Re: [PATCH] xen-mapcache: don''t unmap locked entry during mapcache invalidation

Stefano Stabellini
2012-03-15T17:33:22Z

On Thu, 15 Mar 2012, Olaf Hering wrote:
> On Thu, Mar 15, Stefano Stabellini wrote:
>
> > On Thu, 15 Mar 2012, Julien Grall wrote:
> > > When an IOREQ_TYPE_INVALIDATE is sent to QEMU, it invalidates all
entry
> > > of the map cache even if it''s locked.
> > >
> > > QEMU is not able to know that entry was invalidated, so when an
IO
> > > access is requested a segfault occured.
> >
> > The problem here is the long term mappings in QEMU that cannot easily
be
> > re-created.
> > I am not sure whether this can cause any trouble to things like
> > xenpaging.
>
> Yes, I was wondering about this as well. If the request is made, then
> its expected that all mappings disappear because they can point to
> ballooned gfns. IF that case is safe, then all is fine.

The long lived mappings shouldn''t involve any ballooned gfns, unless
something went terribly wrong.


> In case of xenpaging its not an issue because xenpaging just asks qemu
> to release mappings so that the usage count of mfns drops to 1, so that
> they can be nominated for eviction. If that fails, not a big deal as it
> just means that no more pages can be evicted. xenpaging will retry.

Great, sounds like there is no problem then.