linux-kernel - Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use shareability to guard faulting

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Message-ID: <CAGtprH8xZLqx514XxvNSb2PfK53zGiCP9ARcbv9rbpF=OpBaRg@mail.gmail.com>
Date: Wed, 2 Jul 2025 17:52:40 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Michael Roth <michael.roth@....com>
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Subject: Re: [RFC PATCH v2 02/51] KVM: guest_memfd: Introduce and use
 shareability to guard faulting

On Wed, Jul 2, 2025 at 5:46 PM Vishal Annapurve <vannapurve@...gle.com> wrote:
> ...
> >
> > 2) There are some use-cases for HugeTLB + CoCo that have come to my
> >    attention recently that put a lot of weight on still being able to
> >    maximize mapping/hugepage size when accessing shared mem from userspace,
> >    e.g. for certain DPDK workloads that accessed shared guest buffers
> >    from host userspace. We don't really have a story for this, and I
> >    wouldn't expect us to at this stage, but I think it ties into #1 so
> >    might be worth considering in that context.
>
> Major problem I see here is that if anything in the kernel does a GUP
> on shared memory ranges (which is very likely to happen), it would be
> difficult to get them to let go of the whole hugepage before it can be
> split safely.

The scenario I was alluding to here:
guest trying to convert a subpage from a shared range backed by
hugepage to private.