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

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Date:   Wed, 2 Nov 2016 18:48:20 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Jike Song <jike.song@...el.com>
CC:     Alexey Kardashevskiy <aik@...abs.ru>, <alex.williamson@...hat.com>,
        <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kevin.tian@...el.com>, <qemu-devel@...gnu.org>,
        <bjsdjshi@...ux.vnet.ibm.com>
Subject: Re: [Qemu-devel] [PATCH v9 04/12] vfio iommu: Add support for
 mediated devices



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.

Thanks,
Kirti.