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

Olaf Hering
2012-03-15T17:10:57Z

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.

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.

Olaf