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    SubjectRe: [PATCH 07/22] ext4: Create bitmap checksum helper functions
    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.

    - Ted


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