Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory

Sean Christopherson
2022-06-14T19:09:32Z

On Tue, Jun 14, 2022, Andy Lutomirski wrote:
> On Tue, Jun 14, 2022 at 12:32 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> >
> > On Thu, Jun 09, 2022 at 08:29:06PM +0000, Sean Christopherson wrote:
> > > On Wed, Jun 08, 2022, Vishal Annapurve wrote:
> > >
> > > One argument is that userspace can simply rely on cgroups to detect misbehaving
> > > guests, but (a) those types of OOMs will be a nightmare to debug and (b) an OOM
> > > kill from the host is typically considered a _host_ issue and will be treated as
> > > a missed SLO.
> > >
> > > An idea for handling this in the kernel without too much complexity would be to
> > > add F_SEAL_FAULT_ALLOCATIONS (terrible name) that would prevent page faults from
> > > allocating pages, i.e. holes can only be filled by an explicit fallocate(). Minor
> > > faults, e.g. due to NUMA balancing stupidity, and major faults due to swap would
> > > still work, but writes to previously unreserved/unallocated memory would get a
> > > SIGSEGV on something it has mapped. That would allow the userspace VMM to prevent
> > > unintentional allocations without having to coordinate unmapping/remapping across
> > > multiple processes.
> >
> > Since this is mainly for shared memory and the motivation is catching
> > misbehaved access, can we use mprotect(PROT_NONE) for this? We can mark
> > those range backed by private fd as PROT_NONE during the conversion so
> > subsequence misbehaved accesses will be blocked instead of causing double
> > allocation silently.

PROT_NONE, a.k.a. mprotect(), has the same vma downsides as munmap().

> This patch series is fairly close to implementing a rather more
> efficient solution. I'm not familiar enough with hypervisor userspace
> to really know if this would work, but:
>
> What if shared guest memory could also be file-backed, either in the
> same fd or with a second fd covering the shared portion of a memslot?
> This would allow changes to the backing store (punching holes, etc) to
> be some without mmap_lock or host-userspace TLB flushes? Depending on
> what the guest is doing with its shared memory, userspace might need
> the memory mapped or it might not.

That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea. The issue,
unless I'm misreading code, is that punching a hole in the shared memory backing
store doesn't prevent reallocating that hole on fault, i.e. a helper process that
keeps a valid mapping of guest shared memory can silently fill the hole.

What we're hoping to achieve is a way to prevent allocating memory without a very
explicit action from userspace, e.g. fallocate().