Re: [PATCH 15/21] fs: xfs: Support atomic write for statx

[Dave Chinner]

On Fri, Sep 29, 2023 at 10:27:20AM +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 | 51 +++++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_iops.h |  4 ++++
>  2 files changed, 55 insertions(+)
> 
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 1c1e6171209d..5bff80748223 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -546,6 +546,46 @@ xfs_stat_blksize(
>  	return PAGE_SIZE;
>  }
>  
> +void xfs_ip_atomic_write_attr(struct xfs_inode *ip,
> +			xfs_filblks_t *unit_min_fsb,
> +			xfs_filblks_t *unit_max_fsb)

Formatting.

Also, we don't use variable name shorthand for function names -
xfs_get_atomic_write_hint(ip) to match xfs_get_extsz_hint(ip)
would be appropriate, right?



> +{
> +	xfs_extlen_t		extsz_hint = xfs_get_extsz_hint(ip);
> +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> +	struct block_device	*bdev = target->bt_bdev;
> +	struct xfs_mount	*mp = ip->i_mount;
> +	xfs_filblks_t		atomic_write_unit_min,
> +				atomic_write_unit_max,
> +				align;
> +
> +	atomic_write_unit_min = XFS_B_TO_FSB(mp,
> +		queue_atomic_write_unit_min_bytes(bdev->bd_queue));
> +	atomic_write_unit_max = XFS_B_TO_FSB(mp,
> +		queue_atomic_write_unit_max_bytes(bdev->bd_queue));

These should be set in the buftarg at mount time, like we do with
sector size masks. Then we don't need to convert them to fsbs on
every single lookup.

> +	/* for RT, unset extsize gives hint of 1 */
> +	/* for !RT, unset extsize gives hint of 0 */
> +	if (extsz_hint && (XFS_IS_REALTIME_INODE(ip) ||
> +	    (ip->i_diflags2 & XFS_DIFLAG2_FORCEALIGN)))

Logic is non-obvious. The compound is (rt || force), not
(extsz && rt), so it took me a while to actually realise I read this
incorrectly.

	if (extsz_hint &&
	    (XFS_IS_REALTIME_INODE(ip) ||
	     (ip->i_diflags2 & XFS_DIFLAG2_FORCEALIGN))) {

> +		align = extsz_hint;
> +	else
> +		align = 1;

And now the logic looks wrong to me. We don't want to use extsz hint
for RT inodes if force align is not set, this will always use it
regardless of the fact it has nothing to do with force alignment.

Indeed, if XFS_DIFLAG2_FORCEALIGN is not set, then shouldn't this
always return min/max = 0 because atomic alignments are not in us on
this inode?

i.e. the first thing this code should do is:

	*unit_min_fsb = 0;
	*unit_max_fsb = 0;
	if (!(ip->i_diflags2 & XFS_DIFLAG2_FORCEALIGN))
		return;

Then we can check device support:

	if (!buftarg->bt_atomic_write_max)
		return;

Then we can check for extent size hints. If that's not set:

	align = xfs_get_extsz_hint(ip);
	if (align <= 1) {
		unit_min_fsb = 1;
		unit_max_fsb = 1;
		return;
	}

And finally, if there is an extent size hint, we can return that.

> +	if (atomic_write_unit_max == 0) {
> +		*unit_min_fsb = 0;
> +		*unit_max_fsb = 0;
> +	} else if (atomic_write_unit_min == 0) {
> +		*unit_min_fsb = 1;
> +		*unit_max_fsb = min_t(xfs_filblks_t, atomic_write_unit_max,
> +					align);

Why is it valid for a device to have a zero minimum size? If it can
set a maximum, it should -always- set a minimum size as logical
sector size is a valid lower bound, yes?

> +	} else {
> +		*unit_min_fsb = min_t(xfs_filblks_t, atomic_write_unit_min,
> +					align);
> +		*unit_max_fsb = min_t(xfs_filblks_t, atomic_write_unit_max,
> +					align);
> +	}

Nothing here guarantees the power-of-2 sizes that the RWF_ATOMIC
user interface requires....

It also doesn't check that the extent size hint is aligned with
atomic write units.

It also doesn't check either against stripe unit alignment....

> +}
> +
>  STATIC int
>  xfs_vn_getattr(
>  	struct mnt_idmap	*idmap,
> @@ -614,6 +654,17 @@ 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) {
> +			xfs_filblks_t unit_min_fsb, unit_max_fsb;
> +
> +			xfs_ip_atomic_write_attr(ip, &unit_min_fsb,
> +				&unit_max_fsb);
> +			stat->atomic_write_unit_min = XFS_FSB_TO_B(mp, unit_min_fsb);
> +			stat->atomic_write_unit_max = XFS_FSB_TO_B(mp, unit_max_fsb);

That's just nasty. We pull byte units from the bdev, convert them to
fsb to round them, then convert them back to byte counts. We should
be doing all the work in one set of units....

> +			stat->attributes |= STATX_ATTR_WRITE_ATOMIC;
> +			stat->attributes_mask |= STATX_ATTR_WRITE_ATOMIC;
> +			stat->result_mask |= STATX_WRITE_ATOMIC;

If the min/max are zero, then atomic writes are not supported on
this inode, right? Why would we set any of the attributes or result
mask to say it is supported on this file?


-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx