A useful VFS change ...

[Richard Sharpe]

Hi folks,

In discussions with Dustin an idea was proposed for a way to make VFS
writing easier for those who are accessing user-space file systems (of
which we are seeing more and more these days.)

One of the problems is that a user-space file system cannot, in
general, fall through to vfs_defaults, because, except in limited
cases, these call into the kernel via syscalls with FDs that make no
sense to the kernel.

This requires that the VFS write write code for every VFS function
with many of them simply returning ENOTSUP.

By adding a set of flags to the structure we could simplify things.
Perhaps we should go back to the old idea of OPAQUE vs TRANSPARENT VFS
modules. The default is TRANSPARENT, but if the writer sets the OPAQUE
bit, the infrastructure will return ENOTSUP if a call is made to a VFS
function that is not implemented.

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)