From: "Darrick J. Wong" <djwong@us.ibm.com>
Subject: Re: [PATCH 07/22] ext4: Create bitmap checksum helper functions
Date: Mon, 5 Dec 2011 12:31:41 -0800
Message-ID: <20111205203141.GF7137@tux1.beaverton.ibm.com>
References: <20111128232615.19194.80081.stgit@elm3c44.beaverton.ibm.com> <20111128232703.19194.55084.stgit@elm3c44.beaverton.ibm.com> <20111205163329.GC32031@thunk.org>
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To: "Ted Ts'o" <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Sunil Mushran <sunil.mushran@oracle.com>,
	Martin K Petersen <martin.petersen@oracle.com>,
	Greg Freemyer <greg.freemyer@gmail.com>,
	Amir Goldstein <amir73il@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Andi Kleen <andi@firstfloor.org>,
	Mingming Cao <cmm@us.ibm.com>,
	Joel Becker <jlbec@evilplan.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-ext4@vger.kernel.org, Coly Li <colyli@gmail.com>
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On Mon, Dec 05, 2011 at 11:33:29AM -0500, Ted Ts'o wrote:
> On Mon, Nov 28, 2011 at 03:27:03PM -0800, Darrick J. Wong wrote:
> > +static __u32 ext4_bitmap_csum(struct super_block *sb, ext4_group_t group,
> > +			      struct buffer_head *bh, int sz)
> > +{
> > +	__u32 csum;
> > +	struct ext4_sb_info *sbi = EXT4_SB(sb);
> > +
> > +	group = cpu_to_le32(group);
> > +	csum = ext4_chksum(sbi, sbi->s_uuid_csum, (__u8 *)&group,
> > +			   sizeof(group));
> > +	csum = ext4_chksum(sbi, csum, (__u8 *)bh->b_data, sz);
> > +
> > +	return csum;
> > +}
> 
> Note: it's strictly speaking not necessary to mix in the group and
> s_csum_seed here.  It's useful for the inode table blocks (ITB's)
> because the checksum for a particular ITB is located *in* the ITB
> itself.  So if an ITB gets written to the wrong place, and in
> particular, on top of another ITB, we want to be able to know which
> cloned copy was written to the wrong place on disk.
> 
> But in the case of the inode and block allocation bitmaps, the
> checksums are stored in the block group descriptors; so if the bitmap
> is written to the wrong place (and on top of another bitmap), the
> checksum will fail to verify, independent of whether we've mixed in
> the fs-specific csum seed and the group number.
>
> So I'd suggest dropping this, which will shave a few cycles off of the
> checksum calculation, and it will also simplify the code since we
> won't need this particular function.

Ok, I can get rid of the function, but I'll keep using sbi->s_uuid_csum as the
crc32c seed since it's basically free.

--D
> 
> 						- Ted
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