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

[David Hildenbrand <david@redhat.com>]

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