Re: [PATCH 1/4] btrfs: use BTRFS_PATH_AUTO_FREE in insert_balance_item()

[David Sterba]

On Tue, Apr 08, 2025 at 06:29:30AM -0600, Yangtao Li wrote:
> All cleanup paths lead to btrfs_path_free so we can define path with the
> automatic free callback.
> 
> Signed-off-by: Yangtao Li <frank.li@xxxxxxxx>
> ---
>  fs/btrfs/volumes.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index c8c21c55be53..a962efaec4ea 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3730,7 +3730,7 @@ static int insert_balance_item(struct btrfs_fs_info *fs_info,
>  	struct btrfs_trans_handle *trans;
>  	struct btrfs_balance_item *item;
>  	struct btrfs_disk_balance_args disk_bargs;
> -	struct btrfs_path *path;
> +	BTRFS_PATH_AUTO_FREE(path);
>  	struct extent_buffer *leaf;
>  	struct btrfs_key key;
>  	int ret, err;
> @@ -3740,10 +3740,8 @@ static int insert_balance_item(struct btrfs_fs_info *fs_info,
>  		return -ENOMEM;
>  
>  	trans = btrfs_start_transaction(root, 0);
> -	if (IS_ERR(trans)) {
> -		btrfs_free_path(path);
> +	if (IS_ERR(trans))
>  		return PTR_ERR(trans);
> -	}
>  
>  	key.objectid = BTRFS_BALANCE_OBJECTID;
>  	key.type = BTRFS_TEMPORARY_ITEM_KEY;
> @@ -3767,7 +3765,6 @@ static int insert_balance_item(struct btrfs_fs_info *fs_info,
>  	btrfs_set_balance_sys(leaf, item, &disk_bargs);
>  	btrfs_set_balance_flags(leaf, item, bctl->flags);
>  out:
> -	btrfs_free_path(path);

The pattern where btrfs_free_path() is not called at the end and is
followed by other code is probably not worth converting. It would be
possible to replace it by

	btrfs_release_path(path);
	path = NULL;

that drops the locks and refs from the path but this does not save us
much compared to the straightforward BTRFS_PATH_AUTO_FREE() conversions.
Also release will still keep the object allocated although we don't need
it. As a principle, resources, locks and critical sections should be as
short as possible.

Unfortunatelly I've probably fished all the trivial and almost-trivial
conversions, we don't want 100% use of BTRFS_PATH_AUTO_FREE(), only when
it improves the code.

You may still find cases worth converting, the typical pattern is
btrfs_path_alloc() somewhere near top of the function and
btrfs_free_path() called right before a return.