Subject: Re: [PATCH v14 09/22] vfio iommu type1: Add task structure to
 vfio_dma
To: Alex Williamson <alex.williamson@redhat.com>
References: <1479329194-10247-1-git-send-email-kwankhede@nvidia.com>
 <1479329194-10247-10-git-send-email-kwankhede@nvidia.com>
 <6d256aeb-cd3b-0827-667f-054cdacc45ca@ozlabs.ru>
 <20161116231206.411c68ef@t450s.home>
Cc: Kirti Wankhede <kwankhede@nvidia.com>, pbonzini@redhat.com,
        kraxel@redhat.com, cjia@nvidia.com, qemu-devel@nongnu.org,
        kvm@vger.kernel.org, kevin.tian@intel.com, jike.song@intel.com,
        bjsdjshi@linux.vnet.ibm.com, linux-kernel@vger.kernel.org
From: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-ID: <78bf03ef-8bdd-847d-9d49-c58f4efcf556@ozlabs.ru>
Date: Thu, 17 Nov 2016 17:42:11 +1100
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On 17/11/16 17:12, Alex Williamson wrote:
> On Thu, 17 Nov 2016 16:41:14 +1100
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
>> On 17/11/16 07:46, Kirti Wankhede wrote:
>>> Add task structure to vfio_dma structure. Task structure is used for:
>>> - During DMA_UNMAP, same task who mapped it or other task who shares same
>>> address space is allowed to unmap, otherwise unmap fails.
>>> QEMU maps few iova ranges initially, then fork threads and from the child
>>> thread calls DMA_UNMAP on previously mapped iova. Since child shares same
>>> address space, DMA_UNMAP is successful.
>>> - Avoid accessing struct mm while process is exiting by acquiring
>>> reference of task's mm during page accounting.
>>> - It is also used to get task mlock capability and rlimit for mlock.
>>>
>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>>> Signed-off-by: Neo Jia <cjia@nvidia.com>
>>> Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>  
>>
>>
>> I keep whinging that @mm should be referenced, not @current but you keep
>> referencing @current even if you only need @mm and you are not telling why
>> - and I am wondering what I am missing here? Something else will be used
>> from @task later, besides just @mm?
> 
> Yes, we reference @current from vfio_dma_do_map() and this is stored
> on the struct vfio_dma.  A reference to current is held because the
> external page pinning in vfio_pin_page_external() needs to test the
> capabilities of the task for CAP_IPC_LOCK to know whether locked memory

Ah, that's it - capable(CAP_IPC_LOCK) is checking @current, missed that.


> limits are in effect for the task even when it's not @current (ie. an
> asynchronous call from the vendor driver regardless of what task is
> currently running).  There are also various get_task_mm() taken
> temporarily when we're working with the mm of that task.  Do you spot
> any issues with this behavior? Thanks,

No, now I am fine, thanks!


-- 
Alexey