What's going on here?

-# ./btrfstune -x /dev/xvdb
btrfs_reserve_extent search_start 0 data 52
find_free_extent search_start 0 search_end 18446744073709551615
num_bytes 16384
find_free_extent search_start 0 (check_failed)
find_free_extent new_group search_start 65996496830464
find_free_extent search_start 0 (check_failed)
find_free_extent new_group search_start 65996496830464
find_free_extent search_start 0 (check_failed)
find_free_extent new_group search_start 65996496830464
find_free_extent error -28
btrfs_reserve_extent: find_free_extent ret is -28
extent-tree.c:2694: btrfs_reserve_extent: Assertion `ret` failed.
./btrfstune(btrfs_reserve_extent+0xbdc)[0x419441]
./btrfstune(btrfs_alloc_free_block+0x50)[0x4195e0]
./btrfstune(__btrfs_cow_block+0x19d)[0x408dab]
./btrfstune(btrfs_cow_block+0xec)[0x409805]
./btrfstune[0x40efc0]
./btrfstune(btrfs_commit_transaction+0xec)[0x410c25]
./btrfstune(main+0x55f)[0x438e71]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f13e244b700]
./btrfstune(_start+0x29)[0x407c29]

btrfstune.c

@@ -64,8 +64,11 @@ static int update_seeding_flag(struct btrfs_root *root, int set_flag)
 		fprintf(stderr, "Warning: Seeding flag cleared.\n");
 	}
 
+	fprintf(stderr, "now at btrfs_start_transaction\n");
 	trans = btrfs_start_transaction(root, 1);
+	fprintf(stderr, "now at btrfs_set_super_flags\n");
 	btrfs_set_super_flags(disk_super, super_flags);
+	fprintf(stderr, "now at btrfs_commit_transaction\n");
 	ret = btrfs_commit_transaction(trans, root);
 
 	return ret;

extent-tree.c

@@ -2529,6 +2529,7 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 				     int data)
 {
 	int ret;
+	fprintf(stderr, "find_free_extent search_start %llu search_end %llu num_bytes %llu\n", search_start, search_end, num_bytes);
 	u64 orig_search_start = search_start;
 	struct btrfs_root * root = orig_root->fs_info->extent_root;
 	struct btrfs_fs_info *info = root->fs_info;
@@ -2558,6 +2559,7 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 
 check_failed:
 	search_start = stripe_align(root, search_start);
+	fprintf(stderr, "find_free_extent search_start %llu (check_failed)\n", search_start);
 	if (!block_group) {
 		block_group = btrfs_lookup_first_block_group(info,
 							     search_start);
@@ -2619,6 +2621,7 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 	return 0;
 
 new_group:
+	fprintf(stderr, "find_free_extent new_group search_start %llu \n", search_start);
 	block_group = btrfs_lookup_first_block_group(info, search_start);
 	if (!block_group) {
 		search_start = orig_search_start;
@@ -2639,6 +2642,7 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 	goto check_failed;
 
 error:
+	fprintf(stderr, "find_free_extent error %d\n", ret);
 	return ret;
 }
 
@@ -2681,10 +2685,12 @@ int btrfs_reserve_extent(struct btrfs_trans_handle *trans,
 	}
 
 	WARN_ON(num_bytes < root->sectorsize);
+	fprintf(stderr, "btrfs_reserve_extent search_start %llu data %d\n", search_start, data);
 	ret = find_free_extent(trans, root, num_bytes, empty_size,
 			       search_start, search_end, hint_byte, ins,
 			       trans->alloc_exclude_start,
 			       trans->alloc_exclude_nr, data);
+	fprintf(stderr, "btrfs_reserve_extent: find_free_extent ret is %d\n", ret);
 	BUG_ON(ret);
 	clear_extent_dirty(&root->fs_info->free_space_cache,
 			   ins->objectid, ins->objectid + ins->offset - 1,