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

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Message-ID: <diqzh5zv4keo.fsf@ackerleytng-ctop.c.googlers.com>
Date: Tue, 01 Jul 2025 15:26:55 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, 
	"Shutemov, Kirill" <kirill.shutemov@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "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>, 
	"quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "michael.roth@....com" <michael.roth@....com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"Peng, Chao P" <chao.p.peng@...el.com>, "Du, Fan" <fan.du@...el.com>, 
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Weiny, Ira" <ira.weiny@...el.com>, 
	"Li, Zhiquan1" <zhiquan1.li@...el.com>, "Annapurve, Vishal" <vannapurve@...gle.com>, 
	"jroedel@...e.de" <jroedel@...e.de>, "Miao, Jun" <jun.miao@...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

Ackerley Tng <ackerleytng@...gle.com> writes:

> Yan Zhao <yan.y.zhao@...el.com> writes:
>
>> On Mon, Jun 30, 2025 at 12:25:49PM -0700, Ackerley Tng wrote:
>>> "Edgecombe, Rick P" <rick.p.edgecombe@...el.com> writes:
>>> 
>>> > On Mon, 2025-06-30 at 19:13 +0800, Yan Zhao wrote:
>>> >> > > ok! Lets go f/g. Unless Yan objects.
>>> >> I'm ok with f/g. But I have two implementation specific questions:
>>> >> 
>>> >> 1. How to set the HWPoison bit in TDX?
>>> 
>>> I was thinking to set the HWpoison flag based on page type. If regular
>>> 4K page, set the flag. If THP page (not (yet) supported by guest_memfd),
>>> set the has_hwpoison flag, and if HugeTLB page, call
>>> folio_set_hugetlb_hwpoison().
>> Could you elaborate on how to call folio_set_hugetlb_hwpoison()?
>>
>
> Sorry I meant "in TDX" as in the part of the kernel that performs the
> unmap. I'm assuming something like
>
> int ret = tdx_do_unmap(page)
> if (ret)
> 	set_hwpoison_based_on_folio_type(page_folio(page))
>
> And set_hwpoison_based_on_folio_type() would have to be written to know
> how to set the HWpoison flag based on type of the folio.
>
> I think I might have used the wrong terminology elsewhere. Sorry about
> that. I don't mean to call folio_set_hugetlb_hwpoison() from within the
> TDX module. I meant to set HWpoison in the kernel, based on return value
> to the kernel from the TDX module.
>
>>> But if we go with Rick's suggestion below, then we don't have to figure
>>> this out.
>>> 
>>> >> 2. Should we set this bit for non-guest-memfd pages (e.g. for S-EPT pages) ?
>>> >
>>> > Argh, I guess we can keep the existing ref count based approach for the other
>>> > types of TDX owned pages?
>>> >
>>> 
>>> Wait TDX can only use guest_memfd pages, right? Even if TDX can use
>>> non-guest_memfd pages, why not also set HWpoison for non-guest_memfd
>>> pages?
>> As in https://lore.kernel.org/all/aGJxU95VvQvQ3bj6@yzhao56-desk.sh.intel.com/,
>> I don't find a proper interface for TDX to set HWpoison bit on non-guset_memfd
>> pages.
>>
>> Neither memory_failure() nor memory_failure_queue() seem fit.

Missed out a response on this.

Vishal explained to me that non-guest_memfd pages can be used by TDX for
the TDX module itself.

For those, I think it's still okay to set HWpoison, because the kernel
page freeing process will leak HWpoison-ed pages. free_pages_prepare()
will check for HWpoison and skip freeing:

__folio_put() ->
  free_frozen_pages() ->
    __free_frozen_pages() ->
      free_pages_prepare()