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

[David Hildenbrand]

On 02.05.23 18:06, Jason Gunthorpe wrote:
> On Tue, May 02, 2023 at 05:45:40PM +0200, David Hildenbrand wrote:
> > On 02.05.23 17:36, Jason Gunthorpe wrote:
> > > On Tue, May 02, 2023 at 11:32:57AM -0400, Peter Xu wrote:
> > > > > How does s390 avoid mmu notifiers without having lots of problems?? It
> > > > > is not really optional to hook the invalidations if you need to build
> > > > > a shadow page table..
> > > >
> > > > Totally no idea on s390 details, but.. per my read above, if the firmware
> > > > needs to make sure the page is always available (so no way to fault it in
> > > > on demand), which means a longterm pinning seems appropriate here.
> > > >
> > > > Then if pinned a must, there's no need for mmu notifiers (as the page will
> > > > simply not be invalidated anyway)?
> > >
> > > And what if someone deliberately changes the mapping?  memory hotplug
> > > in the VM, or whatever?
> >
> > Besides s390 not supporting memory hotplug in VMs (yet): if the guest wants
> > a different guest physical address, I guess that's the problem of the guest,
> > and it can update it:
> >
> > KVM_S390_ZPCIOP_REG_AEN is triggered from QEMU via
> > s390_pci_kvm_aif_enable(), triggered by the guest via a special instruction.
> >
> > If the hypervisor changes the mapping, it's just the same thing as mixing
> > e.g. MADV_DONTNEED with longterm pinning in vfio: don't do it. And if you do
> > it, you get to keep the mess you created for your VM.
> >
> > Linux will make sure to not change the mapping: for example, page migration
> > of a pinned page will fail.
> >
> > But maybe I am missing something important here.
>
> It missses the general architectural point why we have all these
> shootdown mechanims in other places - plares are not supposed to make
> these kinds of assumptions. When the userspace unplugs the memory from
> KVM or unmaps it from VFIO it is not still being accessed by the
> kernel.

Yes. Like having memory in a vfio iommu v1 and doing the same (mremap, 
munmap, MADV_DONTNEED, ...). Which is why we disable MADV_DONTNEED 
(e.g., virtio-balloon) in QEMU with vfio.

> Functional bug or not, it is inconsistent with how this is designed to
> work.

Sorry to say, I *really* don't see how that is supposed to work with a 
page that *cannot* be faulted back in on demand.

--
Thanks,

David / dhildenb