Details on the UAPI of implementing notifications on pipes

[David Howells]

So to implement notifications on top of pipes, I've hacked it together a bit
in the following ways:

 (1) I'm passing O_TMPFILE to the pipe2() system call to indicate that you
     want a notifications pipe.  This prohibits splice and co. from being
     called on it as I don't want to have to try to fix iov_iter_revert() to
     handle kernel notifications being intermixed with splices.

     The choice of O_TMPFILE was just for convenience, but it needs to be
     something different.  I could, for instance, add a constant,
     O_NOTIFICATION_PIPE with the same *value* as O_TMPFILE.  I don't think
     it's likely that it will make sense to use O_TMPFILE with a pipe, but I
     also don't want to eat up another O_* constant just for this.

     Unfortunately, pipe2() doesn't have any other arguments into from which I
     can steal a bit.

 (2) I've added a pair of ioctls to configure the notifications bits.  They're
     ioctls as I just reused the ioctl code from my devmisc driver.  Should I
     use fcntl() instead, such as is done for F_SETPIPE_SZ?

     The ioctls do two things: set the ring size to a number of slots (so
     similarish to F_SETPIPE_SZ) and set filters.

Any thoughts on how better to represent these bits?

Thanks,
David