- *
- * Since 2.6.6 (approx), a free page has ->_count = -1. This is so that we
- * can use atomic_add_negative(-1, page->_count) to detect when the page
- * becomes free and so that we can also use atomic_inc_and_test to atomically
- * detect when we just tried to grab a ref on a page which some other CPU has
- * already deemed to be freeable.
- *
- * NO code should make assumptions about this internal detail! Use the provided
- * macros which retain the old rules: page_count(page) == 0 is a free page.