linux-kernel - Re: [PATCH 2/4] KVM: Put mm immediately after async #PF worker completes remote gup()

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Date: Fri, 26 Jan 2024 17:23:48 +0100
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, David Matlack
 <dmatlack@...gle.com>, Xu Yilun <yilun.xu@...ux.intel.com>, Sean
 Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 2/4] KVM: Put mm immediately after async #PF worker
 completes remote gup()

Sean Christopherson <seanjc@...gle.com> writes:

> Put the async #PF worker's reference to the VM's address space as soon as
> the worker is done with the mm.  This will allow deferring getting a
> reference to the worker itself without having to track whether or not
> getting a reference succeeded.
>
> Note, if the vCPU is still alive, there is no danger of the worker getting
> stuck with tearing down the host page tables, as userspace also holds a
> reference (obviously), i.e. there is no risk of delaying the page-present
> notification due to triggering the slow path in mmput().
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  virt/kvm/async_pf.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index 876927a558ad..d5dc50318aa6 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -64,6 +64,7 @@ static void async_pf_execute(struct work_struct *work)
>  	get_user_pages_remote(mm, addr, 1, FOLL_WRITE, NULL, &locked);
>  	if (locked)
>  		mmap_read_unlock(mm);
> +	mmput(mm);
>  
>  	if (IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC))
>  		kvm_arch_async_page_present(vcpu, apf);
> @@ -85,8 +86,6 @@ static void async_pf_execute(struct work_struct *work)
>  	trace_kvm_async_pf_completed(addr, cr2_or_gpa);
>  
>  	__kvm_vcpu_wake_up(vcpu);
> -
> -	mmput(mm);
>  }
>  
>  static void kvm_flush_and_free_async_pf_work(struct kvm_async_pf *work)

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly