Re: [RFC][PATCH] Drop FOLL_GET in GUP when doing async_pf in kvm

[chai wen <chaiw.fnst@cn.fujitsu.com>]

On 10/10/2013 11:15 PM, Gleb Natapov wrote:
> On Thu, Oct 10, 2013 at 09:20:15PM +0800, chai wen wrote:
>> Hi Gleb
>>
>> Thanks for you explanation about async_pf in kvm.
>> Page pinning is not mandatory in kvm async_pf processing and probably should
>> be dropped later.this patch drops the FOLL_GET flag when doing GUP in async_pf
>> and uses a bool (marking result of GUP) to replace the *page in kvm_async_pf
>> structure to simplify some following async_pf check/clear processing.
> Thanks, but the patch need to be on top of
> git://git.kernel.org/pub/scm/virt/kvm/kvm.git queue branch.
>
> Some more comment bellow.
>
>> Thanks.
>>
>> Suggested-by:  gleb@redhat.com
>> Signed-off-by: chai wen <chaiw.fnst@cn.fujitsu.com>
>> ---
>>  arch/x86/kvm/x86.c       |    5 ++---
>>  include/linux/kvm_host.h |    2 +-
>>  virt/kvm/async_pf.c      |   16 ++++++----------
>>  3 files changed, 9 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index e5ca72a..0ff7d02 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7282,8 +7282,7 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>>  {
>>  	int r;
>>  
>> -	if ((vcpu->arch.mmu.direct_map != work->arch.direct_map) ||
>> -	      is_error_page(work->page))
>> +	if ((vcpu->arch.mmu.direct_map != work->arch.direct_map))
>>  		return;
>>  
>>  	r = kvm_mmu_reload(vcpu);
>> @@ -7393,7 +7392,7 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
>>  	struct x86_exception fault;
>>  
>>  	trace_kvm_async_pf_ready(work->arch.token, work->gva);
>> -	if (is_error_page(work->page))
>> +	if (!work->got_page)
>>  		work->arch.token = ~0; /* broadcast wakeup */
>>  	else
>>  		kvm_del_async_pf_gfn(vcpu, work->arch.gfn);
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index 0fbbc7a..232a7d0 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -189,7 +189,7 @@ struct kvm_async_pf {
>>  	gva_t gva;
>>  	unsigned long addr;
>>  	struct kvm_arch_async_pf arch;
>> -	struct page *page;
>> +	bool got_page;
> I would call it wakeup_all.
>
>>  	bool done;
>>  };
>>  
>> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
>> index 8a39dda..0cafdc7 100644
>> --- a/virt/kvm/async_pf.c
>> +++ b/virt/kvm/async_pf.c
>> @@ -56,7 +56,7 @@ void kvm_async_pf_vcpu_init(struct kvm_vcpu *vcpu)
>>  
>>  static void async_pf_execute(struct work_struct *work)
>>  {
>> -	struct page *page = NULL;
>> +	long nrpages = 0;
>>  	struct kvm_async_pf *apf =
>>  		container_of(work, struct kvm_async_pf, work);
>>  	struct mm_struct *mm = apf->mm;
>> @@ -68,13 +68,13 @@ static void async_pf_execute(struct work_struct *work)
>>  
>>  	use_mm(mm);
>>  	down_read(&mm->mmap_sem);
>> -	get_user_pages(current, mm, addr, 1, 1, 0, &page, NULL);
>> +	nrpages = get_user_pages(current, mm, addr, 1, 1, 0, NULL, NULL);
>>  	up_read(&mm->mmap_sem);
>>  	unuse_mm(mm);
>>  
>>  	spin_lock(&vcpu->async_pf.lock);
>>  	list_add_tail(&apf->link, &vcpu->async_pf.done);
>> -	apf->page = page;
>> +	apf->got_page = (1==nrpages);
> Space before and after ==, but this is not needed at all.  It does
> not matter if GUP failed or not here. We need to wake up guest process
> that is waiting on a page, so just set wakeup_all to false in kvm_setup_async_pf().
Hi Gleb
Thanks for you reply.
It does not matter if GUP failed or not here. But in the original logic of
kvm_arch_async_page_ready and kvm_arch_async_page_present, there is
is_error_page
checking in them. I am not sure wheher this ‘logic’ should be dropped.
And in kvm_async_pf_wakeup_all we used a dummy bad page pointer to generate
boradcast wakeup before. if wakeup_all(bool) is marking the result of
gup should it be false in this function ?

thanks
>
>>  	apf->done = true;
>>  	spin_unlock(&vcpu->async_pf.lock);
>>  
>> @@ -114,8 +114,6 @@ void kvm_clear_async_pf_completion_queue(struct kvm_vcpu *vcpu)
>>  			list_entry(vcpu->async_pf.done.next,
>>  				   typeof(*work), link);
>>  		list_del(&work->link);
>> -		if (!is_error_page(work->page))
>> -			kvm_release_page_clean(work->page);
>>  		kmem_cache_free(async_pf_cache, work);
>>  	}
>>  	spin_unlock(&vcpu->async_pf.lock);
>> @@ -135,14 +133,12 @@ void kvm_check_async_pf_completion(struct kvm_vcpu *vcpu)
>>  		list_del(&work->link);
>>  		spin_unlock(&vcpu->async_pf.lock);
>>  
>> -		if (work->page)
>> +		if (work->got_page)
>>  			kvm_arch_async_page_ready(vcpu, work);
>>  		kvm_arch_async_page_present(vcpu, work);
>>  
>>  		list_del(&work->queue);
>>  		vcpu->async_pf.queued--;
>> -		if (!is_error_page(work->page))
>> -			kvm_release_page_clean(work->page);
>>  		kmem_cache_free(async_pf_cache, work);
>>  	}
>>  }
>> @@ -165,7 +161,7 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn,
>>  	if (!work)
>>  		return 0;
>>  
>> -	work->page = NULL;
>> +	work->got_page = false;
> Sp here do:
> 	work->wakeup_all = false;
>
>>  	work->done = false;
>>  	work->vcpu = vcpu;
>>  	work->gva = gva;
>> @@ -206,7 +202,7 @@ int kvm_async_pf_wakeup_all(struct kvm_vcpu *vcpu)
>>  	if (!work)
>>  		return -ENOMEM;
>>  
>> -	work->page = KVM_ERR_PTR_BAD_PAGE;
>> +	work->got_page = false;
> And here:
> 	work->wakeup_all = true;
>>  	INIT_LIST_HEAD(&work->queue); /* for list_del to work */
>>  
>>  	spin_lock(&vcpu->async_pf.lock);
>> -- 
>> 1.7.1
> --
> 			Gleb.
> --
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-- 
Regards

chai wen