We say that in the passive voice, subject has action performed on it by someone or something else.

Active voice: The hungry students ate the pizza.
Verb: ate
Who/what did the eating : hungry students
So, hungry students is the subject in this case.

Passive voice: The pizza was eaten by hungry students.
Verb: was eaten
Who/what was eaten : Pizza
Isn't pizza the subject in this case
Also, the subjects are never objects of preposition, by is the preposition in this case, so how can we say that the subject is pizza

Since the subject cant be pizza , so how do we identify passive voice.