s390/mm: Add mmap_assert_write_locked() check to crst_table_upgrade()

Add mmap_assert_write_locked() check to crst_table_upgrade() in order to
verify that no concurrent page table upgrades of an mm can happen. This
allows to remove the VM_BUG_ON() check which checks for the potential
inconsistent result of concurrent updates.

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/mm/pgalloc.c

@@ -56,6 +56,8 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
 	unsigned long *pgd = NULL, *p4d = NULL, *__pgd;
 	unsigned long asce_limit = mm->context.asce_limit;
 
+	mmap_assert_write_locked(mm);
+
 	/* upgrade should only happen from 3 to 4, 3 to 5, or 4 to 5 levels */
 	VM_BUG_ON(asce_limit < _REGION2_SIZE);
 
@@ -79,13 +81,6 @@ int crst_table_upgrade(struct mm_struct *mm, unsigned long end)
 
 	spin_lock_bh(&mm->page_table_lock);
 
-	/*
-	 * This routine gets called with mmap_lock lock held and there is
-	 * no reason to optimize for the case of otherwise. However, if
-	 * that would ever change, the below check will let us know.
-	 */
-	VM_BUG_ON(asce_limit != mm->context.asce_limit);
-
 	if (p4d) {
 		__pgd = (unsigned long *) mm->pgd;
 		p4d_populate(mm, (p4d_t *) p4d, (pud_t *) __pgd);