Re: [PATCH 4/6] fs: xfs: Support atomic write for statx

[John Garry]

On 13/02/2024 17:37, Darrick J. Wong wrote:
> > We use this in the iomap and statx code
> >
> > > > +	struct xfs_inode *ip,
> > > > +	unsigned int *unit_min,
> > > > +	unsigned int *unit_max)
> > > Weird indenting here.
> > hmmm... I thought that this was the XFS style
> >
> > Can you show how it should look?
> The parameter declarations should line up with the local variables:
>
> void
> xfs_get_atomic_write_attr(
> 	struct xfs_inode	*ip,
> 	unsigned int		*unit_min,
> 	unsigned int		*unit_max)
> {
> 	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> 	struct block_device	*bdev = target->bt_bdev;
> 	struct request_queue	*q = bdev->bd_queue;
> 	struct xfs_mount	*mp = ip->i_mount;
> 	unsigned int		awu_min, awu_max, align;
> 	xfs_extlen_t		extsz = xfs_get_extsz(ip);
>
> > > > +{
> > > > +	xfs_extlen_t		extsz = xfs_get_extsz(ip);
> > > > +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> > > > +	struct block_device	*bdev = target->bt_bdev;
> > > > +	unsigned int		awu_min, awu_max, align;
> > > > +	struct request_queue	*q = bdev->bd_queue;
> > > > +	struct xfs_mount	*mp = ip->i_mount;
> > > > +
> > > > +	/*
> > > > +	 * Convert to multiples of the BLOCKSIZE (as we support a minimum
> > > > +	 * atomic write unit of BLOCKSIZE).
> > > > +	 */
> > > > +	awu_min = queue_atomic_write_unit_min_bytes(q);
> > > > +	awu_max = queue_atomic_write_unit_max_bytes(q);
> > > > +
> > > > +	awu_min &= ~mp->m_blockmask;
> > > Why do you round/down/  the awu_min value here?
> > This is just to ensure that we returning *unit_min >= BLOCKSIZE
> >
> > For example, if awu_min, max 1K, 64K from the bdev, we now have 0 and 64K.
> > And below this gives us awu_min, max of 4k, 64k.
> >
> > Maybe there is a more logical way of doing this.
> 	awu_min = roundup(queue_atomic_write_unit_min_bytes(q),
> 			  mp->m_sb.sb_blocksize);
>
> ?

Yeah, I think that all this can be simplified to be made more obvious.

> > > > +	awu_max &= ~mp->m_blockmask;
> > > Actually -- since the atomic write units have to be powers of 2, why is
> > > rounding needed here at all?
> > Sure, but the bdev can report a awu_min < BLOCKSIZE
> >
> > > > +
> > > > +	align = XFS_FSB_TO_B(mp, extsz);
> > > > +
> > > > +	if (!awu_max || !xfs_inode_atomicwrites(ip) || !align ||
> > > > +	    !is_power_of_2(align)) {
> > > ...and if you take my suggestion to make a common helper to validate the
> > > atomic write unit parameters, this can collapse into:
> > >
> > > 	alloc_unit_bytes = xfs_inode_alloc_unitsize(ip);
> > > 	if (!xfs_inode_has_atomicwrites(ip) ||
> > > 	    !bdev_validate_atomic_write(bdev, alloc_unit_bytes))  > 		/* not supported, return zeroes */
> > > 		*unit_min = 0;
> > > 		*unit_max = 0;
> > > 		return;
> > > 	}
> > >
> > > 	*unit_min = max(alloc_unit_bytes, awu_min);
> > > 	*unit_max = min(alloc_unit_bytes, awu_max);
> > Again, we need to ensure that *unit_min >= BLOCKSIZE
> The file allocation unit and hence the return value of
> xfs_inode_alloc_unitsize is always a multiple of sb_blocksize.

Right, but this value is coming from HW and we are just ensuring that 
the awu_min which we report is >= BLOCKSIZE. xfs_inode_alloc_unitsize() 
return value will really guide unit_max.

Anyway, again I can make this all more obvious.

Thanks,
John