Re: [PATCH 0/2] iommu: Remove iommu_sva_ops::mm_exit()

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Thu, 9 Apr 2020 16:50:58 +0200
Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:

> On Thu, Apr 09, 2020 at 07:14:24AM -0700, Jacob Pan wrote:
> > On Thu, 9 Apr 2020 08:39:05 +0200
> > Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:
> >   
> > > On Wed, Apr 08, 2020 at 04:48:02PM -0700, Jacob Pan wrote:  
> > > > On Wed, 8 Apr 2020 19:32:18 -0300
> > > > Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > >     
> > > > > On Wed, Apr 08, 2020 at 02:35:52PM -0700, Jacob Pan wrote:    
> > > > > > > On Wed, Apr 08, 2020 at 11:35:52AM -0700, Jacob Pan
> > > > > > > wrote:      
> > > > > > > > Hi Jean,
> > > > > > > > 
> > > > > > > > On Wed,  8 Apr 2020 16:04:25 +0200
> > > > > > > > Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:
> > > > > > > >         
> > > > > > > > > The IOMMU SVA API currently requires device drivers to
> > > > > > > > > implement an mm_exit() callback, which stops device
> > > > > > > > > jobs that do DMA. This function is called in the
> > > > > > > > > release() MMU notifier, when an address space that is
> > > > > > > > > shared with a device exits.
> > > > > > > > > 
> > > > > > > > > It has been noted several time during discussions
> > > > > > > > > about SVA that cancelling DMA jobs can be slow and
> > > > > > > > > complex, and doing it in the release() notifier might
> > > > > > > > > cause synchronization issues (patch 2 has more
> > > > > > > > > background). Device drivers must in any case call
> > > > > > > > > unbind() to remove their bond, after stopping DMA
> > > > > > > > > from a more favorable context (release of a file
> > > > > > > > > descriptor).
> > > > > > > > > 
> > > > > > > > > So after mm exits, rather than notifying device
> > > > > > > > > drivers, we can hold on to the PASID until unbind(),
> > > > > > > > > ask IOMMU drivers to silently abort DMA and Page
> > > > > > > > > Requests in the meantime. This change should relieve
> > > > > > > > > the mmput() path.        
> > > > > > > >
> > > > > > > > I assume mm is destroyed after all the FDs are
> > > > > > > > closed        
> > > > > > > 
> > > > > > > FDs do not hold a mmget(), but they may hold a mmgrab(),
> > > > > > > ie anything using mmu_notifiers has to hold a grab until
> > > > > > > the notifier is destroyed, which is often triggered by FD
> > > > > > > close. 
> > > > > > Sorry, I don't get this. Are you saying we have to hold a
> > > > > > mmgrab() between svm_bind/mmu_notifier_register and
> > > > > > svm_unbind/mmu_notifier_unregister?      
> > > > > 
> > > > > Yes. This is done automatically for the caller inside the
> > > > > mmu_notifier implementation. We now even store the mm_struct
> > > > > pointer inside the notifier.
> > > > > 
> > > > > Once a notifier is registered the mm_struct remains valid
> > > > > memory until the notifier is unregistered.
> > > > >     
> > > > > > Isn't the idea of mmu_notifier is to avoid holding the mm
> > > > > > reference and rely on the notifier to tell us when mm is
> > > > > > going away?      
> > > > > 
> > > > > The notifier only holds a mmgrab(), not a mmget() - this
> > > > > allows exit_mmap to proceed, but the mm_struct memory remains.
> > > > > 
> > > > > This is also probably why it is a bad idea to tie the
> > > > > lifetime of something like a pasid to the mmdrop as a evil
> > > > > user could cause a large number of mm structs to be released
> > > > > but not freed, probably defeating cgroup limits and so forth
> > > > > (not sure) 
> > > > > > It seems both Intel and AMD iommu drivers don't hold mmgrab
> > > > > > after mmu_notifier_register.      
> > > > > 
> > > > > It is done internally to the implementation.
> > > > >     
> > > > > > > So the exit_mmap() -> release() may happen before the FDs
> > > > > > > are destroyed, but the final mmdrop() will be during some
> > > > > > > FD release when the final mmdrop() happens.      
> > > > > > 
> > > > > > Do you mean mmdrop() is after FD release?       
> > > > > 
> > > > > Yes, it will be done by the mmu_notifier_unregister(), which
> > > > > should be called during FD release if the iommu lifetime is
> > > > > linked to some FD.   
> > > > > > If so, unbind is called in FD release should take care of
> > > > > > everything, i.e. stops DMA, clear PASID context on IOMMU,
> > > > > > flush PRS queue etc.      
> > > > > 
> > > > > Yes, this is the proper way, when the DMA is stopped and no
> > > > > use of the PASID remains then you can drop the mmu notifier
> > > > > and release the PASID entirely. If that is linked to the
> > > > > lifetime of the FD then forget completely about the mm_struct
> > > > > lifetime, it doesn't matter..   
> > > > Got everything above, thanks a lot.
> > > > 
> > > > If everything is in order with the FD close. Why do we need to 
> > > > "ask IOMMU drivers to silently abort DMA and Page Requests in
> > > > the meantime." in mm_exit notifier? This will be done orderly
> > > > in unbind anyway.    
> > > 
> > > When the process is killed, mm release can happen before fds are
> > > released. If you look at do_exit() in kernel/exit.c:
> > > 
> > > 	exit_mm()
> > > 	  mmput()  
> > > 	   -> mmu release notifier    
> > > 	...
> > > 	exit_files()
> > > 	  close_files()
> > > 	    fput()
> > > 	exit_task_work()
> > > 	  __fput()  
> > > 	   -> unbind()    
> > >   
> > So unbind is coming anyway, the difference in handling in mmu
> > release notifier is whether we silently drop DMA fault vs.
> > reporting fault?  
> 
> What I meant is, between mmu release notifier and unbind(), we can't
> print any error from DMA fault on dmesg, because an mm exit is easily
> triggered by userspace. Look at the lifetime of the bond:
> 
> bind()
>  |
>  : Here any DMA fault is handled by mm, and on error we don't print
>  : anything to dmesg. Userspace can easily trigger faults by issuing
> DMA : on unmapped buffers.
>  |
> mm exit -> clear pgd, invalidate IOTLBs
>  |
>  : Here the PASID descriptor doesn't have the pgd anymore, but we
> don't : print out any error to dmesg either. DMA is likely still
> running but : any fault has to be ignored.
>  :
>  : We also can't free the PASID yet, since transactions are still
> coming : in with this PASID.
>  |
> unbind() -> clear context descriptor, release PASID and mmu notifier
>  |
>  : Here the PASID descriptor is clear. If DMA is still running the
> device : driver really messed up and we have to print out any fault.
> 
> For that middle state I had to introduce a new pasid descriptor state
> in the SMMU driver, to avoid reporting errors between mm exit and
> unbind().
> 
I agree. Silent error in this window is the right way to go. Also, per
Jason's point, mm destroy should have similar behavior than vm unmap.

