Not entirely, but I think my question was just more narrow. Considering the case that started this off (machines that used to work, now hanging on boot because of lack of entropy, triggered by too few disk interrupts), I don't get how this generalizes. If you take that same system, and remove the disk (replacing it with netboot or whatever), how would it ever boot, at least not without making some changes to how early entropy is seeded? I guess in the netboot case you end up with the timing of network interrupts that you can feed into the entropy pool, but this whole thing just feels off to me.

If a system was relying on a specific, inefficient set of disk access to give it just enough entropy to get through early boot, and that making the FS code more efficient caused it to fail to have enough entropy, I'd suggest that (likely unbeknownst to the system administrators), this system was already pretty broken. I get why Torvalds decided to bucket this under the "we don't break userspace" umbrella, and appreciate the care he takes to that sort of thing, but suggesting that "FS access patterns during early boot" is sorta a part of the kernel's ABI is... a bit too incredible for me to take seriously.