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

[David Hildenbrand]

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.

--
Thanks,

David / dhildenb