Re: [PATCH 9/9] xfs: allow bulkstat_single of special inodes

[Brian Foster <bfoster@redhat.com>]

On Wed, Jun 26, 2019 at 01:46:31PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Create a new ireq flag (for single bulkstats) that enables userspace to
> ask us for a special inode number instead of interpreting @ino as a
> literal inode number.  This enables us to query the root inode easily.
> 

Seems reasonable, though what's the use case for this? A brief
description in the commit log would be helpful.

> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Allison Collins <allison.henderson@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_fs.h |   11 ++++++++++-
>  fs/xfs/xfs_ioctl.c     |   10 ++++++++++
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_fs.h b/fs/xfs/libxfs/xfs_fs.h
> index 77c06850ac52..1489bce07d66 100644
> --- a/fs/xfs/libxfs/xfs_fs.h
> +++ b/fs/xfs/libxfs/xfs_fs.h
> @@ -482,7 +482,16 @@ struct xfs_ireq {
>  	uint64_t	reserved[2];	/* must be zero			*/
>  };
>  
> -#define XFS_IREQ_FLAGS_ALL	(0)
> +/*
> + * The @ino value is a special value, not a literal inode number.  See the
> + * XFS_IREQ_SPECIAL_* values below.
> + */
> +#define XFS_IREQ_SPECIAL	(1 << 0)
> +
> +#define XFS_IREQ_FLAGS_ALL	(XFS_IREQ_SPECIAL)
> +
> +/* Operate on the root directory inode. */
> +#define XFS_IREQ_SPECIAL_ROOT	(1)
>  
>  /*
>   * ioctl structures for v5 bulkstat and inumbers requests
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index f71341cd8340..3bb5f980fabf 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -961,6 +961,16 @@ xfs_ireq_setup(
>  	    memchr_inv(hdr->reserved, 0, sizeof(hdr->reserved)))
>  		return -EINVAL;
>  
> +	if (hdr->flags & XFS_IREQ_SPECIAL) {
> +		switch (hdr->ino) {
> +		case XFS_IREQ_SPECIAL_ROOT:
> +			hdr->ino = mp->m_sb.sb_rootino;
> +			break;

Do you envision other ->ino magic values? I'm curious about the need for
the special flag along with a magic inode value as opposed to just a
"root dir" flag or some such.

Brian

> +		default:
> +			return -EINVAL;
> +		}
> +	}
> +
>  	if (XFS_INO_TO_AGNO(mp, hdr->ino) >= mp->m_sb.sb_agcount)
>  		return -EINVAL;
>  
>