linux-kernel - Re: [PATCH 1/2] KVM: async_pf: kill the unnecessary use_mm/unuse_mm async_pf

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Date:	Tue, 22 Apr 2014 22:15:27 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>, Avi Kivity <avi.kivity@...il.com>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
CC:	Dominik Dingel <dingel@...ux.vnet.ibm.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: async_pf: kill the unnecessary use_mm/unuse_mm
 async_pf_execute()

On 21/04/14 15:25, Oleg Nesterov wrote:
> async_pf_execute() has no reasons to adopt apf->mm, gup(current, mm)
> should work just fine even if current has another or NULL ->mm.
> 
> Recently kvm_async_page_present_sync() was added insedie the "use_mm"
> section, but it seems that it doesn't need current->mm too.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Indeed, use/unuse_mm should only be necessary for copy_to/from_user etc.
This is fine for s390, but it seems that x86 kvm_arch_async_page_not_present
might call apf_put_user which might call copy_to_user, so this is not ok, I guess.


> ---
>  virt/kvm/async_pf.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index 10df100..0ced4f3 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -80,12 +80,10 @@ static void async_pf_execute(struct work_struct *work)
> 
>  	might_sleep();
> 
> -	use_mm(mm);
>  	down_read(&mm->mmap_sem);
>  	get_user_pages(current, mm, addr, 1, 1, 0, NULL, NULL);
>  	up_read(&mm->mmap_sem);
>  	kvm_async_page_present_sync(vcpu, apf);
> -	unuse_mm(mm);
> 
>  	spin_lock(&vcpu->async_pf.lock);
>  	list_add_tail(&apf->link, &vcpu->async_pf.done);
> 

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