Re: [PATCH v8 11/15] xfs: commit CoW-based atomic writes atomically

[Christoph Hellwig <hch@lst.de>]

On Wed, Apr 23, 2025 at 07:58:50AM -0700, Darrick J. Wong wrote:
> > > +xfs_calc_default_atomic_ioend_reservation(
> > > +	struct xfs_mount	*mp,
> > > +	struct xfs_trans_resv	*resp)
> > > +{
> > > +	if (xfs_has_reflink(mp))
> > > +		resp->tr_atomic_ioend = resp->tr_itruncate;
> > > +	else
> > > +		memset(&resp->tr_atomic_ioend, 0,
> > > +				sizeof(resp->tr_atomic_ioend));
> > > +}
> > 
> > What is the point of zeroing out the structure for the non-reflink
> > case?  Just as a poision for not using it when not supported as no
> > code should be doing that?  Just thinking of this because it is a
> > potentially nasty landmine for the zoned atomic support.
> 
> Yes.  I thought about adding a really stupid helper:

Why don't we just always set up the xfs_trans_resv structure?  We
do that for all kinds of other transactions not supported as well,
don't we?

> static inline bool xfs_has_sw_atomic_write(struct xfs_mount *mp)
> {
> 	return xfs_has_reflink(mp);
> }
> 
> But that seemed too stupid so I left it out.  Maybe it wasn't so dumb,
> since that would be where you'd enable ZNS support by changing that to:
> 
> 	return xfs_has_reflink(mp) || xfs_has_zoned(mp);

But that helper might actually be useful in various places, so
independent of the above I'm in favor of it.