/*
 * Check that all of the backup GDT blocks are held in the primary GDT block.
 * It is assumed that they are stored in group order.  Returns the number of
 * groups in current filesystem that have BACKUPS,or -ve error code.
 */
static int verify_reserved_gdb(struct super_block *sb,ext4_group_t end,struct buffer_head *primary)
{
    const ext4_fsblk_t blk = primary->b_blocknr;
    unsigned three = 1;
    unsigned five = 5;
    unsigned seven = 7;
    unsigned grp;
    __le32 *p = (__le32 *)primary->b_data;
    int gdbackups = 0;
 
    while ((grp = ext4_list_backups(sb,&three,&five,&seven)) < end) {
        if (le32_to_cpu(*p++) !=
            grp * EXT4_BLOCKS_PER_GROUP(sb) + blk){
            ext4_warning(sb,"reserved GDT %llu"
                     " missing grp %d (%llu)",blk,grp,grp *
                     (ext4_fsblk_t)EXT4_BLOCKS_PER_GROUP(sb) +
                     blk);
            return -EINVAL;
        }
        if (++gdbackups > EXT4_ADDR_PER_BLOCK(sb))
            return -EFBIG;
    }
 
    return gdbackups;
}

有人可以向我解释为什么这个变量是这样初始化的,这个函数在做什么？