mm: clarify the radix_tree exceptional cases

Make the radix_tree exceptional cases, mostly in filemap.c, clearer.

It's hard to devise a suitable snappy name that illuminates the use by
shmem/tmpfs for swap, while keeping filemap/pagecache/radix_tree
generality.  And akpm points out that /* radix_tree_deref_retry(page) */
comments look like calls that have been commented out for unknown
reason.

Skirt the naming difficulty by rearranging these blocks to handle the
transient radix_tree_deref_retry(page) case first; then just explain the
remaining shmem/tmpfs swap case in a comment.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mincore.c

diff --git a/mm/mincore.c b/mm/mincore.c
index 733f182..636a868 100644
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -72,6 +72,7 @@
 	 */
 	page = find_get_page(mapping, pgoff);
 #ifdef CONFIG_SWAP
+	/* shmem/tmpfs may return swap: account for swapcache page too. */
 	if (radix_tree_exceptional_entry(page)) {
 		swp_entry_t swap = radix_to_swp_entry(page);
 		page = find_get_page(&swapper_space, swap.val);