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

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

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.

> > 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]
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