expand_stack(), it is hard to come up with a destructive scenario without
having the vmlist protection in this case.
-The page_table_lock nests with the inode i_shared_sem and the kmem cache
+The page_table_lock nests with the inode i_mmap_lock and the kmem cache
c_spinlock spinlocks. This is okay, since the kmem code asks for pages after
dropping c_spinlock. The page_table_lock also nests with pagecache_lock and
pagemap_lru_lock spinlocks, and no code asks for memory with these locks