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

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Message-ID: <844399f8b16027b06eefc0ef145a3cbb5ae83bc3.camel@intel.com>
Date: Tue, 24 Jun 2025 22:14:48 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "ackerleytng@...gle.com" <ackerleytng@...gle.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
CC: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Du, Fan" <fan.du@...el.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
	<tabba@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>, "Shutemov, Kirill"
	<kirill.shutemov@...el.com>, "michael.roth@....com" <michael.roth@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"Peng, Chao P" <chao.p.peng@...el.com>, "Weiny, Ira" <ira.weiny@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "jroedel@...e.de" <jroedel@...e.de>, "Miao, Jun"
	<jun.miao@...el.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>,
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge
 pages

On Tue, 2025-06-24 at 15:00 -0700, Rick Edgecombe wrote:
> Minor correction here. Yan was concerned about *bugs* happening when freeing
> pages that are accidentally still mapped in the S-EPT. My opinion is that this
> is not especially risky to happen here vs other similar places, but it could be
> helpful if there was a way to catch such bugs. The page flag, or page_ext
> direction came out of a discussion with Dave and Kirill. If it could run all the
> time that would be great, but if not a debug config could be sufficient. For
> example like CONFIG_PAGE_TABLE_CHECK. It doesn't need to support vmemmap
> optimizations because the debug checking doesn't need to run all the time.
> Overhead for debug settings is very normal.

Note, this is separate from the problem of how to handle or notify TDX unmap
errors. That is still an open regardless. But Yan was concerned if we didn't
take a reference when we first mapped it, that it could be more error prone. So
this flag was an alternative to *holding* a reference during the lifetime of S-
EPT mapping.