I removed swap lockmap all together and, to my surprise, I can't
produce any ill behaviour on my system, not even under very heavy
swapping (in low memory condition).

I remember there were some issues when swap lockmap was removed in
2.1.89, so it was reintroduced later (processes were dying randomly).

Question is, why is everything running so smoothly now, even without
swap lockmap?

See for yourself, patch is attached (it is against testing/pre-8).