Re: [PATCH] xfs: update XFS_IOC_DIOINFO memory alignment value

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

On Thu, Jul 11, 2024 at 10:36:37AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> As of v6.0, the DIO memory buffer alignment is no longer aligned to
> the logical sector size of the underlying block device. There is now
> a specific DMA alignment parameter that memory buffers should be
> aligned to. statx(STATX_DIOALIGN) gets this right, but
> XFS_IOC_DIOINFO does not - it still uses the older fixed alignment
> defined by the block device logical sector size.
> 
> This was found because the s390 DASD driver increased DMA alignment
> to PAGE_SIZE in commit bc792884b76f ("s390/dasd: Establish DMA
> alignment") and DIO aligned to logical sector sizes have started
> failing on kernels with that commit. Fixing the "userspace fails
> because device alignment constraints increased" issue is not XFS's
> problem, but we really should be reporting the correct device memory
> alignment in XFS_IOC_DIOINFO.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_ioctl.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index f0117188f302..71eba4849e03 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1368,7 +1368,8 @@ xfs_file_ioctl(
>  		struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
>  		struct dioattr		da;
>  
> -		da.d_mem =  da.d_miniosz = target->bt_logical_sectorsize;
> +		da.d_mem = bdev_dma_alignment(target->bt_bdev);

bdev_dma_alignment returns a mask, so I think you want to add one here?

Though at this point, perhaps DIOINFO should query the STATX_DIOALIGN
information so xfs doesn't have to maintain this anymore?

(Or just make a helper that statx and DIOINFO can both call?)

--D

> +		da.d_miniosz = target->bt_logical_sectorsize;
>  		da.d_maxiosz = INT_MAX & ~(da.d_miniosz - 1);
>  
>  		if (copy_to_user(arg, &da, sizeof(da)))
> -- 
> 2.45.1
> 
>