Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for mediated devices

From: Kirti Wankhede
Date: Wed Nov 02 2016 - 09:21:27 EST

On 11/2/2016 6:30 PM, Jike Song wrote:
> On 11/02/2016 08:41 PM, Kirti Wankhede wrote:
>> On 11/2/2016 5:51 PM, Jike Song wrote:
>>> On 11/02/2016 12:09 PM, Alexey Kardashevskiy wrote:
>>>> Or you could just reference and use @mm as KVM and others do. Or there is
>>>> anything else you need from @current than just @mm?
>>> I agree. If @mm is the only thing needed, there is really no reason to
>>> refer to the @task :-)
>> In vfio_lock_acct(), that is for page accounting, if mm->mmap_sem is
>> already held then page accounting is deferred, where task structure is
>> used to get mm and work is deferred only if mm exist:
>> mm = get_task_mm(task);
>> That is where this module need task structure.
> Kirti,
> By calling get_task_mm you hold a ref on @mm and save it in iommu,
> whenever you want to do something like vfio_lock_acct(), use that mm
> (as you said, if mmap_sem not accessible then defer it to a work, but
> still @mm is the whole information), and put it after the usage.
> I still can't see any reason that the @task have to be saved. It's
> always the @mm all the time. Did I miss anything?

If the process is terminated by SIGKILL, as Alexey mentioned in this
mail thread earlier exit_mm() is called first and then all files are
closed. From exit_mm(), task->mm is set to NULL. So from teardown path,
we should call get_task_mm(task) to get current status intsead of using
stale pointer.