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

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

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()

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]
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu