mm: WARN_ON_ONCE if f_op->mmap() change vma's start address

During reviewing the source code, I found a comment which mention that
after f_op->mmap(), vma's start address can be changed.  I didn't verify
that it is really possible, because there are so many f_op->mmap()
implementation.  But if there are some mmap() which change vma's start
address, it is possible error situation, because we already prepare prev
vma, rb_link and rb_parent and these are related to original address.

So add WARN_ON_ONCE for finding that this situtation really happens.
Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1488,7 +1488,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		 *
 		 * Answer: Yes, several device drivers can do it in their
 		 *         f_op->mmap method. -DaveM
+		 * Bug: If addr is changed, prev, rb_link, rb_parent should
+		 *      be updated for vma_link()
 		 */
+		WARN_ON_ONCE(addr != vma->vm_start);
+
 		addr = vma->vm_start;
 		pgoff = vma->vm_pgoff;
 		vm_flags = vma->vm_flags;