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/filemap.c

@@ -700,10 +700,14 @@ struct page *find_get_page(struct address_space *mapping, pgoff_t offset)
 		if (unlikely(!page))
 			goto out;
 		if (radix_tree_exception(page)) {
-			if (radix_tree_exceptional_entry(page))
-				goto out;
-			/* radix_tree_deref_retry(page) */
+			if (radix_tree_deref_retry(page))
 				goto repeat;
+			/*
+			 * Otherwise, shmem/tmpfs must be storing a swap entry
+			 * here as an exceptional entry: so return it without
+			 * attempting to raise page count.
+			 */
+			goto out;
 		}
 		if (!page_cache_get_speculative(page))
 			goto repeat;
@@ -838,16 +842,22 @@ unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
 			continue;
 
 		if (radix_tree_exception(page)) {
-			if (radix_tree_exceptional_entry(page))
-				continue;
+			if (radix_tree_deref_retry(page)) {
 				/*
-			 * radix_tree_deref_retry(page):
-			 * can only trigger when entry at index 0 moves out of
-			 * or back to root: none yet gotten, safe to restart.
+				 * Transient condition which can only trigger
+				 * when entry at index 0 moves out of or back
+				 * to root: none yet gotten, safe to restart.
 				 */
 				WARN_ON(start | i);
 				goto restart;
 			}
+			/*
+			 * Otherwise, shmem/tmpfs must be storing a swap entry
+			 * here as an exceptional entry: so skip over it -
+			 * we only reach this from invalidate_mapping_pages().
+			 */
+			continue;
+		}
 
 		if (!page_cache_get_speculative(page))
 			goto repeat;
@@ -904,15 +914,21 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 			continue;
 
 		if (radix_tree_exception(page)) {
-			if (radix_tree_exceptional_entry(page))
-				break;
+			if (radix_tree_deref_retry(page)) {
 				/*
-			 * radix_tree_deref_retry(page):
-			 * can only trigger when entry at index 0 moves out of
-			 * or back to root: none yet gotten, safe to restart.
+				 * Transient condition which can only trigger
+				 * when entry at index 0 moves out of or back
+				 * to root: none yet gotten, safe to restart.
 				 */
 				goto restart;
 			}
+			/*
+			 * Otherwise, shmem/tmpfs must be storing a swap entry
+			 * here as an exceptional entry: so stop looking for
+			 * contiguous pages.
+			 */
+			break;
+		}
 
 		if (!page_cache_get_speculative(page))
 			goto repeat;
@@ -973,14 +989,20 @@ unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
 			continue;
 
 		if (radix_tree_exception(page)) {
-			BUG_ON(radix_tree_exceptional_entry(page));
+			if (radix_tree_deref_retry(page)) {
 				/*
-			 * radix_tree_deref_retry(page):
-			 * can only trigger when entry at index 0 moves out of
-			 * or back to root: none yet gotten, safe to restart.
+				 * Transient condition which can only trigger
+				 * when entry at index 0 moves out of or back
+				 * to root: none yet gotten, safe to restart.
 				 */
 				goto restart;
 			}
+			/*
+			 * This function is never used on a shmem/tmpfs
+			 * mapping, so a swap entry won't be found here.
+			 */
+			BUG();
+		}
 
 		if (!page_cache_get_speculative(page))
 			goto repeat;

mm/mincore.c

@@ -72,6 +72,7 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff)
 	 */
 	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);

mm/shmem.c

@@ -332,10 +332,14 @@ static unsigned shmem_find_get_pages_and_swap(struct address_space *mapping,
 		if (unlikely(!page))
 			continue;
 		if (radix_tree_exception(page)) {
-			if (radix_tree_exceptional_entry(page))
-				goto export;
-			/* radix_tree_deref_retry(page) */
+			if (radix_tree_deref_retry(page))
 				goto restart;
+			/*
+			 * Otherwise, we must be storing a swap entry
+			 * here as an exceptional entry: so return it
+			 * without attempting to raise page count.
+			 */
+			goto export;
 		}
 		if (!page_cache_get_speculative(page))
 			goto repeat;