Re: [PATCH v6 10/13] xfs: iomap COW-based atomic write support

[Christoph Hellwig <hch@lst.de>]

On Mon, Mar 17, 2025 at 10:18:58AM +0000, John Garry wrote:
> On 17/03/2025 07:26, Christoph Hellwig wrote:
>>> +static bool
>>> +xfs_bmap_valid_for_atomic_write(
>>
>> This is misnamed.  It checks if the hardware offload an be used.
>
> ok, so maybe:
>
> xfs_bmap_atomic_write_hw_possible()?

That does sound better.

> Fine, so it will be something like "atomic writes are required to be 
> naturally aligned for disk blocks, which is a block layer rule to ensure 
> that we won't straddle any boundary or violate write alignment 
> requirement".

Much better!  Maybe spell out the actual block layer rule, though?

>>
>> Should the atomic and cow be together for coherent naming?
>> But even if the naming is coherent it isn't really
>> self-explanatory, so please add a little top of the function
>> comment introducing it.
>
> I can add a comment, but please let me know of any name suggestion

/*
 * Handler for atomic writes implemented by writing out of place through
 * the COW fork.  If possible we try to use hardware provided atomicy
 * instead, which is handled directly in xfs_direct_write_iomap_begin.
 */

>
>>
>>> +	error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
>>> +			&nimaps, 0);
>>> +	if (error)
>>> +		goto out_unlock;
>>
>> Why does this need to read the existing data for mapping?  You'll
>> overwrite everything through the COW fork anyway.
>>
>
> We next call xfs_reflink_allocate_cow(), which uses the imap as the basis 
> to carry the offset and count.

Is xfs_reflink_allocate_cow even the right helper to use?  We know we
absolutely want a a COW fork extent, we know there can't be delalloc
extent to convert as we flushed dirty data, so most of the logic in it
is pointless.