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

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.

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