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

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Message-ID: <CAGtprH9_McMDepbuvWMLRvHooPdtE4RHog=Dgr_zFXT5s49nXA@mail.gmail.com>
Date: Mon, 28 Apr 2025 17:17:16 -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 Wed, Apr 23, 2025 at 8:07 PM Yan Zhao <yan.y.zhao@...el.com> wrote:
>
> Increase folio ref count before mapping a private page, and decrease
> folio ref count after a mapping failure or successfully removing a private
> page.
>
> The folio ref count to inc/dec corresponds to the mapping/unmapping level,
> ensuring the folio ref count remains balanced after entry splitting or
> merging.
>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  arch/x86/kvm/vmx/tdx.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 355b21fc169f..e23dce59fc72 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1501,9 +1501,9 @@ void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int pgd_level)
>         td_vmcs_write64(to_tdx(vcpu), SHARED_EPT_POINTER, root_hpa);
>  }
>
> -static void tdx_unpin(struct kvm *kvm, struct page *page)
> +static void tdx_unpin(struct kvm *kvm, struct page *page, int level)
>  {
> -       put_page(page);
> +       folio_put_refs(page_folio(page), KVM_PAGES_PER_HPAGE(level));
>  }
>
>  static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
> @@ -1517,13 +1517,13 @@ static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
>
>         err = tdh_mem_page_aug(&kvm_tdx->td, gpa, tdx_level, page, &entry, &level_state);
>         if (unlikely(tdx_operand_busy(err))) {
> -               tdx_unpin(kvm, page);
> +               tdx_unpin(kvm, page, level);
>                 return -EBUSY;
>         }
>
>         if (KVM_BUG_ON(err, kvm)) {
>                 pr_tdx_error_2(TDH_MEM_PAGE_AUG, err, entry, level_state);
> -               tdx_unpin(kvm, page);
> +               tdx_unpin(kvm, page, level);
>                 return -EIO;
>         }
>
> @@ -1570,10 +1570,11 @@ int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
>          * a_ops->migrate_folio (yet), no callback is triggered for KVM on page
>          * migration.  Until guest_memfd supports page migration, prevent page
>          * migration.
> -        * TODO: Once guest_memfd introduces callback on page migration,
> -        * implement it and remove get_page/put_page().
> +        * TODO: To support in-place-conversion in gmem in futre, remove
> +        * folio_ref_add()/folio_put_refs().

With necessary infrastructure in guest_memfd [1] to prevent page
migration, is it necessary to acquire extra folio refcounts? If not,
why not just cleanup this logic now?

[1] https://git.kernel.org/pub/scm/virt/kvm/kvm.git/tree/virt/kvm/guest_memfd.c?h=kvm-coco-queue#n441

> Only increase the folio ref count
> +        * when there're errors during removing private pages.
>          */
> -       get_page(page);
> +       folio_ref_add(page_folio(page), KVM_PAGES_PER_HPAGE(level));
>
>         /*
>          * Read 'pre_fault_allowed' before 'kvm_tdx->state'; see matching
> @@ -1647,7 +1648,7 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
>                 return -EIO;
>
>         tdx_clear_page(page, level);
> -       tdx_unpin(kvm, page);
> +       tdx_unpin(kvm, page, level);
>         return 0;
>  }
>
> @@ -1727,7 +1728,7 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
>         if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level) &&
>             !KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm)) {
>                 atomic64_dec(&kvm_tdx->nr_premapped);
> -               tdx_unpin(kvm, page);
> +               tdx_unpin(kvm, page, level);
>                 return 0;
>         }
>
> --
> 2.43.2
>