mm: introduce mm_find_pmd()

Several place need to find the pmd by(mm_struct, address), so introduce a
function to simplify it.

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Ni zhan Chen <nizhan.chen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/migrate.c b/mm/migrate.c
index 77ed2d7..1dc4598 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -91,8 +91,6 @@
 {
 	struct mm_struct *mm = vma->vm_mm;
 	swp_entry_t entry;
- 	pgd_t *pgd;
- 	pud_t *pud;
  	pmd_t *pmd;
 	pte_t *ptep, pte;
  	spinlock_t *ptl;
@@ -103,19 +101,11 @@
 			goto out;
 		ptl = &mm->page_table_lock;
 	} else {
-		pgd = pgd_offset(mm, addr);
-		if (!pgd_present(*pgd))
+		pmd = mm_find_pmd(mm, addr);
+		if (!pmd)
 			goto out;
-
-		pud = pud_offset(pgd, addr);
-		if (!pud_present(*pud))
-			goto out;
-
-		pmd = pmd_offset(pud, addr);
 		if (pmd_trans_huge(*pmd))
 			goto out;
-		if (!pmd_present(*pmd))
-			goto out;
 
 		ptep = pte_offset_map(pmd, addr);