Even making such a warning conditional on kernel version is problematic, because many distros backport major blocks of code, including perhaps btrfs fixes, and the nominally 3.14 or whatever kernel may actually be running btrfs and other fixes from 4.14 or later, by the time they actually drop support for whatever LTS distro version and quit backporting fixes.
This information could be stored in kernel and made available for usermode tools via some proc file. This would be very useful _especially_ considering backporting. Raid56 could be fixed already (or not) by the time it is implemented, but no doubt there will still be other highly experimental capabilities judging by how things go. And this feature itself could easily be backported.
Some machine-readable readiness level (ok/warning/override flag needed/known but disabled in kernel) plus one-line text message displayed to users in cases 2-4 is all we need. If proc file is missing or doesn't contain information about specific capability, tools could default to current behavior (AFAIR there're already warnings in some cases). Message should tersely cover any known issues, including stability, performance, compatibility and general readiness, and may contain links (to btrfs wiki?) for more information. I expect whole file to easily fit in 512 bytes.
-- With Best Regards, Marat Khalili -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
