radix-tree,shmem: introduce radix_tree_iter_next()

shmem likes to occasionally drop the lock, schedule, then reacqire the
lock and continue with the iteration from the last place it left off.
This is currently done with a pretty ugly goto.  Introduce
radix_tree_iter_next() and use it throughout shmem.c.

[koct9i@gmail.com: fix bug in radix_tree_iter_next() for tagged iteration]
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/radix-tree.h

@@ -402,6 +402,22 @@ void **radix_tree_iter_retry(struct radix_tree_iter *iter)
 	return NULL;
 }
 
+/**
+ * radix_tree_iter_next - resume iterating when the chunk may be invalid
+ * @iter:	iterator state
+ *
+ * If the iterator needs to release then reacquire a lock, the chunk may
+ * have been invalidated by an insertion or deletion.  Call this function
+ * to continue the iteration from the next index.
+ */
+static inline __must_check
+void **radix_tree_iter_next(struct radix_tree_iter *iter)
+{
+	iter->next_index = iter->index + 1;
+	iter->tags = 0;
+	return NULL;
+}
+
 /**
  * radix_tree_chunk_size - get current chunk size
  *

mm/shmem.c

@@ -376,7 +376,6 @@ unsigned long shmem_partial_swap_usage(struct address_space *mapping,
 
 	rcu_read_lock();
 
-restart:
 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
 		if (iter.index >= end)
 			break;
@@ -393,8 +392,7 @@ restart:
 
 		if (need_resched()) {
 			cond_resched_rcu();
-			start = iter.index + 1;
-			goto restart;
+			slot = radix_tree_iter_next(&iter);
 		}
 	}
 
@@ -1944,7 +1942,6 @@ static void shmem_tag_pins(struct address_space *mapping)
 	start = 0;
 	rcu_read_lock();
 
-restart:
 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
 		page = radix_tree_deref_slot(slot);
 		if (!page || radix_tree_exception(page)) {
@@ -1961,8 +1958,7 @@ restart:
 
 		if (need_resched()) {
 			cond_resched_rcu();
-			start = iter.index + 1;
-			goto restart;
+			slot = radix_tree_iter_next(&iter);
 		}
 	}
 	rcu_read_unlock();
@@ -1999,7 +1995,6 @@ static int shmem_wait_for_pins(struct address_space *mapping)
 
 		start = 0;
 		rcu_read_lock();
-restart:
 		radix_tree_for_each_tagged(slot, &mapping->page_tree, &iter,
 					   start, SHMEM_TAG_PINNED) {
 
@@ -2033,8 +2028,7 @@ restart:
 continue_resched:
 			if (need_resched()) {
 				cond_resched_rcu();
-				start = iter.index + 1;
-				goto restart;
+				slot = radix_tree_iter_next(&iter);
 			}
 		}
 		rcu_read_unlock();