mm: use radix_tree_iter_retry()

Instead of a 'goto restart', we can now use radix_tree_iter_retry() to
restart from our current position.  This will make a difference when
there are more ways to happen across an indirect pointer.  And it
eliminates some confusing gotos.

[vbabka@suse.cz: remove now-obsolete-and-misleading comment]
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -1255,7 +1255,6 @@ unsigned find_get_entries(struct address_space *mapping,
 		return 0;
 
 	rcu_read_lock();
-restart:
 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
 		struct page *page;
 repeat:
@@ -1263,8 +1262,10 @@ repeat:
 		if (unlikely(!page))
 			continue;
 		if (radix_tree_exception(page)) {
-			if (radix_tree_deref_retry(page))
-				goto restart;
+			if (radix_tree_deref_retry(page)) {
+				slot = radix_tree_iter_retry(&iter);
+				continue;
+			}
 			/*
 			 * A shadow entry of a recently evicted page, a swap
 			 * entry from shmem/tmpfs or a DAX entry.  Return it
@@ -1317,7 +1318,6 @@ unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
 		return 0;
 
 	rcu_read_lock();
-restart:
 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
 		struct page *page;
 repeat:
@@ -1327,13 +1327,8 @@ repeat:
 
 		if (radix_tree_exception(page)) {
 			if (radix_tree_deref_retry(page)) {
-				/*
-				 * 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(iter.index);
-				goto restart;
+				slot = radix_tree_iter_retry(&iter);
+				continue;
 			}
 			/*
 			 * A shadow entry of a recently evicted page,
@@ -1384,7 +1379,6 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
 		return 0;
 
 	rcu_read_lock();
-restart:
 	radix_tree_for_each_contig(slot, &mapping->page_tree, &iter, index) {
 		struct page *page;
 repeat:
@@ -1395,12 +1389,8 @@ repeat:
 
 		if (radix_tree_exception(page)) {
 			if (radix_tree_deref_retry(page)) {
-				/*
-				 * 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;
+				slot = radix_tree_iter_retry(&iter);
+				continue;
 			}
 			/*
 			 * A shadow entry of a recently evicted page,
@@ -1460,7 +1450,6 @@ unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
 		return 0;
 
 	rcu_read_lock();
-restart:
 	radix_tree_for_each_tagged(slot, &mapping->page_tree,
 				   &iter, *index, tag) {
 		struct page *page;
@@ -1471,12 +1460,8 @@ repeat:
 
 		if (radix_tree_exception(page)) {
 			if (radix_tree_deref_retry(page)) {
-				/*
-				 * 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;
+				slot = radix_tree_iter_retry(&iter);
+				continue;
 			}
 			/*
 			 * A shadow entry of a recently evicted page.
@@ -1539,7 +1524,6 @@ unsigned find_get_entries_tag(struct address_space *mapping, pgoff_t start,
 		return 0;
 
 	rcu_read_lock();
-restart:
 	radix_tree_for_each_tagged(slot, &mapping->page_tree,
 				   &iter, start, tag) {
 		struct page *page;
@@ -1549,12 +1533,8 @@ repeat:
 			continue;
 		if (radix_tree_exception(page)) {
 			if (radix_tree_deref_retry(page)) {
-				/*
-				 * 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;
+				slot = radix_tree_iter_retry(&iter);
+				continue;
 			}
 
 			/*
@@ -2171,10 +2151,11 @@ repeat:
 		if (unlikely(!page))
 			goto next;
 		if (radix_tree_exception(page)) {
-			if (radix_tree_deref_retry(page))
-				break;
-			else
-				goto next;
+			if (radix_tree_deref_retry(page)) {
+				slot = radix_tree_iter_retry(&iter);
+				continue;
+			}
+			goto next;
 		}
 
 		if (!page_cache_get_speculative(page))

mm/shmem.c

@@ -383,13 +383,10 @@ restart:
 
 		page = radix_tree_deref_slot(slot);
 
-		/*
-		 * This should only be possible to happen at index 0, so we
-		 * don't need to reset the counter, nor do we risk infinite
-		 * restarts.
-		 */
-		if (radix_tree_deref_retry(page))
-			goto restart;
+		if (radix_tree_deref_retry(page)) {
+			slot = radix_tree_iter_retry(&iter);
+			continue;
+		}
 
 		if (radix_tree_exceptional_entry(page))
 			swapped++;
@@ -1951,8 +1948,10 @@ restart:
 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
 		page = radix_tree_deref_slot(slot);
 		if (!page || radix_tree_exception(page)) {
-			if (radix_tree_deref_retry(page))
-				goto restart;
+			if (radix_tree_deref_retry(page)) {
+				slot = radix_tree_iter_retry(&iter);
+				continue;
+			}
 		} else if (page_count(page) - page_mapcount(page) > 1) {
 			spin_lock_irq(&mapping->tree_lock);
 			radix_tree_tag_set(&mapping->page_tree, iter.index,
@@ -2006,8 +2005,10 @@ restart:
 
 			page = radix_tree_deref_slot(slot);
 			if (radix_tree_exception(page)) {
-				if (radix_tree_deref_retry(page))
-					goto restart;
+				if (radix_tree_deref_retry(page)) {
+					slot = radix_tree_iter_retry(&iter);
+					continue;
+				}
 
 				page = NULL;
 			}