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

From: David Hildenbrand
Date: Wed Jun 19 2024 - 08:17:09 EST

On 19.06.24 11:11, Fuad Tabba wrote:
Hi John and David,

Thank you for your comments.

On Wed, Jun 19, 2024 at 8:38 AM David Hildenbrand <david@xxxxxxxxxx> wrote:

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.

As you know, initially we went down the route of guest memory and
invested a lot of time on it, including presenting our proposal at LPC
last year. But there was resistance to expanding it to support more
than what was initially envisioned, e.g., sharing guest memory in
place migration, and maybe even huge pages, and its implications such
as being able to conditionally mmap guest memory.

Yes, and I think we might have to revive that discussion, unfortunately. I started thinking about this, but did not reach a conclusion. Sharing my thoughts.

The minimum we might need to make use of guest_memfd (v1 or v2 ;) ) not just for private memory should be:

(1) Have private + shared parts backed by guest_memfd. Either the same,
or a fd pair.
(2) Allow to mmap only the "shared" parts.
(3) Allow in-place conversion between "shared" and "private" parts.
(4) Allow migration of the "shared" parts.

A) Convert shared -> private?
* Must not be GUP-pinned
* Must not be mapped
* Must not reside on ZONE_MOVABLE/MIGRATE_CMA
* (must rule out any other problematic folio references that could
read/write memory, might be feasible for guest_memfd)

B) Convert private -> shared?
* Nothing to consider

C) Map something?
* Must not be private

For ordinary (small) pages, that might be feasible. (ZONE_MOVABLE/MIGRATE_CMA might be feasible, but maybe we could just not support them initially)

The real fun begins once we want to support huge pages/large folios and can end up having a mixture of "private" and "shared" per huge page. But really, that's what we want in the end I think.

Unless we can teach the VM to not convert arbitrary physical memory ranges on a 4k basis to a mixture of private/shared ... but I've been told we don't want that. Hm.


There are two big problems with that that I can see:

1) References/GUP-pins are per folio

What if some shared part of the folio is pinned but another shared part that we want to convert to private is not? Core-mm will not provide the answer to that: the folio maybe pinned, that's it. *Disallowing* at least long-term GUP-pins might be an option.

To get stuff into an IOMMU, maybe a per-fd interface could work, and guest_memfd would track itself which parts are currently "handed out", and with which "semantics" (shared vs. private).

[IOMMU + private parts might require that either way? Because, if we dissallow mmap, how should that ever work with an IOMMU otherwise].

2) Tracking of mappings will likely soon be per folio.

page_mapped() / folio_mapped() only tell us if any part of the folio is mapped. Of course, what always works is unmapping the whole thing, or walking the rmap to detect if a specific part is currently mapped.


Then, there is the problem of getting huge pages into guest_memfd (using hugetlb reserves, but not using hugetlb), but that should be solvable.


As raised in previous discussions, I think we should then allow the whole guest_memfd to be mapped, but simply SIGBUS/... when trying to access a private part. We would track private/shared internally, and track "handed out" pages to IOMMUs internally. FOLL_LONGTERM would be disallowed.

But that's only the high level idea I had so far ... likely ignore way too many details.

Is there broader interest to discuss that and there would be value in setting up a meeting and finally make progress with that?

I recall quite some details with memory renting or so on pKVM ... and I have to refresh my memory on that.


To be honest, personally (speaking only for myself, not necessarily
for Elliot and not for anyone else in the pKVM team), I still would
prefer to use guest_memfd(). I think that having one solution for
confidential computing that rules them all would be best. But we do
need to be able to share memory in place, have a plan for supporting
huge pages in the near future, and migration in the not-too-distant
future.

Yes, huge pages are also of interest for RH. And memory-overconsumption due to having partially used huge pages in private/shared memory is not desired.


We are currently shipping pKVM in Android as it is, warts and all.
We're also working on upstreaming the rest of it. Currently, this is
the main blocker for us to be able to upstream the rest (same probably
applies to Gunyah).

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

At a high level: We want to prevent a misbehaving host process from
crashing the system when attempting to access (deliberately or
accidentally) protected guest memory. As it currently stands in pKVM
and Gunyah, the hypervisor does prevent the host from accessing
(private) guest memory. In certain cases though, if the host attempts
to access that memory and is prevented by the hypervisor (either out
of ignorance or out of malice), the host kernel wouldn't be able to
recover, causing the whole system to crash.

guest_memfd() prevents such accesses by not allowing confidential
memory to be mapped at the host to begin with. This works fine for us,
but there's the issue of being able to share memory in place, which
implies mapping it conditionally (among others that I've mentioned).

The approach we're taking with this proposal is to instead restrict
the pinning of protected memory. If the host kernel can't pin the
memory, then a misbehaving process can't trick the host into accessing
it.

Got it, thanks. So once we pinned it, nobody else can pin it. But we can still map it?



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

The exclusive pin would be acquired for private guest pages, in
addition to a normal pin. It would be released when the private memory
is released, or if the guest shares that memory.

Understood.


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?

The exclusive pin would be rejected if there's any other pin
(exclusive or normal). Normal pins would be rejected if there's an
exclusive pin.

Makes sense, thanks.


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

I can't :)

:)


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).

Other references would crash the userspace process, but the host
kernel can handle them, and shouldn't cause the system to crash. The
way things are now in Android/pKVM, a userspace process can crash the
system as a whole.

Okay, so very Android/pKVM specific :/


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?

I'm not sure I understand the question. We use anonymous memory for pKVM.


"we want to support grabbing shmem user pages instead of using KVM's guestmemfd" indicated to me that you primarily care about shmem with FOLL_EXCLUSIVE?

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?

A FOLL_EXCLUSIVE would fail if there's any other pin. A normal pin
(!FOLL_EXCLUSIVE) would fail if there's a FOLL_EXCLUSIVE pin. It's the
PIN to end all pins!


[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.

We are gating it behind a CONFIG flag :)

;)


Also, since pin is already overloading the refcount, having the
exclusive pin there helps in ensuring atomic accesses and avoiding
races.


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)

We are planning on submitting a proposal for LPC (see you in Vienna!) :)

Great!

--
Cheers,

David / dhildenb