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

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Message-ID: <5356DA2F.8080606@de.ibm.com>
Date:	Tue, 22 Apr 2014 23:07:59 +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 22/04/14 22:15, Christian Borntraeger wrote:
> 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.

wanted to say kvm_arch_async_page_not_present, but I have to correct myself.
x86 does the "page is there" in the cpu loop, not in the worker. The cpu look 
d oes have a valid mm. So this patch should be also ok.
 
> 
>> ---
>>  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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