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

[John Garry]

On 28/02/2025 01:19, Darrick J. Wong wrote:
> > +	if (ret == -EAGAIN && !(iocb->ki_flags & IOCB_NOWAIT) &&
> > +	    !(dio_flags & IOMAP_DIO_ATOMIC_SW)) {
> > +		xfs_iunlock(ip, iolock);
> > +		dio_flags = IOMAP_DIO_ATOMIC_SW | IOMAP_DIO_FORCE_WAIT;
> 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.

Cheers,
John