A320 First Officer reported receiving an ECAM for smoke in the aft cargo compartment. Extinguishing agent was discharged and flight returned to departure airport.

Narrative

On Day 0; while operating ZZZ-ZZZ1; during the climb phase of flight; we received an ECAM for SMOKE AFT CARGO SMOKE. I was PM during this leg and started ECAM actions when directed. After turning as CAB FANS per the procedure; the initial ECAM disappeared; leading us to believe it was a spurious alert. A short moment later; I noticed what I felt was the presence of a smoke odor in the cockpit. I alerted the Captain to the smell and he agreed that it did not smell right. At this time; I called to the FWD flight attendant and asked them if everything was normal in the back. The flight attendant advised that people were starting to stand up and they were trying to figure out why. I asked if there was smoke in the cabin; and then she noticed it and advised there was smoke coming from the approximate area of rows XX-XY. Almost simultaneously; we got the AFT CARGO SMOKE ECAM again; we followed through with the entire ECAM; discharging the aft cargo agent into the bay. The Captain and I decided to [request priority handling] at this time and start heading back to ZZZ. I advised ATC and requested immediate turn back to the airport. I advised the flight attendants what was happening. At this time we received the HYD Y RSVR LO LVL ECAM; and performed ECAM actions. Once we had started our turn and descent back towards the airport; I advised the passengers that we would be returning out of an abundance of caution" back to ZZZ. ATC gave us routing directly to the ILS for RWY ZZ; the Captain advised dispatch with ACARS and we continued to the airport. I continued to communicate with the FA's and requested they secure the cabin immediately for landing. The flight attendants advised they were moving passengers away from the rows to get them away from the smoke. The flight attendants advised me when the cabin was secured prior to landing. I updated the passengers during our descent; appreciably 10 minutes out; advising we would be landing in 10 minutes and to expect a normal landing. We landed without further incident and were followed back to pad #2 by Airport Rescue and Firefighting (ARFF)."