linux-ext4 - Re: [PATCH 06/22] ext4: Calculate and verify inode checksums

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Date:	Mon, 5 Dec 2011 11:24:06 -0500
From:	Ted Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <djwong@...ibm.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 06/22] ext4: Calculate and verify inode checksums

On Mon, Nov 28, 2011 at 03:26:56PM -0800, Darrick J. Wong wrote:
> +	/* Precompute second piece of csum */
> +	if (EXT4_HAS_RO_COMPAT_FEATURE(sb,
> +			EXT4_FEATURE_RO_COMPAT_METADATA_CSUM)) {
> +		__u32 csum;
> +		struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> +		__le32 inum = cpu_to_le32(inode->i_ino);
> +		__le32 gen = cpu_to_le32(inode->i_generation);
> +		csum = ext4_chksum(sbi, sbi->s_uuid_csum, (__u8 *)&inum,
> +				   sizeof(inum));
> +		ei->i_uuid_inum_csum = ext4_chksum(sbi, csum, (__u8 *)&gen,
> +						   sizeof(gen));
> +	}

Why do we include a copy of i_generation in the precomputed initial
part of the checksum?  Since i_generation is in the raw, on-disk
version of the inode, what's the rationale for including it here?  It
shouldn't *hurt*, but it a few extra CPU cycles, and I'm not seeing
how it helps.

						- Ted
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