RESTOR: Knowledge Recovery in Machine Unlearning

TL;DR

RESTOR is a framework that evaluates machine unlearning algorithms by measuring their ability to forget specific data and recover the original knowledge state, revealing insights into their mechanisms and effectiveness.

Contribution

This work introduces RESTOR, a novel evaluation framework for machine unlearning that assesses both forgetting and knowledge recovery, addressing limitations of previous heuristics.

Findings

Some algorithms focus only on forgetting, not recovery

Localizing unlearning targets improves performance

RESTOR uncovers mechanisms of popular unlearning methods

Abstract

Large language models trained on web-scale corpora can memorize undesirable data containing misinformation, copyrighted material, or private or sensitive information. Recently, several machine unlearning algorithms have been proposed to eliminate the effect of such datapoints from trained models -- that is, to approximate a model that had never been trained on these datapoints in the first place. However, evaluating the effectiveness of unlearning algorithms remains an open challenge. Previous work has relied on heuristics -- such as verifying that the model can no longer reproduce the specific information targeted for removal while maintaining accuracy on unrelated test data. These approaches inadequately capture the complete effect of reversing the influence of datapoints on a trained model. In this work, we propose the RESTOR framework for machine unlearning evaluation, which assesses the ability of unlearning algorithms for targeted data erasure, by evaluating the ability of models to forget the knowledge introduced in these datapoints, while simultaneously recovering the model's knowledge state had it never encountered these datapoints. RESTOR helps uncover several novel insights about popular unlearning algorithms, and the mechanisms through which they operate -- for instance, identifying that some algorithms merely emphasize forgetting but not recovering knowledge, and that localizing unlearning targets can enhance unlearning performance.