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

Zhangfei Gao
2022-04-26T04:28:26Z

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);
}

>
> Thanks,
> Jean
>
> [1] https://lore.kernel.org/linux-iommu/20220421052121.3464100-9-baolu.lu@linux.intel.com/