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

[Matthew Rosato]

On 5/2/23 11:19 AM, Lorenzo Stoakes wrote:
> On Tue, May 02, 2023 at 05:09:06PM +0200, David Hildenbrand wrote:
>> 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.
>>
> 
> Yes apologies - thank you VERY much for doing this Matthew, but apologies, I
> made rather a clanger in patch 2 which would mean fast patch degrading to slow
> path would pass even for file-backed.
> 
> Will respin a v7 + cc you on that, if you could be so kind as to test that?
> 

Sure, will do!