pmd, vma, &ptl) != 1)
+		return 0;
 	/*
 	 * For architectures like ppc64 we look at deposited pgtable
 	 * when calling pmdp_huge_get_and_clear. So do the
@@ -1472,16 +1471,11 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 			tlb->fullmm);
 	tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
 	if (vma_is_dax(vma)) {
-			if (is_huge_zero_pmd(orig_pmd)) {
-				pgtable = NULL;
-			} else {
 		spin_unlock(ptl);
-				return 1;
-			}
-		} else {
-			pgtable = pgtable_trans_huge_withdraw(tlb->mm, pmd);
-		}
-		if (is_huge_zero_pmd(orig_pmd)) {
+		if (is_huge_zero_pmd(orig_pmd))
+			put_huge_zero_page();
+	} else if (is_huge_zero_pmd(orig_pmd)) {
+		pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
 		atomic_long_dec(&tlb->mm->nr_ptes);
 		spin_unlock(ptl);
 		put_huge_zero_page();
@@ -1491,15 +1485,12 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
 		add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
 		VM_BUG_ON_PAGE(!PageHead(page), page);
+		pte_free(tlb->mm, pgtable_trans_huge_withdraw(tlb->mm, pmd));
 		atomic_long_dec(&tlb->mm->nr_ptes);
 		spin_unlock(ptl);
 		tlb_remove_page(tlb, page);
 	}
-		if (pgtable)
-			pte_free(tlb->mm, pgtable);
-		ret = 1;
-	}
-	return ret;
+	return 1;
 }
 
 int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,