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

[Matthew Rosato]

On 5/2/23 9:04 AM, Christian Borntraeger wrote:
> 
> 
> Am 02.05.23 um 14:54 schrieb Lorenzo Stoakes:
>> On Tue, May 02, 2023 at 02:46:28PM +0200, Christian Borntraeger wrote:
>>> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes:
>>>> Writing to file-backed dirty-tracked mappings via GUP is inherently broken
>>>> as we cannot rule out folios being cleaned and then a GUP user writing to
>>>> them again and possibly marking them dirty unexpectedly.
>>>>
>>>> This is especially egregious for long-term mappings (as indicated by the
>>>> use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as
>>>> we have already done in the slow path.
>>>
>>> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery?
>>> It would no longer work with file backed memory, correct?
>>>
>>> See
>>> arch/s390/kvm/pci.c
>>>
>>> kvm_s390_pci_aif_enable
>>> which does have
>>> FOLL_WRITE | FOLL_LONGTERM
>>> to
>>>
>>
>> Does this memory map a dirty-tracked file? It's kind of hard to dig into where
>> the address originates from without going through a ton of code. In worst case
>> if the fast code doesn't find a whitelist it'll fall back to slow path which
>> explicitly checks for dirty-tracked filesystem.
> 
> It does pin from whatever QEMU uses as backing for the guest.
>>
>> We can reintroduce a flag to permit exceptions if this is really broken, are you
>> able to test? I don't have an s390 sat around :)
> 
> Matt (Rosato on cc) probably can. In the end, it would mean having
>   <memoryBacking>
>     <source type="file"/>
>   </memoryBacking>
> 
> In libvirt I guess.

I am running with this series applied using a QEMU guest with memory-backend-file (using the above libvirt snippet) for a few different PCI device types and AEN forwarding (e.g. what is setup in kvm_s390_pci_aif_enable) is still working.