quoted

@@ -124,6 +130,7 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
  				       struct page *page)
  {
  	__SetPageBalloon(page);
+	page->mapping = balloon->inode->i_mapping;
  	SetPagePrivate(page);
  	set_page_private(page, (unsigned long)balloon);
  	list_add(&page->lru, &balloon->pages);

@@ -140,6 +147,7 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
  static inline void balloon_page_delete(struct page *page)
  {
  	__ClearPageBalloon(page);
+	page->mapping = NULL;
  	set_page_private(page, 0);
  	if (PagePrivate(page)) {
  		ClearPagePrivate(page);

Order of cleanup here is not the reverse of the order of initialization.
Better make it exactly the reverse.


Also, I have a question: is it enough to lock the page to make changing
the mapping safe? Do all users lock the page too?

I think balloon developers can answer that precisely.

I've just follow this comment:
http://lxr.free-electrons.com/source/include/linux/balloon_compaction.h#L16