mm: improve struct page documentation

Rewrite the documentation to describe what you can use in struct page
rather than what you can't.

Link: http://lkml.kernel.org/r/20180518194519.3820-12-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types.h

@@ -33,29 +33,27 @@ 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. If you allocate the page using alloc_pages(), you
- * can use some of the space in struct page for your own purposes.
+ * who is mapping it.
  *
- * 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
+ * If you allocate the page using alloc_pages(), you can use some of the
+ * space in struct page for your own purposes.  The five words in the main
+ * union are available, except for bit 0 of the first word which must be
+ * kept clear.  Many users use this word to store a pointer to an object
+ * which is guaranteed to be aligned.  If you use the same storage as
+ * page->mapping, you must restore it to NULL before freeing the page.
+ *
+ * If your page will not be mapped to userspace, you can also use the four
+ * bytes in the mapcount union, but you must call page_mapcount_reset()
+ * before freeing it.
+ *
+ * If you want to use the refcount field, it 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.
+ * If you allocate pages of order > 0, you can use some of the fields
+ * in each subpage, but you may need to restore some of their values
+ * afterwards.
  *
  * SLUB uses cmpxchg_double() to atomically update its freelist and
  * counters.  That requires that freelist & counters be adjacent and