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

Darrick J. Wong
2025-04-23T15:58:52Z

On Wed, Apr 23, 2025 at 05:53:40PM +0200, Christoph Hellwig wrote:
> 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?

Works for me. There's really no harm in it mirroring tr_itruncate since
it won't affect the log size calculation.

> > 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.

<nod> John, who should work on the next round, you or me?

--D