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

[David Hildenbrand]

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.

--
Thanks,

David / dhildenb