You're mostly correct, but there are a couple of things to note:

1) Under Linux, getting the `head' pointer requires an ioctl.  This is
both good and bad; it's good in that it doesn't mess with the size of
the buffer or require wasting a page due to alignment, but it's bad in
that it's more overhead.  

2) There is no `tail' pointer in the Linux version.  From my reading
of the code, it will just wrap around and play the buffer again if the
process is too slow.