thp: zap_huge_pmd(): zap huge zero pmd

We don't have a mapped page to zap in huge zero page case.  Let's just clear
pmd and remove it from tlb.
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1085,15 +1085,20 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		pmd_t orig_pmd;
 		pgtable = pgtable_trans_huge_withdraw(tlb->mm);
 		orig_pmd = pmdp_get_and_clear(tlb->mm, addr, pmd);
-		page = pmd_page(orig_pmd);
 		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
-		page_remove_rmap(page);
-		VM_BUG_ON(page_mapcount(page) < 0);
-		add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
-		VM_BUG_ON(!PageHead(page));
-		tlb->mm->nr_ptes--;
-		spin_unlock(&tlb->mm->page_table_lock);
-		tlb_remove_page(tlb, page);
+		if (is_huge_zero_pmd(orig_pmd)) {
+			tlb->mm->nr_ptes--;
+			spin_unlock(&tlb->mm->page_table_lock);
+		} else {
+			page = pmd_page(orig_pmd);
+			page_remove_rmap(page);
+			VM_BUG_ON(page_mapcount(page) < 0);
+			add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
+			VM_BUG_ON(!PageHead(page));
+			tlb->mm->nr_ptes--;
+			spin_unlock(&tlb->mm->page_table_lock);
+			tlb_remove_page(tlb, page);
+		}
 		pte_free(tlb->mm, pgtable);
 		ret = 1;
 	}