mm: expose abnormal new_pte during move_ptes

When executing move_ptes, the new_pte must be NULL, otherwise it will be
overwritten by the old_pte, and cause the abnormal new_pte to be leaked. 
In order to make this problem to be more explicit, let's add WARN_ON_ONCE
when new_pte is not NULL.

[[email protected]: s/WARN_ON_ONCE/VM_WARN_ON_ONCE/]
Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Oleg Nesterov <[email protected]>
Signed-off-by: Pu Lehui <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: "Masami Hiramatsu (Google)" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Pu Lehui

mm/mremap.c

@@ -237,6 +237,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
 
 	for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
 				   new_pte++, new_addr += PAGE_SIZE) {
+		VM_WARN_ON_ONCE(!pte_none(*new_pte));
+
 		if (pte_none(ptep_get(old_pte)))
 			continue;