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

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Date: Wed, 14 Feb 2024 12:26:27 +0000
From: John Garry <john.g.garry@...cle.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
        dchinner@...hat.com, jack@...e.cz, chandan.babu@...cle.com,
        martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com
Subject: Re: [PATCH 4/6] fs: xfs: Support atomic write for statx

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