Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

Zhangfei Gao
2022-04-26T05:19:28Z

On 2022/4/26 下午12:36, Fenghua Yu wrote:
> On Tue, Apr 26, 2022 at 12:28:00PM +0800, Zhangfei Gao wrote:
>> Hi, Jean
>>
>> On 2022/4/26 上午12:13, Jean-Philippe Brucker wrote:
>>> Hi Jacob,
>>>
>>> On Mon, Apr 25, 2022 at 08:34:44AM -0700, Jacob Pan wrote:
>>>> Hi Jean-Philippe,
>>>>
>>>> On Mon, 25 Apr 2022 15:26:40 +0100, Jean-Philippe Brucker
>>>> <jean-philippe@linaro.org> wrote:
>>>>
>>>>> On Mon, Apr 25, 2022 at 07:18:36AM -0700, Dave Hansen wrote:
>>>>>> On 4/25/22 06:53, Jean-Philippe Brucker wrote:
>>>>>>> On Sat, Apr 23, 2022 at 07:13:39PM +0800, zhangfei.gao@foxmail.com
>>>>>>> wrote:
>>>>>>>>>> On 5.17
>>>>>>>>>> fops_release is called automatically, as well as
>>>>>>>>>> iommu_sva_unbind_device. On 5.18-rc1.
>>>>>>>>>> fops_release is not called, have to manually call close(fd)
>>>>>>>>> Right that's weird
>>>>>>>> Looks it is caused by the fix patch, via mmget, which may add
>>>>>>>> refcount of fd.
>>>>>>> Yes indirectly I think: when the process mmaps the queue,
>>>>>>> mmap_region() takes a reference to the uacce fd. That reference is
>>>>>>> released either by explicit close() or munmap(), or by exit_mmap()
>>>>>>> (which is triggered by mmput()). Since there is an mm->fd dependency,
>>>>>>> we cannot add a fd->mm dependency, so no mmget()/mmput() in
>>>>>>> bind()/unbind().
>>>>>>>
>>>>>>> I guess we should go back to refcounted PASIDs instead, to avoid
>>>>>>> freeing them until unbind().
>>>>>> Yeah, this is a bit gnarly for -rc4. Let's just make sure there's
>>>>>> nothing else simple we can do.
>>>>>>
>>>>>> How does the IOMMU hardware know that all activity to a given PASID is
>>>>>> finished? That activity should, today, be independent of an mm or a
>>>>>> fd's lifetime.
>>>>> In the case of uacce, it's tied to the fd lifetime: opening an accelerator
>>>>> queue calls iommu_sva_bind_device(), which sets up the PASID context in
>>>>> the IOMMU. Closing the queue calls iommu_sva_unbind_device() which
>>>>> destroys the PASID context (after the device driver stopped all DMA for
>>>>> this PASID).
>>>>>
>>>> For VT-d, it is essentially the same flow except managed by the individual
>>>> drivers such as DSA.
>>>> If free() happens before unbind(), we deactivate the PASIDs and suppress
>>>> faults from the device. When the unbind finally comes, we finalize the
>>>> PASID teardown. It seems we have a need for an intermediate state where
>>>> PASID is "pending free"?
>>> Yes we do have that state, though I'm not sure we need to make it explicit
>>> in the ioasid allocator.
>>>
>>> Could we move mm_pasid_drop() to __mmdrop() instead of __mmput()? For Arm
>>> we do need to hold the mm_count until unbind(), and mmgrab()/mmdrop() is
>>> also part of Lu's rework [1].
>> Move mm_pasid_drop to __mmdrop looks workable.
>>
>> The nginx works since ioasid is not freed when master exit until nginx stop.
>>
>> The ioasid does not free immediately when fops_release->unbind finished.
>> Instead, __mmdrop happens a bit lazy,  which has no issue though
>> I passed 10000 times exit without unbind test, the pasid allocation is ok.
>>
>> Thanks
>>
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 9796897560ab..60f417f69367 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -792,6 +792,8 @@ void __mmdrop(struct mm_struct *mm)
>>         mmu_notifier_subscriptions_destroy(mm);
>>         check_mm(mm);
>>         put_user_ns(mm->user_ns);
>> +       mm_pasid_drop(mm);
>>         free_mm(mm);
>>  }
>>  EXPORT_SYMBOL_GPL(__mmdrop);
>> @@ -1190,7 +1192,6 @@ static inline void __mmput(struct mm_struct *mm)
>>         }
>>         if (mm->binfmt)
>>                 module_put(mm->binfmt->module);
>> -       mm_pasid_drop(mm);
>>         mmdrop(mm);
>>  }
> Thank you very much, Zhangfei!
>
> I just now sent out an identical patch. It works on X86 as well.
>
> So seems the patch is the right fix.
>
> Either you can send out the patch or I add your Signed-off-by? Either way
> is OK for me.
Thanks Fenghua,
It does not matter. Have added tested-by.
I was in stress test for checking the pasid free, since it was freed lazily.

Thank all for the help, a bit nervous, since it is rc4 now.