mm/doc: Add documentation for folio_test_uptodate

 * might lose their PG_swapbacked flag when they simply can be dropped (e.g. as

 * a result of MADV_FREE).

 *

 * PG_uptodate tells whether the page's contents is valid.  When a read

 * completes, the page becomes uptodate, unless a disk I/O error happened.

 *

 * PG_referenced, PG_reclaim are used for page reclaim for anonymous and

 * file-backed pagecache (see mm/vmscan.c).

 *

u64 stable_page_flags(struct page *page);

/**

 * folio_test_uptodate - Is this folio up to date?

 * @folio: The folio.

 *

 * The uptodate flag is set on a folio when every byte in the folio is

 * at least as new as the corresponding bytes on storage.  Anonymous

 * and CoW folios are always uptodate.  If the folio is not uptodate,

 * some of the bytes in it may be; see the is_partially_uptodate()

 * address_space operation.

 */

static inline bool folio_test_uptodate(struct folio *folio)

{

	bool ret = test_bit(PG_uptodate, folio_flags(folio, 0));