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

[Matthew Rosato]

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.

> 
> Which, again, makes it weird/wrong that this KVM code is pinning it
> again :\
> 
> Jason