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SubjectRe: [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@redhat.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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