Btrfs: add WARN_ONCE to detect unexpected error from merge_extent_mapping

Orabug: 27446653

This is a subtle case, so in order to understand the problem, it'd be
good to know the content of existing and em when any error occurs.

Reviewed-by: Anand Jain <[email protected]>
Signed-off-by: Liu Bo <[email protected]>
Signed-off-by: Somasundaram Krishnasamy <[email protected]>

Author: Liu Bo

fs/btrfs/inode.c

@@ -7227,11 +7227,14 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
 			 */
 			err = merge_extent_mapping(em_tree, existing,
 						   em, start);
-			free_extent_map(existing);
 			if (err) {
+				WARN_ONCE(1, "existing (0x%llx 0x%llx 0x%llx) em (0x%llx 0x%llx 0x%llx)\n",
+					  existing->start, existing->len, existing->block_start,
+					  em->start, em->len, em->block_start);
 				free_extent_map(em);
 				em = NULL;
 			}
+			free_extent_map(existing);
 		} else {
 			free_extent_map(em);
 			em = existing;