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

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Date:	Wed, 23 Apr 2014 21:32:28 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Avi Kivity <avi.kivity@...il.com>, Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	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 04/22, Christian Borntraeger wrote:
>
> 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.

Thanks ;)

Btw, I forgot to mention this in the changelog, but

> >> @@ -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);

it can actually do

	get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL);

"task" is only used to increment task_struct->xxx_flt. I don't think
async_pf_execute() actually needs this (current is PF_WQ_WORKER after
all), but I didn't dare to do another change in the code I can hardly
understand.

Oleg.

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