Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

[David Hildenbrand <david@redhat.com>]

Hi,

On 19.06.24 04:44, John Hubbard wrote:
> On 6/18/24 5:05 PM, Elliot Berman wrote:
>> In arm64 pKVM and QuIC's Gunyah protected VM model, we want to support
>> grabbing shmem user pages instead of using KVM's guestmemfd. These
>> hypervisors provide a different isolation model than the CoCo
>> implementations from x86. KVM's guest_memfd is focused on providing
>> memory that is more isolated than AVF requires. Some specific examples
>> include ability to pre-load data onto guest-private pages, dynamically
>> sharing/isolating guest pages without copy, and (future) migrating
>> guest-private pages.  In sum of those differences after a discussion in
>> [1] and at PUCK, we want to try to stick with existing shmem and extend
>> GUP to support the isolation needs for arm64 pKVM and Gunyah.

The main question really is, into which direction we want and can 
develop guest_memfd. At this point (after talking to Jason at LSF/MM), I 
wonder if guest_memfd should be our new target for guest memory, both 
shared and private. There are a bunch of issues to be sorted out though ...

As there is interest from Red Hat into supporting hugetlb-style huge 
pages in confidential VMs for real-time workloads, and wasting memory is 
not really desired, I'm going to think some more about some of the 
challenges (shared+private in guest_memfd, mmap support, migration of 
!shared folios, hugetlb-like support, in-place shared<->private 
conversion, interaction with page pinning). Tricky.

Ideally, we'd have one way to back guest memory for confidential VMs in 
the future.


Can you comment on the bigger design goal here? In particular:

1) Who would get the exclusive PIN and for which reason? When would we
    pin, when would we unpin?

2) What would happen if there is already another PIN? Can we deal with
    speculative short-term PINs from GUP-fast that could introduce
    errors?

3) How can we be sure we don't need other long-term pins (IOMMUs?) in
    the future?

4) Why are GUP pins special? How one would deal with other folio
    references (e.g., simply mmap the shmem file into a different
    process).

5) Why you have to bother about anonymous pages at all (skimming over s
    some patches), when you really want to handle shmem differently only?

>> To that
>> end, we introduce the concept of "exclusive GUP pinning", which enforces
>> that only one pin of any kind is allowed when using the FOLL_EXCLUSIVE
>> flag is set. This behavior doesn't affect FOLL_GET or any other folio
>> refcount operations that don't go through the FOLL_PIN path.

So, FOLL_EXCLUSIVE would fail if there already is a PIN, but 
!FOLL_EXCLUSIVE would succeed even if there is a single PIN via 
FOLL_EXCLUSIVE? Or would the single FOLL_EXCLUSIVE pin make other pins 
that don't have FOLL_EXCLUSIVE set fail as well?

>>
>> [1]: https://lore.kernel.org/all/20240319143119.GA2736@willie-the-truck/
>>
> 
> Hi!
> 
> Looking through this, I feel that some intangible threshold of "this is
> too much overloading of page->_refcount" has been crossed. This is a very
> specific feature, and it is using approximately one more bit than is
> really actually "available"...

Agreed.

> 
> If we need a bit in struct page/folio, is this really the only way? Willy
> is working towards getting us an entirely separate folio->pincount, I
> suppose that might take too long? Or not?

Before talking about how to implement it, I think we first have to learn 
whether that approach is what we want at all, and how it fits into the 
bigger picture of that use case.

> 
> This feels like force-fitting a very specific feature (KVM/CoCo handling
> of shmem pages) into a more general mechanism that is running low on
> bits (gup/pup).

Agreed.

> 
> Maybe a good topic for LPC!

The KVM track has plenty of guest_memfd topics, might be a good fit 
there. (or in the MM track, of course)

-- 
Cheers,

David / dhildenb