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

Sean Christopherson
2024-06-20T20:30:32Z

On Thu, Jun 20, 2024, David Hildenbrand wrote:
> On 20.06.24 18:36, Jason Gunthorpe wrote:
> > On Thu, Jun 20, 2024 at 04:45:08PM +0200, David Hildenbrand wrote:
> >
> > > If we could disallow pinning any shared pages, that would make life a lot
> > > easier, but I think there were reasons for why we might require it. To
> > > convert shared->private, simply unmap that folio (only the shared parts
> > > could possibly be mapped) from all user page tables.
> >
> > IMHO it should be reasonable to make it work like ZONE_MOVABLE and
> > FOLL_LONGTERM. Making a shared page private is really no different
> > from moving it.
> >
> > And if you have built a VMM that uses VMA mapped shared pages and
> > short-term pinning then you should really also ensure that the VM is
> > aware when the pins go away. For instance if you are doing some virtio
> > thing with O_DIRECT pinning then the guest will know the pins are gone
> > when it observes virtio completions.
> >
> > In this way making private is just like moving, we unmap the page and
> > then drive the refcount to zero, then move it.
> Yes, but here is the catch: what if a single shared subpage of a large folio
> is (validly) longterm pinned and you want to convert another shared subpage
> to private?
>
> Sure, we can unmap the whole large folio (including all shared parts) before
> the conversion, just like we would do for migration. But we cannot detect
> that nobody pinned that subpage that we want to convert to private.
>
> Core-mm is not, and will not, track pins per subpage.
>
> So I only see two options:
>
> a) Disallow long-term pinning. That means, we can, with a bit of wait,
> always convert subpages shared->private after unmapping them and
> waiting for the short-term pin to go away. Not too bad, and we
> already have other mechanisms disallow long-term pinnings (especially
> writable fs ones!).

I don't think disallowing _just_ long-term GUP will suffice, if we go the "disallow
GUP" route than I think it needs to disallow GUP, period. Like the whole "GUP
writes to file-back memory" issue[*], which I think you're alluding to, short-term
GUP is also problematic. But unlike file-backed memory, for TDX and SNP (and I
think pKVM), a single rogue access has a high probability of being fatal to the
entire system.

I.e. except for blatant bugs, e.g. use-after-free, we need to be able to guarantee
with 100% accuracy that there are no outstanding mappings when converting a page
from shared=>private. Crossing our fingers and hoping that short-term GUP will
have gone away isn't enough.

[*] https://lore.kernel.org/all/cover.1683235180.git.lstoakes@gmail.com

> b) Expose the large folio as multiple 4k folios to the core-mm.
>
>
> b) would look as follows: we allocate a gigantic page from the (hugetlb)
> reserve into guest_memfd. Then, we break it down into individual 4k folios
> by splitting/demoting the folio. We make sure that all 4k folios are
> unmovable (raised refcount). We keep tracking internally that these 4k
> folios comprise a single large gigantic page.
>
> Core-mm can track for us now without any modifications per (previously
> subpage,) now small folios GUP pins and page table mappings without
> modifications.
>
> Once we unmap the gigantic page from guest_memfd, we recronstruct the
> gigantic page and hand it back to the reserve (only possible once all pins
> are gone).
>
> We can still map the whole thing into the KVM guest+iommu using a single
> large unit, because guest_memfd knows the origin/relationship of these
> pages. But we would only map individual pages into user page tables (unless
> we use large VM_PFNMAP mappings, but then also pinning would not work, so
> that's likely also not what we want).

Not being to map guest_memfd into userspace with 1GiB mappings should be ok, at
least for CoCo VMs. If the guest shares an entire 1GiB chunk, e.g. for DMA or
whatever, then userspace can simply punch a hole in guest_memfd and allocate 1GiB
of memory from regular memory. Even losing 2MiB mappings should be ok.

For non-CoCo VMs, I expect we'll want to be much more permissive, but I think
they'll be a complete non-issue because there is no shared vs. private to worry
about. We can simply allow any and all userspace mappings for guest_memfd that is
attached to a "regular" VM, because a misbehaving userspace only loses whatever
hardening (or other benefits) was being provided by using guest_memfd. I.e. the
kernel and system at-large isn't at risk.

> The downside is that we won't benefit from vmemmap optimizations for large
> folios from hugetlb, and have more tracking overhead when mapping individual
> pages into user page tables.

Hmm, I suspect losing the vmemmap optimizations would be acceptable, especially
if we could defer the shattering until the guest actually tried to partially
convert a 1GiB/2MiB region, and restore the optimizations when the memory is
converted back.

> OTOH, maybe we really *need* per-page tracking and this might be the
> simplest way forward, making GUP and friends just work naturally with it.