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

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Message-ID: <62b1c20ceed0aa657db5fe73a2f81a37d211f28f.camel@intel.com>
Date: Thu, 19 Jun 2025 00:22:40 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Annapurve, Vishal" <vannapurve@...gle.com>
CC: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao"
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Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge
 pages

On Tue, 2025-06-17 at 21:29 -0700, Vishal Annapurve wrote:
> > 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?
> 
> It's simpler to handle the transient refcounts as there are following options:
> 1) Wait for a small amount of time
> 2) Keep the folio refcounts frozen to zero at all times, which will
> effectively eliminate the scenario of transient refcounts.
> 3) Use raw memory without page structs - unmanaged by kernel.
> 
> > 
> > > 
> > > 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?
> 
> It would be great to avoid having to go through discussion again, if
> we have good reasons to handle it now.

I thought we already came to agreement on whether to spend time pre-designing
for future things. This thread has gotten pretty long, can we stick to the
current problems in an effort to close it?