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From: James Houghton <jthoughton@google.com>
Date: Fri, 19 Apr 2024 13:54:47 -0700
Message-ID: <CADrL8HWAzcq4N-triWiMvtpUKvb7C03+2mQpNBxV9Ci-F5GyBw@mail.gmail.com>
Subject: Re: [PATCH v3 5/7] KVM: x86: Participate in bitmap-based PTE aging
To: David Matlack <dmatlack@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Paolo Bonzini <pbonzini@redhat.com>, 
	Yu Zhao <yuzhao@google.com>, Marc Zyngier <maz@kernel.org>, 
	Oliver Upton <oliver.upton@linux.dev>, Sean Christopherson <seanjc@google.com>, 
	Jonathan Corbet <corbet@lwn.net>, James Morse <james.morse@arm.com>, 
	Suzuki K Poulose <suzuki.poulose@arm.com>, Zenghui Yu <yuzenghui@huawei.com>, 
	Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Masami Hiramatsu <mhiramat@kernel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Shaoqin Huang <shahuang@redhat.com>, 
	Gavin Shan <gshan@redhat.com>, Ricardo Koller <ricarkol@google.com>, 
	Raghavendra Rao Ananta <rananta@google.com>, Ryan Roberts <ryan.roberts@arm.com>, 
	David Rientjes <rientjes@google.com>, Axel Rasmussen <axelrasmussen@google.com>, 
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, 
	kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-mm@kvack.org, 
	linux-trace-kernel@vger.kernel.org
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On Fri, Apr 12, 2024 at 1:44=E2=80=AFPM David Matlack <dmatlack@google.com>=
 wrote:
>
> On 2024-04-01 11:29 PM, James Houghton wrote:
> > Only handle the TDP MMU case for now. In other cases, if a bitmap was
> > not provided, fallback to the slowpath that takes mmu_lock, or, if a
> > bitmap was provided, inform the caller that the bitmap is unreliable.
>
> I think this patch will trigger a lockdep assert in
>
>   kvm_tdp_mmu_age_gfn_range
>     kvm_tdp_mmu_handle_gfn
>       for_each_tdp_mmu_root
>         __for_each_tdp_mmu_root
>           kvm_lockdep_assert_mmu_lock_held
>
> ... because it walks tdp_mmu_roots without holding mmu_lock.

Indeed, thanks. I'll make sure to build with CONFIG_LOCKDEP for the
future versions and check for errors.

>
> Yu's patch[1] added a lockless walk to the TDP MMU. We'd need something
> similar here and also update the comment above tdp_mmu_roots describing
> how tdp_mmu_roots can be read locklessly.

I'll add the macro / function to do the lockless walk of tdp_mmu_roots
and explain why it is safe. Thanks for pointing out this big mistake.

> [1] https://lore.kernel.org/kvmarm/ZItX64Bbx5vdjo9M@google.com/