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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 16, 2024, Jason Gunthorpe wrote:
> On Tue, Jul 16, 2024 at 09:03:00AM -0700, Sean Christopherson wrote:
> 
> > > + To support huge pages, guest_memfd will take ownership of the hugepages, and
> > >   provide interested parties (userspace, KVM, iommu) with pages to be used.
> > >   + guest_memfd will track usage of (sub)pages, for both private and shared
> > >     memory
> > >   + Pages will be broken into smaller (probably 4K) chunks at creation time to
> > >     simplify implementation (as opposed to splitting at runtime when private to
> > >     shared conversion is requested by the guest)
> > 
> > FWIW, I doubt we'll ever release a version with mmap()+guest_memfd support that
> > shatters pages at creation.  I can see it being an intermediate step, e.g. to
> > prove correctness and provide a bisection point, but shattering hugepages at
> > creation would effectively make hugepage support useless.
> 
> Why? If the private memory retains its contiguity seperately but the
> struct pages are removed from the vmemmap, what is the downside?

Oooh, you're talking about shattering only the host userspace mappings.  Now I
understand why there was a bit of a disconnect, I was thinking you (hand-wavy
everyone) were saying that KVM would immediately shatter its own mappings too.

> As I understand it the point is to give a large contiguous range to
> the private world and use only 4k pages to give the hypervisor world
> access to limited amounts of the memory.
> 
> Is there a reason that not having the shared memory elevated to higher
> contiguity a deal breaker?

Nope.  I'm sure someone will ask for it sooner than later, but definitely not a
must have.