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Date: Wed, 23 Apr 2025 17:53:40 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, John Garry <john.g.garry@oracle.com>,
	brauner@kernel.org, viro@zeniv.linux.org.uk, jack@suse.cz,
	cem@kernel.org, linux-fsdevel@vger.kernel.org, dchinner@redhat.com,
	linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org,
	ojaswin@linux.ibm.com, ritesh.list@gmail.com,
	martin.petersen@oracle.com, linux-ext4@vger.kernel.org,
	linux-block@vger.kernel.org, catherine.hoang@oracle.com,
	linux-api@vger.kernel.org
Subject: Re: [PATCH v8 11/15] xfs: commit CoW-based atomic writes atomically
Message-ID: <20250423155340.GA32225@lst.de>
References: <20250422122739.2230121-1-john.g.garry@oracle.com> <20250422122739.2230121-12-john.g.garry@oracle.com> <20250423082307.GA29539@lst.de> <20250423145850.GA25675@frogsfrogsfrogs>
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In-Reply-To: <20250423145850.GA25675@frogsfrogsfrogs>
User-Agent: Mutt/1.5.17 (2007-11-01)

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.