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