linux-ext4 - Re: [PATCH, REWORKED 01/11] Add initial checksum support for the gdt_checksum/uninit

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Message-ID: <20080317180505.GG8368@mit.edu>
Date:	Mon, 17 Mar 2008 14:05:05 -0400
From:	Theodore Tso <tytso@....edu>
To:	Andreas Dilger <adilger@....com>
Cc:	linux-ext4@...r.kernel.org, "Jose R. Santos" <jrs@...ibm.com>,
	Andreas Dilger <adilger@...sterfs.com>
Subject: Re: [PATCH, REWORKED 01/11] Add initial checksum support for the
	gdt_checksum/uninit_group feature

On Tue, Mar 18, 2008 at 01:22:54AM +0800, Andreas Dilger wrote:
> On Mar 17, 2008  09:28 -0400, Theodore Ts'o wrote:
> > +STATIC __u16 ext2fs_group_desc_csum(ext2_filsys fs, dgrp_t group)
  ....
> > +	if (fs->super->s_feature_ro_compat & EXT4_FEATURE_RO_COMPAT_GDT_CSUM) {
> > +		int offset = offsetof(struct ext2_group_desc, bg_checksum);
> > +
  ...
> > +		offset += sizeof(desc->bg_checksum); /* skip checksum */
> > +		assert(offset == sizeof(*desc));
> 
> Note that this assertion needs to be removed when the group descriptor
> becomes larger, unless ext2_group_desc never changes in the future..

Yeah, I looked at this, and was half tempted to remove it.

Indeed ext2_group_desc will never change in the future.  Too many
things would break if we change it.  That's why there is an
ext4_group_desc which looks exactly like ext2_group_desc for the first
32 bytes, and adds the high 32-bits for the various fields in the
second 32-bytes.

But if ext2_group_desc is never going to change, then there's no real
good reason to use a run-time check here.  Better to turn it into a
compile time check, using #error.  The tricky part is doing it in a
way which is ANSI-C compliant (or maybe we just wrap it in a #ifdef
GCC and only do the sanity check if you are compiling with GCC).

    	     	    	   	    - Ted
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