REVIEW 3 major objections 5 minor 37 references
Efficient Machine Unlearning by Model Splitting and Core Sample Selection
T0 review · 3 major / 5 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read MAXRR makes most unlearning requests exact by construction, with no retraining needed for samples outside the core set and the SVM support vectors.
desk verdict A genuinely new unlearning-aware training scheme with a useful core-set design principle, but the exact-unlearning guarantee fails as proved because the core set is selected from the full data and is therefore not independent of the forgotten points. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the dual soft-margin SVM, whose decision function is a weighted combination of training samples that have nonzero dual variables (the support vectors). Proposition 1 establishes that removing any non-support sample from the training set leaves the dual optimum feasible and optimal, so the model literally does not change. Model splitting turns this property into an architecture: the network's last layer is replaced by a linear SVM trained on the feature embeddings. Core sample selection ranks all training samples by how many times they become support vectors across multiple runs, defines $D_k$ as the top $k$, and trains the feature extractor only on $D_k$, making it structurally independent of all non-core samples. Definition 1 generalizes exact unlearning so that equality to any algorithm $A'$ trained on any remaining subset $D_p$ is sufficient; that generalization is what converts 'no retraining needed' into a formal exact-unlearning certificate.
What would settle it
A decisive calculation: take a non-core, non-support-vector sample, apply MAXRR's exact-unlearning protocol for it, and compute the divergence between the resulting distribution and the distribution of the original algorithm retrained on $D \setminus D_f$; a nonzero divergence shows the exactness certificate is an artifact of Definition 1 rather than of matching full retraining. A second check is whether $D_k$ remains unchanged when that sample is excluded from the ranking procedure—if not, the proof's $A'$ is not independent of $D_f$.
Extended reading notes
Core claim
The paper's central claim is that a large fraction of unlearning requests can be made exact by construction. With the MAXRR composition $w_{\mathrm{FE}} \circ w_{\mathrm{SVM}}$, the feature extractor $w_{\mathrm{FE}}$ is trained only on a core set $D_k$ of samples that are most frequently selected as support vectors across several initial runs, and the linear SVM $w_{\mathrm{SVM}}$ is trained on the full dataset's embeddings. Proposition 1 shows that deleting a non-support-vector sample leaves the SVM's optimum unchanged, and the FE is independent of every sample outside $D_k$ by design. Hence for any request $D_f \subseteq D \setminus (S \cup D_k)$ the stored checkpoint is already exactly unlearned in the sense of Definition 1, and for $D_f \subset S \setminus D_k$ only the SVM layer needs retraining, which is still exact. The reported Fashion-MNIST experiments put the exact-unlearning coverage at $|S \cup D_k| = 20.5 \cdot 10^3$ out of $60 \cdot 10^3$ training samples on average.
Load-bearing premise
The load-bearing premise is that the generalized definition of exact unlearning—comparing the unlearned model with any algorithm trained on any subset of the remaining data, rather than with the original algorithm retrained on the full remaining set—is an acceptable operationalization, and that the core set used to train the feature extractor is fixed independently of the samples later requested for deletion; if either gives way, the exactness claim no longer holds.
Editorial extensions
If this is right
- Deleting any sample outside $S \cup D_k$ is free: the stored checkpoint is already exactly unlearned under Definition 1 and no retraining is triggered.
- When the forgotten sample lies in $S \setminus D_k$, the whole unlearning cost is one SVM retraining on the remaining embeddings; the feature extractor, usually the expensive part, is untouched.
- The unlearned model can be more accurate than a fully retrained model, since the core samples that drive feature quality stay in the feature extractor (0.8798 vs 0.8625 average accuracy in the reported core-unlearning experiment).
- Because the guarantee is structural, it transfers to any classifier whose final layer is a linear SVM, not just to the specific architecture tested.
- MAXRR is compatible with stricter unlearning methods as a preprocessing step, so a provider can upgrade to stronger guarantees when needed.
Reading between the lines
- Implicit in the exactness proof is that the core set $D_k$ is chosen without using the samples that will later be forgotten; the paper's ranking procedure runs on the full dataset $D$, so unless $D_k$ is fixed externally or before any request, the $A'$ in Definition 1 may not actually be independent of $D_f$.
- If a regulator adopts Definition 1, the right to be forgotten becomes an architectural property: compliance is achieved by how the model is built, not by post-hoc deletion. If not, MAXRR's contribution reduces to an efficient approximate-unlearning scheme with the same weak-verification caveats as other approaches.
- The exact-unlearning coverage is governed by the overlap $S \cup D_k$; on datasets where support-vector membership is unstable across runs, the core set must grow to keep accuracy, shrinking the fraction of deletion requests that can be served for free. This is a testable prediction for other datasets.
