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@xxxxxx>
> ---
> 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;
> + else
> + stat->dio_offset_align = stat->dio_read_offset_align;
It might be worth making it explicitly clear that logical block size
aligned IO for COW operations will still work. I think that's what
you are trying to say, but it took me a while to work out. Perhaps
something like:
/*
* COW operations are inefficient on sub-fsblock aligned
* ranges. They need to copy the entire block, so the
* minimum IO size we will ever do in this case is a single
* filesystem block.
*
* Even though we support sector sized IO on COW inodes, we
* want to help applications avoid the costly RMW cycle it
* requires for COW inodes. Hence report the native
* filesystem allocation unit size here to indicate the
* smallest alignment that will avoid RMW cycles in the DIO
* write path.
*/
-Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
