Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[David Hildenbrand]

On 02.05.23 15:56, Matthew Rosato wrote:
> On 5/2/23 9:50 AM, Jason Gunthorpe wrote:
> > On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
> > > > Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
> > >
> > > Okay, so IIRC we'll fail starting the domain early, that's good. And if we
> > > pin all guest memory (instead of small pieces dynamically), there is little
> > > existing use for file-backed RAM in such zPCI configurations (because memory
> > > cannot be reclaimed either way if it's all pinned), so likely there are no
> > > real existing users.
> >
> > Right, this is VFIO, the physical HW can't tolerate not having pinned
> > memory, so something somewhere is always pinning it.
>
> I might have mis-explained above.
>
> With iommufd nesting, we will pin everything upfront as a starting point.
>
> The current usage of vfio type1 iommu for s390 does not pin the entirety of guest memory upfront, it happens as guest RPCITs occur / type1 mappings are made.

... so, after the domain started successfully on the libvirt/QEMU side ? :/

It would be great to confirm that. There might be a BUG in patch #2 (see 
my reply to patch #2) that might not allow you to reproduce it right now.

--
Thanks,

David / dhildenb