NXIVM gal an early jail bird

Convicted “slave master” Allison Mack is already serving her time for her role in the Nxivm sex cult — checking into federal prison more than two weeks earlier than she needed to, records show.

The disgraced former “Smallville” actress (above) is listed in online records as inmate 90838-053 at the Federal Correctional Institution in Dublin, Calif. Her release date is listed as “unknown.”

The Federal Bureau of Prisons confirmed to The Post that Mack checked in Monday — even though she did not need to surrender until Sept. 29.

She had pleaded guilty to being the right-hand woman for perverted cult leader Keith Raniere, who was sentenced in October to 120 years in prison for turning numerous women into sex slaves who were then branded.

Mack had faced up to 17¹/2 years behind bars — but in June got a wrist-slap sentence of three years after cooperating with investigators to bring down the twisted cult.