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

[Christian Borntraeger <borntraeger@linux.ibm.com>]

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.