Re: [PATCH 4/6] fs: xfs: Support atomic write for statx

[Darrick J. Wong]

On Wed, Jan 24, 2024 at 02:26:43PM +0000, John Garry wrote:
> Support providing info on atomic write unit min and max for an inode.
> 
> For simplicity, currently we limit the min at the FS block size, but a
> lower limit could be supported in future.
> 
> The atomic write unit min and max is limited by the guaranteed extent
> alignment for the inode.
> 
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> ---
>  fs/xfs/xfs_iops.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_iops.h |  4 ++++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index a0d77f5f512e..0890d2f70f4d 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -546,6 +546,44 @@ xfs_stat_blksize(
>  	return PAGE_SIZE;
>  }
>  
> +void xfs_get_atomic_write_attr(

static void?

> +	struct xfs_inode *ip,
> +	unsigned int *unit_min,
> +	unsigned int *unit_max)

Weird indenting here.

> +{
> +	xfs_extlen_t		extsz = xfs_get_extsz(ip);
> +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> +	struct block_device	*bdev = target->bt_bdev;
> +	unsigned int		awu_min, awu_max, align;
> +	struct request_queue	*q = bdev->bd_queue;
> +	struct xfs_mount	*mp = ip->i_mount;
> +
> +	/*
> +	 * Convert to multiples of the BLOCKSIZE (as we support a minimum
> +	 * atomic write unit of BLOCKSIZE).
> +	 */
> +	awu_min = queue_atomic_write_unit_min_bytes(q);
> +	awu_max = queue_atomic_write_unit_max_bytes(q);
> +
> +	awu_min &= ~mp->m_blockmask;

Why do you round /down/ the awu_min value here?

> +	awu_max &= ~mp->m_blockmask;

Actually -- since the atomic write units have to be powers of 2, why is
rounding needed here at all?

> +
> +	align = XFS_FSB_TO_B(mp, extsz);
> +
> +	if (!awu_max || !xfs_inode_atomicwrites(ip) || !align ||
> +	    !is_power_of_2(align)) {

..and if you take my suggestion to make a common helper to validate the
atomic write unit parameters, this can collapse into:

	alloc_unit_bytes = xfs_inode_alloc_unitsize(ip);
	if (!xfs_inode_has_atomicwrites(ip) ||
	    !bdev_validate_atomic_write(bdev, alloc_unit_bytes)) {
		/* not supported, return zeroes */
		*unit_min = 0;
		*unit_max = 0;
		return;
	}

	*unit_min = max(alloc_unit_bytes, awu_min);
	*unit_max = min(alloc_unit_bytes, awu_max);

--D

> +		*unit_min = 0;
> +		*unit_max = 0;
> +	} else {
> +		if (awu_min)
> +			*unit_min = min(awu_min, align);
> +		else
> +			*unit_min = mp->m_sb.sb_blocksize;
> +
> +		*unit_max = min(awu_max, align);
> +	}
> +}
> +
>  STATIC int
>  xfs_vn_getattr(
>  	struct mnt_idmap	*idmap,
> @@ -619,6 +657,13 @@ xfs_vn_getattr(
>  			stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
>  			stat->dio_offset_align = bdev_logical_block_size(bdev);
>  		}
> +		if (request_mask & STATX_WRITE_ATOMIC) {
> +			unsigned int unit_min, unit_max;
> +
> +			xfs_get_atomic_write_attr(ip, &unit_min, &unit_max);
> +			generic_fill_statx_atomic_writes(stat,
> +				unit_min, unit_max);
> +		}
>  		fallthrough;
>  	default:
>  		stat->blksize = xfs_stat_blksize(ip);
> diff --git a/fs/xfs/xfs_iops.h b/fs/xfs/xfs_iops.h
> index 7f84a0843b24..76dd4c3687aa 100644
> --- a/fs/xfs/xfs_iops.h
> +++ b/fs/xfs/xfs_iops.h
> @@ -19,4 +19,8 @@ int xfs_vn_setattr_size(struct mnt_idmap *idmap,
>  int xfs_inode_init_security(struct inode *inode, struct inode *dir,
>  		const struct qstr *qstr);
>  
> +void xfs_get_atomic_write_attr(struct xfs_inode *ip,
> +		unsigned int *unit_min,
> +		unsigned int *unit_max);
> +
>  #endif /* __XFS_IOPS_H__ */
> -- 
> 2.31.1
> 
>