Re: [PATCH v2 08/13] xfs: Do not free EOF blocks for forcealign

[John Garry]

On 08/07/2024 02:44, Dave Chinner wrote:
> On Sat, Jul 06, 2024 at 09:56:09AM +0200, Christoph Hellwig wrote:
> > On Fri, Jul 05, 2024 at 04:24:45PM +0000, John Garry wrote:
> > > -	if (xfs_inode_has_bigrtalloc(ip))
> > > +
> > > +	/* Only try to free beyond the allocation unit that crosses EOF */
> > > +	if (xfs_inode_has_forcealign(ip))
> > > +		end_fsb = roundup_64(end_fsb, ip->i_extsize);
> > > +	else if (xfs_inode_has_bigrtalloc(ip))
> > >   		end_fsb = xfs_rtb_roundup_rtx(mp, end_fsb);
> >
> > Shouldn't we have a common helper to align things the right way?
>
> Yes, that's what I keep saying. 

Such a change was introduced in 
https://lore.kernel.org/linux-xfs/20240501235310.GP360919@frogsfrogsfrogs/

and, as you can see, Darrick was less than happy with it. That is why I 
kept this method which removed recently added RT code.

Darrick, can we find a better method to factor this code out, like below?

> The common way to do this is:
>
> 	align = xfs_inode_alloc_unitsize(ip);
> 	if (align > mp->m_blocksize)
> 		end_fsb = roundup_64(end_fsb, align);
>
> Wrapping that into a helper might be appropriate, though we'd need
> wrappers for aligning both the start (down) and end (up).
>
> To make this work, the xfs_inode_alloc_unitsize() code needs to grow
> a forcealign check. That overrides the RT rextsize value (force
> align on RT should work the same as it does on data devs) and needs
> to look like this:
>
> 	unsigned int		blocks = 1;
>
> +	if (xfs_inode_has_forcealign(ip)
> +		blocks = ip->i_extsize;
> -	if (XFS_IS_REALTIME_INODE(ip))
> +	else if (XFS_IS_REALTIME_INODE(ip))
>                  blocks = ip->i_mount->m_sb.sb_rextsize;

That's in 09/13

>          return XFS_FSB_TO_B(ip->i_mount, blocks);
>
> > But more importantly shouldn't this also cover hole punching if we
> > really want force aligned boundaries?

so doesn't the xfs_file_fallocate(PUNCH_HOLES) -> 
xfs_flush_unmap_range() -> rounding with xfs_inode_alloc_unitsize() do 
the required job?

> Yes, that's what I keep saying. There is no difference in the
> alignment behaviour needed for "xfs_inode_has_bigrtalloc" and
> "xfs_inode_has_forcealign" except for the source of the allocation
> alignment value.