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

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Message-ID: <aBldhnTK93+eKcMq@yzhao56-desk.sh.intel.com>
Date: Tue, 6 May 2025 08:53:26 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Vishal Annapurve <vannapurve@...gle.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

Sorry for the late reply, I was on leave last week.

On Tue, Apr 29, 2025 at 06:46:59AM -0700, Vishal Annapurve wrote:
> On Mon, Apr 28, 2025 at 5:52 PM Yan Zhao <yan.y.zhao@...el.com> wrote:
> > So, we plan to remove folio_ref_add()/folio_put_refs() in future, only invoking
> > folio_ref_add() in the event of a removal failure.
> 
> In my opinion, the above scheme can be deployed with this series
> itself. guest_memfd will not take away memory from TDX VMs without an
I initially intended to add a separate patch at the end of this series to
implement invoking folio_ref_add() only upon a removal failure. However, I
decided against it since it's not a must before guest_memfd supports in-place
conversion.

We can include it in the next version If you think it's better.

> invalidation. folio_ref_add() will not work for memory not backed by
> page structs, but that problem can be solved in future possibly by
With current TDX code, all memory must be backed by a page struct.
Both tdh_mem_page_add() and tdh_mem_page_aug() require a "struct page *" rather
than a pfn.

> notifying guest_memfd of certain ranges being in use even after
> invalidation completes.
A curious question:
To support memory not backed by page structs in future, is there any counterpart
to the page struct to hold ref count and map count?