mm/huge_memory.c: deposit page table when copying a PMD migration entry

We need to deposit pre-allocated PTE page table when a PMD migration
entry is copied in copy_huge_pmd().  Otherwise, we will leak the
pre-allocated page and cause a NULL pointer dereference later in
zap_huge_pmd().

The missing counters during PMD migration entry copy process are added
as well.

The bug report is here: https://lkml.org/lkml/2017/10/29/214

Link: http://lkml.kernel.org/r/20171030144636.4836-1-zi.yan@sent.com
Fixes: 84c3fc4e ("mm: thp: check pmd migration entry in common path")
Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@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

@@ -941,6 +941,9 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 				pmd = pmd_swp_mksoft_dirty(pmd);
 			set_pmd_at(src_mm, addr, src_pmd, pmd);
 		}
+		add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+		atomic_long_inc(&dst_mm->nr_ptes);
+		pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
 		set_pmd_at(dst_mm, addr, dst_pmd, pmd);
 		ret = 0;
 		goto out_unlock;