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Date: Fri, 25 Sep 2026 07:58:53 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Carlos Maiolino <cem@kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <brauner@kernel.org>, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 06/21] xfs: prepare xfs_rtfile_initialize_blocks for
 larger than FSB blocks
Message-ID: <20260925055853.GD3193@lst.de>
References: <20260924100032.2733101-1-hch@lst.de> <20260924100032.2733101-7-hch@lst.de> <20260924220311.GH2705364@frogsfrogsfrogs>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Thu, Sep 24, 2026 at 03:03:11PM -0700, Darrick J. Wong wrote:
> > +	size_t			copylen = len;
> > +	struct xfs_trans_res	tres = M_RES(mp)->tr_growrtzero;
> >  	struct xfs_trans	*tp;
> >  	struct xfs_buf		*bp;
> > -	const size_t		copylen = mp->m_blockwsize << XFS_WORDLOG;
> >  	int			error;
> >  
> > -	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_growrtzero, 0, 0, 0, &tp);
> > +	tres.tr_logres *= nblks;
> 
> Hmm.  Is it safe to multiply the log reservation by an arbitrary
> block count?  I would think we'd want *some* guarantee that we can't
> create a transaction that's larger than the log can support.
> 
> It might suffice to put in a safeguard like:
> 
> 	/* Log should always be able to handle 64k of logged buffers */
> 	ASSERT(nblks <= XFS_B_TO_FSB(mp, SZ_64K));

Sounds good.  Or recalculate the entire reservation, but that feels a bit
wasteful.

> > +	if (xfs_has_rtgroups(mp))
> > +		copylen -= sizeof(struct xfs_rtbuf_blkinfo);
> 
> This is how we maintain copylen as the amount of non-header data to copy
> out of *data, correct? 

Yes.

> I suppose that means that the checksum file blocks also have a header?

Yes.  We need this to see if the checksums are corrupted, and then with
the follow on series potentially read it from another mirror.