Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

[Vishal Annapurve]

On Tue, Jun 17, 2025 at 11:15 PM Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
>
> On Tue, Jun 17, 2025 at 09:33:02PM -0700, Vishal Annapurve wrote:
> > On Tue, Jun 17, 2025 at 5:49 PM Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> > >
> > > On Wed, Jun 18, 2025 at 08:34:24AM +0800, Edgecombe, Rick P wrote:
> > > > On Tue, 2025-06-17 at 01:09 -0700, Vishal Annapurve wrote:
> > > > > Sorry I quoted Ackerley's response wrongly. Here is the correct reference [1].
> > > >
> > > > I'm confused...
> > > >
> > > > >
> > > > > Speculative/transient refcounts came up a few times In the context of
> > > > > guest_memfd discussions, some examples include: pagetable walkers,
> > > > > page migration, speculative pagecache lookups, GUP-fast etc. David H
> > > > > can provide more context here as needed.
> > > > >
> > > > > Effectively some core-mm features that are present today or might land
> > > > > in the future can cause folio refcounts to be grabbed for short
> > > > > durations without actual access to underlying physical memory. These
> > > > > scenarios are unlikely to happen for private memory but can't be
> > > > > discounted completely.
> > > >
> > > > This means the refcount could be increased for other reasons, and so guestmemfd
> > > > shouldn't rely on refcounts for it's purposes? So, it is not a problem for other
> > > > components handling the page elevate the refcount?
> > > Besides that, in [3], when kvm_gmem_convert_should_proceed() determines whether
> > > to convert to private, why is it allowed to just invoke
> > > kvm_gmem_has_safe_refcount() without taking speculative/transient refcounts into
> > > account? Isn't it more easier for shared pages to have speculative/transient
> > > refcounts?
> >
> > These speculative refcounts are taken into account, in case of unsafe
> > refcounts, conversion operation immediately exits to userspace with
> > EAGAIN and userspace is supposed to retry conversion.
> Hmm, so why can't private-to-shared conversion also exit to userspace with
> EAGAIN?

How would userspace/guest_memfd differentiate between
speculative/transient refcounts and extra refcounts due to TDX unmap
failures?

>
> In the POC
> https://lore.kernel.org/lkml/aE%2Fq9VKkmaCcuwpU@xxxxxxxxxxxxxxxxxxxxxxxxx,
> kvm_gmem_convert_should_proceed() just returns EFAULT (can be modified to
> EAGAIN) to userspace instead.
>
> >
> > Yes, it's more easier for shared pages to have speculative/transient refcounts.
> >
> > >
> > > [3] https://lore.kernel.org/lkml/d3832fd95a03aad562705872cbda5b3d248ca321.1747264138.git.ackerleytng@xxxxxxxxxx/
> > >
> > > > >
> > > > > Another reason to avoid relying on refcounts is to not block usage of
> > > > > raw physical memory unmanaged by kernel (without page structs) to back
> > > > > guest private memory as we had discussed previously. This will help
> > > > > simplify merge/split operations during conversions and help usecases
> > > > > like guest memory persistence [2] and non-confidential VMs.
> > > >
> > > > If this becomes a thing for private memory (which it isn't yet), then couldn't
> > > > we just change things at that point?
> > > >
> > > > Is the only issue with TDX taking refcounts that it won't work with future code
> > > > changes?
> > > >
> > > > >
> > > > > [1] https://lore.kernel.org/lkml/diqz7c2lr6wg.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > > > > [2] https://lore.kernel.org/lkml/20240805093245.889357-1-jgowans@xxxxxxxxxx/
> > > >