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

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Message-ID: <CAGtprH8GfY2NjVM4=iVoWOenoexB1vs8=ALYTC-y7suf__+3iA@mail.gmail.com>
Date: Mon, 12 May 2025 09:53:43 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, x86@...nel.org, rick.p.edgecombe@...el.com, 
	dave.hansen@...el.com, kirill.shutemov@...el.com, tabba@...gle.com, 
	ackerleytng@...gle.com, quic_eberman@...cinc.com, michael.roth@....com, 
	david@...hat.com, vbabka@...e.cz, jroedel@...e.de, thomas.lendacky@....com, 
	pgonda@...gle.com, zhiquan1.li@...el.com, fan.du@...el.com, 
	jun.miao@...el.com, ira.weiny@...el.com, isaku.yamahata@...el.com, 
	xiaoyao.li@...el.com, binbin.wu@...ux.intel.com, chao.p.peng@...el.com
Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages

On Sun, May 11, 2025 at 7:18 PM Yan Zhao <yan.y.zhao@...el.com> wrote:
> ...
> >
> > I might be wrongly throwing out some terminologies here then.
> > VM_PFNMAP flag can be set for memory backed by folios/page structs.
> > udmabuf seems to be working with pinned "folios" in the backend.
> >
> > The goal is to get to a stage where guest_memfd is backed by pfn
> > ranges unmanaged by kernel that guest_memfd owns and distributes to
> > userspace, KVM, IOMMU subject to shareability attributes. if the
> OK. So from point of the reset part of kernel, those pfns are not regarded as
> memory.
>
> > shareability changes, the users will get notified and will have to
> > invalidate their mappings. guest_memfd will allow mmaping such ranges
> > with VM_PFNMAP flag set by default in the VMAs to indicate the need of
> > special handling/lack of page structs.
> My concern is a failable invalidation notifer may not be ideal.
> Instead of relying on ref counts (or other mechanisms) to determine whether to
> start shareabilitiy changes, with a failable invalidation notifier, some users
> may fail the invalidation and the shareability change, even after other users
> have successfully unmapped a range.

Even if one user fails to invalidate its mappings, I don't see a
reason to go ahead with shareability change. Shareability should not
change unless all existing users let go of their soon-to-be-invalid
view of memory.

>
> Auditing whether multiple users of shared memory correctly perform unmapping is
> harder than auditing reference counts.
>
> > private memory backed by page structs and use a special "filemap" to
> > map file offsets to these private memory ranges. This step will also
> > need similar contract with users -
> >    1) memory is pinned by guest_memfd
> >    2) users will get invalidation notifiers on shareability changes
> >
> > I am sure there is a lot of work here and many quirks to be addressed,
> > let's discuss this more with better context around. A few related RFC
> > series are planned to be posted in the near future.
> Ok. Thanks for your time and discussions :)
> ...