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

[Fenghua Yu]

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@xxxxxxxxxx> 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@xxxxxxxxxxx
> > > > > > 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