mm: document how to use struct page

 * it to keep track of whatever it is we are using the page for at the

 * moment. Note that we have no way to track which tasks are using

 * a page, though if it is a pagecache page, rmap structures can tell us

 * who is mapping it.

 * who is mapping it. If you allocate the page using alloc_pages(), you

 * can use some of the space in struct page for your own purposes.

 *

 * Pages that were once in the page cache may be found under the RCU lock

 * even after they have been recycled to a different purpose.  The page

 * cache reads and writes some of the fields in struct page to pin the

 * page before checking that it's still in the page cache.  It is vital

 * that all users of struct page:

 * 1. Use the first word as PageFlags.

 * 2. Clear or preserve bit 0 of page->compound_head.  It is used as

 *    PageTail for compound pages, and the page cache must not see false

 *    positives.  Some users put a pointer here (guaranteed to be at least

 *    4-byte aligned), other users avoid using the field altogether.

 * 3. page->_refcount must either not be used, or must be used in such a

 *    way that other CPUs temporarily incrementing and then decrementing the

 *    refcount does not cause problems.  On receiving the page from

 *    alloc_pages(), the refcount will be positive.

 * 4. Either preserve page->_mapcount or restore it to -1 before freeing it.

 *

 * If you allocate pages of order > 0, you can use the fields in the struct

 * page associated with each page, but bear in mind that the pages may have

 * been inserted individually into the page cache, so you must use the above

 * four fields in a compatible way for each struct page.

 *

 * SLUB uses cmpxchg_double() to atomically update its freelist and

 * counters.  That requires that freelist & counters be adjacent and