[xfs-masters] Re: Interaction between Xen and XFS: stray RW mappings

[Nick Piggin <nickpiggin@xxxxxxxxxxxx>]

On Monday 22 October 2007 14:28, dean gaudet wrote:
> On Sun, 21 Oct 2007, Jeremy Fitzhardinge wrote:
> > dean gaudet wrote:
> > > On Mon, 15 Oct 2007, Nick Piggin wrote:
> > >> Yes, as Dave said, vmap (more specifically: vunmap) is very expensive
> > >> because it generally has to invalidate TLBs on all CPUs.
> > >
> > > why is that?  ignoring 32-bit archs we have heaps of address space
> > > available... couldn't the kernel just burn address space and delay
> > > global TLB invalidate by some relatively long time (say 1 second)?
> >
> > Yes, that's precisely the problem.  xfs does delay the unmap, leaving
> > stray mappings, which upsets Xen.
>
> sounds like a bug in xen to me :)

You could call it a bug I think. I don't know much about Xen though,
whether or not it expects to be able to run an arbitrary OS kernel.

Presumably, the hypervisor _could_ write protect and trap writes to
*all* page table page mappings.