Re: [PATCH v3 04/25] KVM: x86/mmu: Add dedicated API to map guest_memfd pfn into TDP MMU

From: Sean Christopherson <seanjc@google.com>
To: Yan Zhao <yan.y.zhao@intel.com>
Cc: Marc Zyngier <maz@kernel.org>,
	Oliver Upton <oliver.upton@linux.dev>,
	 Tianrui Zhao <zhaotianrui@loongson.cn>,
	Bibo Mao <maobibo@loongson.cn>,
	 Huacai Chen <chenhuacai@kernel.org>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	 Anup Patel <anup@brainfault.org>, Paul Walmsley <pjw@kernel.org>,
	 Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	 Christian Borntraeger <borntraeger@linux.ibm.com>,
	Janosch Frank <frankja@linux.ibm.com>,
	 Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	 "Kirill A. Shutemov" <kas@kernel.org>,
	linux-arm-kernel@lists.infradead.org,  kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, loongarch@lists.linux.dev,
	 linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	 kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
	 x86@kernel.org, linux-coco@lists.linux.dev,
	linux-kernel@vger.kernel.org,  Ira Weiny <ira.weiny@intel.com>,
	Kai Huang <kai.huang@intel.com>,
	 Michael Roth <michael.roth@amd.com>,
	Vishal Annapurve <vannapurve@google.com>,
	 Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Ackerley Tng <ackerleytng@google.com>,
	 Binbin Wu <binbin.wu@linux.intel.com>
Subject: Re: [PATCH v3 04/25] KVM: x86/mmu: Add dedicated API to map guest_memfd pfn into TDP MMU
Date: Wed, 5 Nov 2025 07:26:33 -0800	[thread overview]
Message-ID: <aQtsqXPaZo2SMdJU@google.com> (raw)
In-Reply-To: <aQsBI1/SIXGbf9nA@yzhao56-desk.sh.intel.com>

On Wed, Nov 05, 2025, Yan Zhao wrote:
> On Wed, Nov 05, 2025 at 03:32:29PM +0800, Yan Zhao wrote:
> > On Tue, Nov 04, 2025 at 09:57:26AM -0800, Sean Christopherson wrote:
> > > On Thu, Oct 30, 2025, Yan Zhao wrote:
> > > > On Wed, Oct 22, 2025 at 12:53:53PM +0800, Yan Zhao wrote:
> > > > > On Thu, Oct 16, 2025 at 05:32:22PM -0700, Sean Christopherson wrote:
> > > > > > Link: https://lore.kernel.org/all/20250709232103.zwmufocd3l7sqk7y@amd.com
> > > > > 
> > > > > Hi Sean,                                                                         
> > > > > 
> > > > > Will you post [1] to fix the AB-BA deadlock issue for huge page in-place
> > > > > conversion as well?
> > > 
> > > If you (or anyone) has the bandwidth, please pick it up.  I won't have cycles to
> > > look at that for many weeks (potentially not even this calendar year).
> > Got it!
> > On the other hand, do you think we can address the warning as below?
> > The code is based on [2].
> Hmm, updated the diff.
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 7b4a4474d468..543e1eb9db65 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -853,6 +853,9 @@ static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags)
>         inode->i_size = size;
>         mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
>         mapping_set_inaccessible(inode->i_mapping);
> +       if (flags &GUEST_MEMFD_FLAG_MMAP)
> +               lockdep_set_subclass(&inode->i_mapping->invalidate_lock, 1);
> +
>         /* Unmovable mappings are supposed to be marked unevictable as well. */
>         WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
> 
>  
> > As noted in [3], the only scenario can trigger the warning after [2] is when a
> > process creates a TDX VM with non-in-place-conversion guest_memfd and a normal
> > VM with in-place-conversion guest_memfd. The two invalidate_lock's don't contend
> > with each other theoretically.

Hmm, no, I think we need to hoist gup() call outside of filemap_invalidate_lock(),
because I don't think this is strictly limited to TDX VMs without in-place
conversion.  Even with in-place conversion, I think KVM should allow the source
page to be shared memory, at which point I believe this becomes a legimate AB-BA
issue.

In general, playing lockdep games with so many subsystems involved terrifies me.

> > [2] https://lore.kernel.org/all/cover.1760731772.git.ackerleytng@google.com/
> > [3] https://lore.kernel.org/all/aQMi%2Fn9DVyeaWsVH@yzhao56-desk.sh.intel.com/

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