Re: Re: Next steps with pv_ops for Xen

[Gerd Hoffmann <kraxel@redhat.com>]

Derek Murray wrote:
> I take the blame for that one. I added the hook because, if a process
> were to die whilst holding one or more grants, there were no hooks that
> would make it possible to carry out the grant-unmap. All existing hooks
> on either the device or the VMA were called *after* the PTEs were cleared.

Hmm.  What exactly is the issue here?

This is about *userspace* mappings, right?  As far as I can see from a
quick scan there of the code is an additional kernel space mapping for
the grants and the userspace mapping is optional.  I don't see any
problems with userspace mapping going away without *instant*
notification.  Cleaning up a bit later, called from the
file_ops->release callback maybe, should work ok.

The problem I see with the additional vm_ops callback is that I suspect
you'll have to come up with some *very* good arguments to get it
accepted by the VM (as in "virtual memory") folks and merged mainline.

> It gets better, though. The same hook is used in the version of blktap
> in linux-2.6.18-xen (not, as far as I can see, in the sparse tree for
> xen-3.1-testing):

Oh, I'm thinking more in the direction of killing blktap altogether in
favor of a pure userspace implementation on top of gntdev.

cheers,
  Gerd