EXT2_DESC_PER_BLOCK
offset = block_group & (EXT2_DESC_PER_BLOCK(sb) - 1);
ilog2 (EXT2_DESC_PER_BLOCK(sb));
db_count = (sbi->s_groups_count + EXT2_DESC_PER_BLOCK(sb) - 1) /
EXT2_DESC_PER_BLOCK(sb);