In the opening scene of “ The Smashing Machine ,” Benny Safdie ’s bracing, clear-eyed, and laceratingly humane sports biopic, we see grainy staged video footage of Mark Kerr ( Dwayne Johnson ), the mixed martial arts and Ultimate Fighting Championship star, in his very first amateur bout. Kerr originally won notoriety as a wrestler, and in case anyone has wandered into the movie thinking that it might be about “fake wrestling,” this fight will dispel that delusion: It ends with Kerr crouching on top of his opponent, assaulting him with one merciless bare-knuckle punch after another, reducing his face to a bloody pulp.

As all of this is happening, we hear Kerr’s voice on the soundtrack — it’s a voice that’s disarmingly gentle and sweet — describing, to an interviewer, the high he get

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