ZFS On Linux's kernel modules issues are not like NVidia's
In the Hacker News discussion of my entry on the general risk to ZFS from the shift in its userbase towards ZFS on Linux, a number of people suggested that the risks to ZFS on Linux were actually low since proprietary kernel modules such as NVidia's GPU drivers don't seem to have any problems dealing with things like the kernel making various functions inaccessible or GPL-only. I have two views on this, because I think that the risks are a bit different and less obvious than what they initially look like.
On a purely technical level, ZFS on Linux probably has it easier and is at less risk than NVidia's GPU drivers. Advanced GPU drivers deal with hardware that only they can work with and may need to do various weird sorts of operations for memory mapping, DMA control, and so on that aren't needed or used by existing kernel modules. It's at least possible that the Linux kernel could not support access to this sort of stuff from any kernel module (in tree or out, regardless of license), and someday leave a GPU driver up the creek.
By contrast, most in-tree filesystems are built as loadable modules (for good reasons) so the kernel already provides to modules everything necessary to support a filesystem, and it's likely that ZFS on Linux could survive with just this. What ZFS on Linux needs from the kernel is likely to be especially close to what BTRFS needs, since both are dealing with very similar core issues like talking to multiple disks at once, checksum computation, and so on, and there is very little prospect that BTRFS will either be removed from the kernel tree or only be supported if built into the kernel itself.
But on the political and social level it's another thing entirely. NVidia and other vendors of proprietary kernel modules have already decided that they basically don't care about anything except what they can implement. Licenses and people's views of their actions are irrelevant; if they can do it technically and they need to in order to make their driver work, they will. GPL shim modules to get access to GPL-only kernel symbols are just the starting point.
Most of the people involved in ZFS on Linux are probably not going to feel this way. Sure, ZFS on Linux could implement shim modules and other workarounds if the kernel cuts off full access to necessary things, but I don't think they're going to. ZFS on Linux developers are open source developers in a way that NVidia's driver programmers are not, and if the Linux kernel people yell at them hard enough they will likely go away, not resort to technical hacks to get around the technical barriers.
In other words, the concern with ZFS on Linux is not that it will become technically unviable, because that's unlikely. The concern is that it will become socially unviable, that to continue on a technical level its developers and users would have to become just as indifferent to the social norms of the kernel license as NVidia is.
(And if that did happen, which it might, I think it would make ZFS on Linux much more precarious than it currently is, because ZoL would be relying on its ability to find and keep both good kernel developers and sources of development funding that are willing to flout social norms in a way that they don't have to today.)