Re: [PATCH 3/3] xfs: report the correct read/write dio alignment for reflinked inodes

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Aug 28, 2024 at 08:11:03AM +0300, Christoph Hellwig wrote:
> For I/O to reflinked blocks we always need to write an entire new
> file system block, and the code enforces the file system block alignment
> for the entire file if it has any reflinked blocks.
> 
> Use the new STATX_DIO_READ_ALIGN flag to report the asymmetric read
> vs write alignments for reflinked files.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_iops.c | 37 +++++++++++++++++++++++++++++--------
>  1 file changed, 29 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 1cdc8034f54d93..de2fc12688dc23 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -570,6 +570,33 @@ xfs_stat_blksize(
>  	return PAGE_SIZE;
>  }
>  
> +static void
> +xfs_report_dioalign(
> +	struct xfs_inode	*ip,
> +	struct kstat		*stat)
> +{
> +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> +	struct block_device	*bdev = target->bt_bdev;
> +
> +	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
> +	stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
> +	stat->dio_read_offset_align = bdev_logical_block_size(bdev);
> +
> +	/*
> +	 * On COW inodes we are forced to always rewrite an entire file system
> +	 * block.
> +	 *
> +	 * Because applications assume they can do sector sized direct writes
> +	 * on XFS we provide an emulation by doing a read-modify-write cycle
> +	 * through the cache, but that is highly inefficient.  Thus report the
> +	 * natively supported size here.
> +	 */
> +	if (xfs_is_cow_inode(ip))
> +		stat->dio_offset_align = ip->i_mount->m_sb.sb_blocksize;

xfs_inode_alloc_unitsize(), since we can only cow full allocation units.

(Not necessary today, but we might as well make it work for rtreflink
from the start.)

--D

> +	else
> +		stat->dio_offset_align = stat->dio_read_offset_align;
> +}
> +
>  STATIC int
>  xfs_vn_getattr(
>  	struct mnt_idmap	*idmap,
> @@ -635,14 +662,8 @@ xfs_vn_getattr(
>  		stat->rdev = inode->i_rdev;
>  		break;
>  	case S_IFREG:
> -		if (request_mask & STATX_DIOALIGN) {
> -			struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> -			struct block_device	*bdev = target->bt_bdev;
> -
> -			stat->result_mask |= STATX_DIOALIGN;
> -			stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
> -			stat->dio_offset_align = bdev_logical_block_size(bdev);
> -		}
> +		if (request_mask & (STATX_DIOALIGN | STATX_DIO_READ_ALIGN))
> +			xfs_report_dioalign(ip, stat);
>  		fallthrough;
>  	default:
>  		stat->blksize = xfs_stat_blksize(ip);
> -- 
> 2.43.0
> 
>