Thanks for the keyword! Searching for "rowhammer anvil" led me to https://web.eecs.umich.edu/~genkin/papers/another-flip-rowha... which links to https://lwn.net/Articles/704920/ which is the article I was thinking of. I can't find much else about that kernel patch other than https://patchwork.kernel.org/patch/9400475/ and https://patchwork.kernel.org/patch/9401819/ so that approach was probably abandoned.