I am basing this on the assumption that trolls in the Potterverse require at least somewhat regular eating. They are said to have a fondness for human flesh, and presumably creatures of that size don’t just exist on nothing.
When Harry and Hermione get past McGonagall’s protection of the Philosopher’s Stone (the chess set), they are met with Quirrell’s protection, a giant troll (thankfully knocked out).
So if we’re assuming that the troll has been in this dungeon-like place for the entire school year, ever since the Stone was moved there—then who has been feeding it?
The obvious answer would be Quirrell himself, since it was his protection—but that doesn’t add up, for at least three reasons:
Quirrell had to do the whole hooded-stranger-with-a-dragon’s-egg tap-and-dance to figure out how to get past Fluffy, so obviously he wouldn’t have been regularly getting past him to feed the troll.
When Quirrell does get to his troll on his way to the Stone, he injures the flying key charmed by Flitwick—if he’d been down there to feed the troll regularly, he would surely either have bent that key completely out of shape by now, or had a way to get to it more easily and safely.
Quirrell is terrified of trolls, if you base your belief on his reaction to the troll on Halloween, where the mere presence of one in the dungeons is enough to send him fleeing to the Great Hall and faint from, ostensibly, fear. Not the kind of person you’d set to feed a troll in a distant, underground location.
Then again, one may wonder why Quirrell’s (faked) fear of trolls, including the theatrical faint in the Great Hall at Halloween, appears to be accepted by the staff (excluding Snape) to begin with. Presumably at least some of the staff—or at the very least Dumbledore himself—are aware that Quirrell has a ‘gift’ with trolls, and indeed managed to get one even bigger than the Halloween one into this dungeon-like place under the school. This must have happened long before Halloween, and it makes little sense to believe that he has enough bravado with trolls to get one into the protection rooms, but not enough to deal with one in the school dungeons. Either he’s scared witless of them, or he’s good with them.
Is there anything in the Potterverse (even non-canonical) that gives any indication of this?