Re: [PATCH v2 3/3] xfs: Add alignment check for DAX mount

[Dave Chinner <david@fromorbit.com>]

On Mon, May 02, 2016 at 12:42:58PM -0600, Toshi Kani wrote:
> When a partition is not aligned by 4KB, mount -o dax succeeds,
> but any read/write access to the filesystem fails, except for
> metadata update.
> 
> Call bdev_direct_access to check the alignment when -o dax is
> specified.
> 
> Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Boaz Harrosh <boaz@plexistor.com>
> ---
>  fs/xfs/xfs_super.c |   23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 187e14b..b7ee323 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -1557,15 +1557,30 @@ xfs_fs_fill_super(
>  		sb->s_flags |= MS_I_VERSION;
>  
>  	if (mp->m_flags & XFS_MOUNT_DAX) {
> +		struct blk_dax_ctl dax = {
> +			.sector = 0,
> +			.size = PAGE_SIZE,
> +		};
>  		xfs_warn(mp,
> -	"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
> +		"DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
>  		if (sb->s_blocksize != PAGE_SIZE) {
>  			xfs_alert(mp,
>  		"Filesystem block size invalid for DAX Turning DAX off.");
>  			mp->m_flags &= ~XFS_MOUNT_DAX;
> -		} else if (!sb->s_bdev->bd_disk->fops->direct_access) {
> -			xfs_alert(mp,
> -		"Block device does not support DAX Turning DAX off.");
> +		} else if ((error = bdev_direct_access(sb->s_bdev, &dax)) < 0) {
> +			switch (error) {
> +			case -EOPNOTSUPP:
> +				xfs_alert(mp,
> +			"Block device does not support DAX Turning DAX off.");
> +				break;
> +			case -EINVAL:
> +				xfs_alert(mp,
> +			"Partition alignment invalid for DAX Turning DAX off.");
> +				break;
> +			default:
> +				xfs_alert(mp,
> +			"DAX access failed (%d) DAX Turning DAX off.", error);
> +			}

Please write a helper along the lines of:

	error = blkdev_supports_dax(sb->s_bdev, sb->s_blocksize);

and encapsulate all this, including the specific error messages in the
helper (i.e. "Block device %s does not support DAX."). Then the rest
of the filesystem code looks something like this:

	if (mp->m_flags & XFS_MOUNT_DAX) {
		error = blkdev_supports_dax(sb->s_bdev, sb->s_blocksize);
		if (error) {
			xfs_alert(mp,
		"DAX unsupported by block device. Turning off DAX.");
			mp->m_flags &= ~XFS_MOUNT_DAX;
		}
	}

And each filesystem can choose to do what it wants with the error
without having to care exactly why DAX is not supported.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com