- Appendix B already shows confidence-based MIAs cannot verify genuinely exact unlearning of easy samples, so the approximate-mode verification results in Figs. 3-4 should be read as evidence of behavioral similarity, not as proof that residual information is gone.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes MAXRR, an unlearning-aware training procedure for SVM-headed classifiers. The model is split into a feature extractor (FE) and a linear SVM; the FE is trained on a core subset D_k of the most influential samples, determined by support-vector frequency over multiple runs on the full dataset, while the SVM is trained on all data. The authors introduce a generalized exact-unlearning definition (Definition 1) that allows comparison to any algorithm A' trained on any subset D_p of the remaining data, and they claim that for any request D_f disjoint from S ∪ D_k, the model is "per design exactly unlearned." When D_f intersects D_k, they fall back to approximate unlearning and verify it with a confidence-based membership inference attack. The paper reports Fashion MNIST experiments showing competitive accuracy and similar MIA verification results between MAXRR and full retraining.
Significance. If the exact-unlearning claim were correct, MAXRR would be a genuinely useful recipe: most deletion requests become free or cheap for SVM-headed classifiers, and the paper would contribute a clean decomposition of the training procedure into a reusable feature extractor and a retrainable last layer. The writing is clear, the algorithm is simple to implement, and the authors honestly acknowledge the limitations of MIA-based verification, including in Appendix B the fact that their MIA cannot detect even exact unlearning of non-core samples. However, the central claim rests on a weakened definition and on an independence argument that does not hold as written; the substantive exact-unlearning guarantee is therefore not established, and the approximate case is supported only by a verification method the paper itself shows to be unreliable. The contribution is potentially salvageable, but the current manuscript substantially overstates what is proven.
major comments (3)
- [Section IV-B, exact unlearning paragraph] The proof that any D_f ⊆ D\(S∪D_k) is exactly unlearned asserts that "the feature extractor w_FE is entirely independent of D\D_k." This is not correct under the procedure described in Section IV: the core set D_k is selected by ranking support-vector frequencies computed over the full dataset D, so the identity of D_k and hence the trained w_FE are functions of D, including any later-requested D_f. The comparison algorithm A'(D_p) constructed in the next paragraph reuses the same D_k; because D_k was inferred from a dataset that contains D_f, that A' is not independent of D_f under the usual meaning of Definition 1. Removing D_f before the ranking stage could select a different core set and a different feature extractor, so U(A(D),D,D_f) is not generally equal to A(D\D_f) even when D_f contains no core samples and no support vectors. The authors need either to prove that the core-selection procedure is independent of every possible deletion request, e.g. by choosing D_k without using the deletable data, or to explicitly restrict the claim to a fixed precomputed D_k and explain what privacy guarantee that provides.
- [Definition 1 and the abstract] The abstract states that MAXRR supports "exact unlearning" and that its properties "closely match" full retraining, without the crucial caveat that its exactness is relative to Definition 1, a substantial weakening of the standard notion. Definition 1 allows the unlearned model to be compared to any algorithm A' on any subset D_p of the remaining data, and Section IV-B then chooses A' and D_p so that U equals A'(D_p) by construction. This makes the exactness guarantee definitional rather than a property validated against an independent baseline, and it raises the question of what privacy property Definition 1 actually provides. The paper should either adopt the standard comparison A(D\D_f) (in which the core-selection dependence is fatal), or clearly motivate Definition 1 as a meaningful operationalization and advertise the guarantee as such in the abstract and introduction.
- [Section IV-B(b) and Appendix B] The evidence for the approximate-unlearning claim is not load-bearing enough. For the case D_f ∩ D_k ≠ ∅, the paper verifies unlearning by showing that a confidence-based MIA yields similar classifications for MAXRR and for a fully retrained model (Figures 3 and 4). However, Appendix B shows that the same MIA cannot reliably detect whether non-core samples were unlearned even when the unlearning is exact. A verification method that fails to separate an exactly unlearned model from a non-unlearned one cannot support the claim that an approximate method "closely matches" full retraining. The authors should either provide a stronger verification argument (a different attack, a distributional distance, or a formal approximate-unlearning bound) or explicitly weaken the claim to an empirical utility comparison rather than an unlearning guarantee.
minor comments (5)
- [Section I, first paragraph] The phrase "theright to be forgotten" in the abstract and introduction is missing a space; this is a simple typographical error.
- [Section II, decision function] The decision function uses "sgin(x)" where the text presumably means "sign(x)"; please correct this typo.
- [Section III, Proposition 1 proof] The proof argues that after removing D_f, an improved solution on D\D_f would also be feasible for D and contradict optimality; this is correct for the objective value, but the argument should explicitly handle the case of multiple optimal dual solutions, since the decision boundary depends on b and the chosen support-vector set.
- [Section IV-A] The text says "Assume access to a ranking" but does not specify whether the ranking is part of the training algorithm and whether it may access samples that will later be forgotten; clarifying this would make the exactness claim easier to evaluate.
- [Figures 3 and 4] The axis label "Percentage of runs both algorithms output same verification result" is confusing because agreement between MAXRR and perfect unlearning is not the same as successful unlearning; please rephrase and clarify in the caption and text.
