Re: [PATCH 3/3] xfs: make struct xfs_buf_log_format have a consistent size

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jan 07, 2020 at 08:18:25PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Increase XFS_BLF_DATAMAP_SIZE by 1 to fill in the implied padding at the
> end of struct xfs_buf_log_format.  This makes the size consistent so
> that we can check it in xfs_ondisk.h, and will be needed once we start
> logging attribute values.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_log_format.h |    9 +++++----
>  fs/xfs/xfs_ondisk.h            |    1 +
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_log_format.h b/fs/xfs/libxfs/xfs_log_format.h
> index 8ef31d71a9c7..5d8eb8978c33 100644
> --- a/fs/xfs/libxfs/xfs_log_format.h
> +++ b/fs/xfs/libxfs/xfs_log_format.h
> @@ -462,11 +462,12 @@ static inline uint xfs_log_dinode_size(int version)
>  #define	XFS_BLF_GDQUOT_BUF	(1<<4)
>  
>  /*
> - * This is the structure used to lay out a buf log item in the
> - * log.  The data map describes which 128 byte chunks of the buffer
> - * have been logged.
> + * This is the structure used to lay out a buf log item in the log.  The data
> + * map describes which 128 byte chunks of the buffer have been logged.  Note
> + * that XFS_BLF_DATAMAP_SIZE is an odd number so that the structure size will
> + * be consistent between 32-bit and 64-bit platforms.
>   */
> -#define XFS_BLF_DATAMAP_SIZE	((XFS_MAX_BLOCKSIZE / XFS_BLF_CHUNK) / NBWORD)
> +#define XFS_BLF_DATAMAP_SIZE	(1 + ((XFS_MAX_BLOCKSIZE / XFS_BLF_CHUNK) / NBWORD))

I don't understand the explanation.  Why would the size differ for
32-bit vs 64-bit architectures when it only uses fixed size types?