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

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Message-ID: <20250423155340.GA32225@lst.de>
Date: Wed, 23 Apr 2025 17:53:40 +0200
From: Christoph Hellwig <hch@....de>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christoph Hellwig <hch@....de>, John Garry <john.g.garry@...cle.com>,
	brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
	cem@...nel.org, linux-fsdevel@...r.kernel.org, dchinner@...hat.com,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	ojaswin@...ux.ibm.com, ritesh.list@...il.com,
	martin.petersen@...cle.com, linux-ext4@...r.kernel.org,
	linux-block@...r.kernel.org, catherine.hoang@...cle.com,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v8 11/15] xfs: commit CoW-based atomic writes atomically

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.