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

[Yan Zhao]

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?

[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/
>