iomap: export iomap_iter_advance() and return remaining length

As a final step for generic iter advance, export the helper and
update it to return the remaining length of the current iteration
after the advance. This will usually be 0 in the iomap_iter() case,
but will be useful for the various operations that iterate on their
own and will be updated to advance as they progress.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250207143253.314068-7-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

@@ -15,22 +15,16 @@ static inline void iomap_iter_reset_iomap(struct iomap_iter *iter)
 }
 
 /*
- * Advance to the next range we need to map.
- *
- * If the iomap is marked IOMAP_F_STALE, it means the existing map was not fully
- * processed - it was aborted because the extent the iomap spanned may have been
- * changed during the operation. In this case, the iteration behaviour is to
- * remap the unprocessed range of the iter, and that means we may need to remap
- * even when we've made no progress (i.e. count = 0). Hence the "finished
- * iterating" case needs to distinguish between (count = 0) meaning we are done
- * and (count = 0 && stale) meaning we need to remap the entire remaining range.
+ * Advance the current iterator position and output the length remaining for the
+ * current mapping.
  */
-static inline int iomap_iter_advance(struct iomap_iter *iter, s64 count)
+int iomap_iter_advance(struct iomap_iter *iter, u64 *count)
 {
-	if (WARN_ON_ONCE(count > iomap_length(iter)))
+	if (WARN_ON_ONCE(*count > iomap_length(iter)))
 		return -EIO;
-	iter->pos += count;
-	iter->len -= count;
+	iter->pos += *count;
+	iter->len -= *count;
+	*count = iomap_length(iter);
 	return 0;
 }
 
@@ -93,7 +87,7 @@ int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops)
 	 * 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);
+	ret = iomap_iter_advance(iter, &processed);
 	if (!ret && iter->len > 0)
 		ret = 1;
 	if (ret > 0 && !iter->processed && !stale)

include/linux/iomap.h

@@ -236,6 +236,7 @@ struct iomap_iter {
 };
 
 int iomap_iter(struct iomap_iter *iter, const struct iomap_ops *ops);
+int iomap_iter_advance(struct iomap_iter *iter, u64 *count);
 
 /**
  * iomap_length_trim - trimmed length of the current iomap iteration