Re: [PATCH] btrfs: make btrfs_super_block::log_root_transid deprecated

[Filipe Manana <fdmanana@kernel.org>]

On Tue, Jun 07, 2022 at 07:50:59PM +0800, Qu Wenruo wrote:
> When using "btrfs inspect-internal dump-super" to inspect an fs with
> dirty log, it always shows the log_root_transid as 0:
> 
>  log_root                30474240
>  log_root_transid        0 <<<
>  log_root_level          0
> 
> It turns out that, btrfs_super_block::log_root_transid is never really
> utilized (even no read for it).
> 
> This can date back to the introduction of btrfs into upstream kernel.
> 
> In fact, when reading log tree root, we always use
> btrfs_super_block::generation + 1 as the expected generation.
> So here we're completely safe to mark this member deprecated.
> 
> In theory we can easily reuse this member for other purposes, but to be
> extra safe, here we follow the leafsize way, by adding "__unused_" for
> log_root_transid.
> And we can safely remove the accessors, since there is no such callers
> from the very beginning.
> 
> Cc: Chris Mason <clm@fb.com>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/ctree.h | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 0e49b1a0c071..71ba3b9fd8c2 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -248,8 +248,13 @@ struct btrfs_super_block {
>  	__le64 chunk_root;
>  	__le64 log_root;
>  
> -	/* this will help find the new super based on the log root */
> -	__le64 log_root_transid;
> +	/*
> +	 * This member is never utlized from the very beginning of btrfs, thus

utlized -> utilized (or used)

> +	 * it's always 0 no matter kernel versions.

no matter kernel versions -> no matter which kernel version

Other than that, it looks fine.
Thanks.

Reviewed-by: Filipe Manana <fdmanana@suse.com>

> +	 * We always use generation + 1 to read log tree root.
> +	 * So here we mark it deprecated.
> +	 */
> +	__le64 __unused_log_root_transid;
>  	__le64 total_bytes;
>  	__le64 bytes_used;
>  	__le64 root_dir_objectid;
> @@ -2475,8 +2480,6 @@ BTRFS_SETGET_STACK_FUNCS(super_chunk_root_level, struct btrfs_super_block,
>  			 chunk_root_level, 8);
>  BTRFS_SETGET_STACK_FUNCS(super_log_root, struct btrfs_super_block,
>  			 log_root, 64);
> -BTRFS_SETGET_STACK_FUNCS(super_log_root_transid, struct btrfs_super_block,
> -			 log_root_transid, 64);
>  BTRFS_SETGET_STACK_FUNCS(super_log_root_level, struct btrfs_super_block,
>  			 log_root_level, 8);
>  BTRFS_SETGET_STACK_FUNCS(super_total_bytes, struct btrfs_super_block,
> -- 
> 2.36.1
>