iomap: lift iter termination logic from iomap_iter_advance()

The iter termination logic in iomap_iter_advance() is only needed by
iomap_iter() to determine whether to proceed with the next mapping
for an ongoing operation. The old logic sets ret to 1 and then
terminates if the operation is complete (iter->len == 0) or the
previous iteration performed no work and the mapping has not been
marked stale. The stale check exists to allow operations to
retry the current mapping if an inconsistency has been detected.

To further genericize iomap_iter_advance(), lift the termination
logic into iomap_iter() and update the former to return success (0)
or an error code. iomap_iter() continues on successful advance and
non-zero iter->len or otherwise terminates in the no progress (and
not stale) or error cases.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250207143253.314068-6-bfoster@redhat.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Brian Foster

fs/iomap/iter.c

@@ -27,17 +27,11 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
  */
 static inline int iomap_iter_advance(struct iomap_iter *iter, s64 count)
 {
-	bool stale = iter->iomap.flags & IOMAP_F_STALE;
-	int ret = 1;
-
 	if (WARN_ON_ONCE(count > iomap_length(iter)))
 		return -EIO;
 	iter->pos += count;
 	iter->len -= count;
-	if (!iter->len || (!count && !stale))
-		ret = 0;
-
-	return ret;
+	return 0;
 }
 
 static inline void iomap_iter_done(struct iomap_iter *iter)
@@ -69,6 +63,7 @@ static inline void iomap_iter_done(struct iomap_iter *iter)
  */
 int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 {
+	bool stale = iter->iomap.flags & IOMAP_F_STALE;
 	s64 processed;
 	int ret;
 
@@ -91,8 +86,18 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 		return processed;
 	}
 
-	/* advance and clear state from the previous iteration */
+	/*
+	 * Advance the iter and clear state from the previous iteration. Use
+	 * iter->len to determine whether to continue onto the next mapping.
+	 * Explicitly terminate in the case where the current iter has not
+	 * advanced at all (i.e. no work was done for some reason) unless the
+	 * mapping has been marked stale and needs to be reprocessed.
+	 */
 	ret = iomap_iter_advance(iter, processed);
+	if (!ret && iter->len > 0)
+		ret = 1;
+	if (ret > 0 && !iter->processed && !stale)
+		ret = 0;
 	iomap_iter_reset_iomap(iter);
 	if (ret <= 0)
 		return ret;