From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [EXT4 set 8][PATCH 1/1]Add journal checksums
Date: Wed, 11 Jul 2007 10:29:17 -0700
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Cc: Girish Shilamkar <girish@clusterfs.com>, cmm@us.ibm.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-ext4@vger.kernel.org
To: Andreas Dilger <adilger@clusterfs.com>
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On Wed, 11 Jul 2007 07:01:08 -0600 Andreas Dilger <adilger@clusterfs.com> wrote:

> > > > -	/* AKPM: buglet - add `i' to tmp! */
> > > 
> > > Damn.  After, what, seven years, someone actually fixed it?
> > > 
> > > >  	for (i = 0; i < bh->b_size; i += 512) {
> > > > -		journal_header_t *tmp = (journal_header_t*)bh->b_data;
> > > > +		struct commit_header *tmp =
> > > > +			(struct commit_header *)(bh->b_data + i);
> > > >  		tmp->h_magic = cpu_to_be32(JBD2_MAGIC_NUMBER);
> > > >  		tmp->h_blocktype = cpu_to_be32(JBD2_COMMIT_BLOCK);
> > > >  		tmp->h_sequence = cpu_to_be32(commit_transaction->t_tid);
> > > > +
> > > > +		if (JBD2_HAS_COMPAT_FEATURE(journal,
> > > > +					    JBD2_FEATURE_COMPAT_CHECKSUM)) {
> > > > +			tmp->h_chksum_type 	= JBD2_CRC32_CHKSUM;
> > > > +			tmp->h_chksum_size 	= JBD2_CRC32_CHKSUM_SIZE;
> > > > +			tmp->h_chksum[0] 	= cpu_to_be32(crc32_sum);
> > > > +		}
> > > >  	}
> > > 
> > > And in doing so, changed the on-disk format of the journal commit blocks.
> > > 
> > > Surely this was worth a mention in the changelog, if not a standalone patch?
> > > 
> > > I don't think this is worth doing, really.  Why not just leave the format
> > > as it was, take the loop out and run this code once rather than eight
> > > times?
> 
> Well, we aren't using the rest of the commit block in any case.  I think
> the original intention was that we'd get 8 copies of the commit block so
> we would be sure to get a good one.
> 
> I don't know whether we'd rather have 8 copies of the commit block, or
> more potential to expand the commit block?  I don't personally have any
> preference, since the checksum should be a more robust way of checking
> validity than having multiple copies, so we may as well remove the loop
> and stick with a single copy for now.

We've never altered any commit block sectors apart from the zeroeth one
(eight times) due to the above bug.  So I'd suggest that we should formalise
the old bug and leave the format as-is.  That'll leave lots of space spare in
the commit block.