I think Jeremy is accurate in saying the below.
Jeremy Allison wrote:
What compelling reason is there for doing this in the kernel?
Because without kernel support there is no way someone can publish a new metadata type and have it automatically supported by all application data files (ie. most apps ignore it, and only apps that are aware of it can see it). Without kernel support you have to have all apps agree on a data format. And that's harder to do than getting linux kernel VFS engineers to agree on things :-).
Jeremy.
