While ECC memory is probably important and probably better than nothing, if there's one thing we've learned about Rowhammer it's that the obvious mitigations that "should" stop or detect it often fail to a clever attacker.

Just the first thing that popped into my head, but: say you watch the ECC correctable error rate over time, and somehow (not so easy!) determine which process is causing those errors. You forcibly kill the process and log a message about it, and also terminate/notify processes potentially affected (say, send them a SIGBUS or something and unmap the pages containing the affected data).

I, a "clever" attacker, use this to leak out your secrets- I do my hammering juuust right so that, if some secret bit is 0, your hammering flips ECC bits, while if it's 1 your hammering doesn't affect things. Lovely little side-channel.

Universal memory encryption and authentication seems to be the only sure way out of the cycle of "attack, mitigation, attack the mitigation", and it's already starting to roll out.