mm: document how to use struct page

Be really explicit about what bits / bytes are reserved for users that
want to store extra information about the pages they allocate.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Christoph Lameter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/mm_types.h

@@ -31,7 +31,29 @@ struct hmm;
  * 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