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

John Garry
2024-07-08T07:37:20Z

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.
>