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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <seanjc@google.com>, Paolo Bonzini
 <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, David Matlack
 <dmatlack@google.com>, Xu Yilun <yilun.xu@linux.intel.com>, Sean
 Christopherson <seanjc@google.com>
Subject: Re: [PATCH 3/4] KVM: Get reference to VM's address space in the
 async #PF worker
In-Reply-To: <20240110011533.503302-4-seanjc@google.com>
References: <20240110011533.503302-1-seanjc@google.com>
 <20240110011533.503302-4-seanjc@google.com>
Date: Fri, 26 Jan 2024 17:21:48 +0100
Message-ID: <87sf2k83qb.fsf@redhat.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain

Sean Christopherson <seanjc@google.com> writes:

> Get a reference to the target VM's address space in async_pf_execute()
> instead of gifting a reference from kvm_setup_async_pf().  Keeping the
> address space alive just to service an async #PF is counter-productive,
> i.e. if the process is exiting and all vCPUs are dead, then NOT doing
> get_user_pages_remote() and freeing the address space asap is
> desirable.

It took me a while to realize why all vCPU fds are managed by the same
mm which did KVM_CREATE_VM as (AFAIU) fds can be passed around. Turns
out, we explicitly forbid this in kvm_vcpu_ioctl():

        if (vcpu->kvm->mm != current->mm || vcpu->kvm->vm_dead)
                return -EIO;

so this indeed means that grabbing current->mm in kvm_setup_async_pf()
can be avoided. I'm not sure whether it's just me or a "all vCPUs are
quired to be managed by the same mm" comment somewhere would be helpful.

>
> Handling the mm reference entirely within async_pf_execute() also
> simplifies the async #PF flows as a whole, e.g. it's not immediately
> obvious when the worker task vs. the vCPU task is responsible for putting
> the gifted mm reference.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  include/linux/kvm_host.h |  1 -
>  virt/kvm/async_pf.c      | 32 ++++++++++++++++++--------------
>  2 files changed, 18 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 7e7fd25b09b3..bbfefd7e612f 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -238,7 +238,6 @@ struct kvm_async_pf {
>  	struct list_head link;
>  	struct list_head queue;
>  	struct kvm_vcpu *vcpu;
> -	struct mm_struct *mm;
>  	gpa_t cr2_or_gpa;
>  	unsigned long addr;
>  	struct kvm_arch_async_pf arch;
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index d5dc50318aa6..c3f4f351a2ae 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -46,8 +46,8 @@ static void async_pf_execute(struct work_struct *work)
>  {
>  	struct kvm_async_pf *apf =
>  		container_of(work, struct kvm_async_pf, work);
> -	struct mm_struct *mm = apf->mm;
>  	struct kvm_vcpu *vcpu = apf->vcpu;
> +	struct mm_struct *mm = vcpu->kvm->mm;
>  	unsigned long addr = apf->addr;
>  	gpa_t cr2_or_gpa = apf->cr2_or_gpa;
>  	int locked = 1;
> @@ -56,16 +56,24 @@ static void async_pf_execute(struct work_struct *work)
>  	might_sleep();
>  
>  	/*
> -	 * This work is run asynchronously to the task which owns
> -	 * mm and might be done in another context, so we must
> -	 * access remotely.
> +	 * Attempt to pin the VM's host address space, and simply skip gup() if
> +	 * acquiring a pin fail, i.e. if the process is exiting.  Note, KVM
> +	 * holds a reference to its associated mm_struct until the very end of
> +	 * kvm_destroy_vm(), i.e. the struct itself won't be freed before this
> +	 * work item is fully processed.
>  	 */
> -	mmap_read_lock(mm);
> -	get_user_pages_remote(mm, addr, 1, FOLL_WRITE, NULL, &locked);
> -	if (locked)
> -		mmap_read_unlock(mm);
> -	mmput(mm);
> +	if (mmget_not_zero(mm)) {
> +		mmap_read_lock(mm);
> +		get_user_pages_remote(mm, addr, 1, FOLL_WRITE, NULL, &locked);
> +		if (locked)
> +			mmap_read_unlock(mm);
> +		mmput(mm);
> +	}
>  
> +	/*
> +	 * Notify and kick the vCPU even if faulting in the page failed, e.g.
> +	 * so that the vCPU can retry the fault synchronously.
> +	 */
>  	if (IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC))
>  		kvm_arch_async_page_present(vcpu, apf);
>  
> @@ -129,10 +137,8 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>  #ifdef CONFIG_KVM_ASYNC_PF_SYNC
>  		flush_work(&work->work);
>  #else
> -		if (cancel_work_sync(&work->work)) {
> -			mmput(work->mm);
> +		if (cancel_work_sync(&work->work))
>  			kmem_cache_free(async_pf_cache, work);
> -		}
>  #endif
>  		spin_lock(&vcpu->async_pf.lock);
>  	}
> @@ -211,8 +217,6 @@ bool kvm_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  	work->cr2_or_gpa = cr2_or_gpa;
>  	work->addr = hva;
>  	work->arch = *arch;
> -	work->mm = current->mm;
> -	mmget(work->mm);
>  
>  	INIT_WORK(&work->work, async_pf_execute);

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

-- 
Vitaly