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

[Matthew Rosato]

On 5/2/23 10:57 AM, David Hildenbrand wrote:
> On 02.05.23 15:35, Matthew Rosato wrote:
>> 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.
>>
> 
> That's ... unexpected. :)
> 
> Either this series doesn't work as expected or you end up using a filesystem that is still compatible. But I guess most applicable filesystems (ext4, btrfs, xfs) all have a page_mkwrite callback and should, therefore, disallow long-term pinning with this series.
> 

The memory backend file is on ext4 in my tests.  A quick trace shows that pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable is still returning positive implying pages are being pinned.