Circularity Check
Exact unlearning is defined into existence: Definition 1 allows any A' on any subset, and the exactness proof sets A' to MAXRR's own pipeline; core set D_k is fit on the full dataset, so independence from D_f is not established.
-
self definitional
[Section II, Definition 1; Section IV-B (Exact Unlearning)]
"Definition 1 (Generalized exact unlearning). A model A(D) is said to unlearn D_f ⊂ D if there exists an algorithm A′ independent of D_f, and a data subset D_p ⊆ D\D_f such that D(P(U(A(D),D,D_f))∥P(A′(D_p))) = 0. ... Upon an unlearning request, the procedure reuses w_FE and retrains the SVM on the remaining data D_p = D\D_f. This is functionally equivalent to running A′(D_p), which applies Algorithm 1 on D_k ⊂ D_p to obtain w_FE, and then trains a new SVM w⋆_SVM on D_p."
The success criterion lets the paper choose A' and D_p freely, so the equality holds by construction once A' is defined to be the paper's own pipeline (Algorithm 1 on D_k plus SVM on D_p). The guarantee is not measured against A(D\D_f) or any fixed external retraining procedure; the definition itself creates the guarantee. The asserted independence of A' from D_f is load-bearing and is not established, since A' uses D_k, which was selected from the full dataset D.
-
fitted input called prediction
[Section IV (Core Sample Selection) and Section IV-B (Exact Unlearning)]
"Using this insight, we rank samples by their frequency of being selected as support vectors. Let f_ℓ denote the frequency for each sample x_ℓ. Define D_k to be the subset of the k most frequently occurring support vectors (i.e., core samples)... For any unlearning request D_f ⊆ D\(S∪D_k), the model is per design exactly unlearned in the sense of Definition 1. This is because the feature extractor w_FE is entirely independent of D\D_k, and the SVM w⋆_SVM depends only on the support vectors S."
D_k is a fitted artifact of the full dataset D: the ranking of support-vector frequencies comes from multiple training runs on D. A later request D_f is part of that same D, so w_FE (trained on D_k) is not independent of D_f; if D_f had been absent during core selection, a different D_k' could have been chosen. The 'prediction' that non-core requests leave w_FE unchanged is therefore forced by a core set that was itself fit to data containing the forgotten samples. The exactness claim is conditional on a D_k whose identity the request helped determine.
full rationale
MAXRR's empirical accuracy comparisons against a fully retrained baseline (Table I, Figs. 3-6) are genuine external evaluations and are not circular. However, the formal exact-unlearning claim is circular. Definition 1 weakens the standard requirement by allowing any algorithm A' and any subset D_p of the remaining data; the proof in Section IV-B then exhibits A'(D_p) as exactly the output of MAXRR's own training/unlearning composition. Since the definition asks only for existence of such an A', the equality is true by construction. The additional assertion that A' is independent of D_f fails for the full training procedure because D_k is selected from D via repeated SVM training on all of D, so D_f can influence which samples become core samples. Appendix B confirms that the MIA verification cannot even certify the exact case, which is consistent with a guarantee that is definitional rather than externally testable. No self-citation chain is present; the circularity is in the definition and in the fitted core set.
Assumptions & free parameters
free parameters (3)
- Core set size k =
20,000
- Number of runs for support-vector frequency =
100
- SVM regularization C =
not reported
assumptions (4)
- standard math Removing non-support-vector samples from an SVM training set leaves the optimal dual solution unchanged, as stated in Proposition 1.
- ad hoc to paper The generalized exact unlearning definition, Definition 1, allowing any algorithm A' and any subset D_p, is an acceptable operationalization of unlearning.
- domain assumption Support-vector frequency across runs identifies the most important samples for the feature extractor.
- domain assumption Fashion MNIST and LeNet-5 are representative of practical machine learning deployments.
Cite this review
Pith. "Pith review of Efficient Machine Unlearning by Model Splitting and Core Sample Selection." pith.science (2026). https://pith.science/paper/TA7PCW2Y
@misc{pith2026250507026,
author = {Pith},
title = {Pith review of: Efficient Machine Unlearning by Model Splitting and Core Sample Selection},
year = {2026},
howpublished = {\url{https://pith.science/paper/TA7PCW2Y}},
note = {Machine review of arXiv:2505.07026}
}
read the original abstract
Machine unlearning is essential for meeting legal obligations such as the right to be forgotten, which requires the removal of specific data from machine learning models upon request. While several approaches to unlearning have been proposed, existing solutions often struggle with efficiency and, more critically, with the verification of unlearning - particularly in the case of weak unlearning guarantees, where verification remains an open challenge. We introduce a generalized variant of the standard unlearning metric that enables more efficient and precise unlearning strategies. We also present an unlearning-aware training procedure that, in many cases, allows for exact unlearning. We term our approach MaxRR. When exact unlearning is not feasible, MaxRR still supports efficient unlearning with properties closely matching those achieved through full retraining.
Figures
Figures from the paper (2 more)
Reference graph
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