The Savage Attacker feat (PHB 169) says:
Once per turn when you roll damage for a melee weapon attack, you can reroll the weapon’s damage dice and use either total.
Critical hits (PHB 196):
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.
For example, if you score a critical hit with a dagger, roll 2d4 for the damage, rather than 1d4, and then add your relevant ability modifier. If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.
If a character with Savage Attacker gets a crit, what happens? Sticking with their dagger example, I see at least two interpretations:
You have two instances of d4 weapon damage. Since Savage Attacker applies once per turn, you can choose either of the dice (after rolling) to reroll.
Your weapon deals 2d4 damage, so you can choose to reroll the 2d4 and take the higher total.
In the event, I ruled option 2, but in retrospect I think maybe option 1 might make more sense. Some people on enworld claimed 2, without any discussion of the possibilities (though there the OP was pretty confused).
How is it supposed to work?