Re: [PATCH v3 09/12] xfs: Add xfs_file_dio_write_atomic()

[John Garry <john.g.garry@oracle.com>]

On 28/02/2025 15:39, Darrick J. Wong wrote:
>>> One last little nit here: if the filesystem doesn't have reflink, you
>>> can't use copy on write as a fallback.
>>>
>>> 		/*
>>> 		 * The atomic write fallback uses out of place writes
>>> 		 * implemented with the COW code, so we must fail the
>>> 		 * atomic write if that is not supported.
>>> 		 */
>>> 		if (!xfs_has_reflink(ip->i_mount))
>>> 			return -EOPNOTSUPP;
>>> 		dio_flags = IOMAP_DIO_ATOMIC_SW | IOMAP_DIO_FORCE_WAIT;
>>>
>> Currently the awu max is limited to 1x FS block if no reflink, and then we
>> check the write length against awu max in xfs_file_write_iter() for
>> IOCB_ATOMIC. And the xfs iomap would not request a SW-based atomic write for
>> 1x FS block. So in a around-about way we are checking it.
>>
>> So let me know if you would still like that additional check - it seems
>> sensible to add it.
> Yes, please.  The more guardrails the better, particularly when someone
> gets around to enabling software-only RWF_ATOMIC.

ok, but I think that adding it at the start of 
xfs_atomic_write_sw_iomap_begin() is a better place (to add it).

It seems a bit neater (than adding it here) with the retry handling and 
locking/unlocking, and would save adding it in another possible future 
callsite for xfs_atomic_write_sw_iomap_begin().

Cheers,
John