What was found
FLT 1791, AIR TURNBACK & EMERGENCY DECLARED - AFTER TAKEOFF CLIMBING THROUGH 500 FT, RECEIVED "LAV SMOKE" MESSAGE & ALARM. AFT F/A REPORTED SMOKE STARTED IN FWD CABIN THEN AFT THEN MID. SMOKE WAS COMING IN THROUGH SIDEWALL VENTS. SMOKE DISSIPATED ON DESCENT, "LAV SMOKE" MESSAGE & ALARM CONTINUOUS THROUGH FLT. F/A REPORTED OIL ODOR ASSOCIATED WITH SMOKE. PERFORMED TSM TASK 21-20-00-810-801A AND FOUND NO DEFECTS IN CABIN. INSPECTED NR 1 AND NR 2 PACK IAW 21-51-05 AND FOUND NR 2 HEAT EXCHANGER CONTAMINATED. PERFORMED ENG PNEUMATIC BLEED CHECK IAW AMM 36-11-00 AND FOUND STRONG ODOR COMING FROM PACK NR 2 WITH ENG BLEEDS OR APU BLEED. PLACED PACK NR 2 ON MDDR 118101 IAW MEL 21-51-00 COND NR 1.