On Mon, Dec 05, 2011 at 11:24:06AM -0500, Ted Ts'o wrote:
> 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.
i_uuid_inum_csum seeds crc32c for all the metadata objects that are attached to
an inode (extents, dir blocks, xattrs), not just the inode itself. That way
you can ensure that a piece of metadata corresponds to a specific incarnation
of an inode, not just any of the inode's incarnations since mkfs time.
--D
>
> - Ted
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