Thanks,

Jacob

> Thanks,
> Jean
> 
> > If a process crash during unbind, something already went seriously
> > wrong, DMA fault is expected.
> > I think having some error indication is useful, compared to
> > "silently drop"
> > 
> > Thanks,
> > 
> > Jacob
> >   
> > > Thanks,
> > > Jean
> > >   
> > > >     
> > > > > > Enforcing unbind upon FD close might be a precarious path,
> > > > > > perhaps that is why we have to deal with out of order
> > > > > > situation?      
> > > > > 
> > > > > How so? You just put it in the FD release function :)
> > > > >     
> > > > I was thinking some driver may choose to defer unbind in some
> > > > workqueue etc.
> > > >     
> > > > > > > > In VT-d, because of enqcmd and lazy PASID free we plan
> > > > > > > > to hold on to the PASID until mmdrop.
> > > > > > > > https://lore.kernel.org/patchwork/patch/1217762/        
> > > > > > > 
> > > > > > > Why? The bind already gets a mmu_notifier which has
> > > > > > > refcounts and the right lifetime for PASID.. This code
> > > > > > > could already be simplified by using the
> > > > > > > mmu_notifier_get()/put() stuff. 
> > > > > > Yes, I guess mmu_notifier_get()/put() is new :)
> > > > > > +Fenghua      
> > > > > 
> > > > > I was going to convert the intel code when I did many other
> > > > > drivers, but it was a bit too complex..
> > > > > 
> > > > > But the approach is straightforward. Get rid of the mm search
> > > > > list and use mmu_notifier_get(). This returns a singlton
> > > > > notifier for the mm_struct and handles refcounting/etc
> > > > > 
> > > > > Use mmu_notifier_put() during a unbind, it will callback to
> > > > > free_notifier() to do the final frees (ie this is where the
> > > > > pasid should go away)
> > > > > 
> > > > > For the SVM_FLAG_PRIVATE_PASID continue to use
> > > > > mmu_notifier_register, however this can now be mixed with
> > > > > mmu_notifier_put() so the cleanup is the same. A separate ops
> > > > > static struct is needed to create a unique key though
> > > > > 
> > > > > Jason    
> > > > 
> > > > [Jacob Pan]    
> > 
> > [Jacob Pan]  

[Jacob Pan]
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