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

From: Fenghua Yu
Date: Fri Apr 29 2022 - 09:50:55 EST

Hi, Baolu,

On Fri, Apr 29, 2022 at 03:53:57PM +0800, Baolu Lu wrote:
> On 2022/4/28 16:39, Jean-Philippe Brucker wrote:
> > > The address space is what the OOM killer is after. That gets refcounted
> > > with mmget()/mmput()/mm->mm_users. The OOM killer is satiated by the
> > > page freeing done in __mmput()->exit_mmap().
> > >
> > > Also, all the VMAs should be gone after exit_mmap(). So, even if
> > > vma->vm_file was holding a reference to a device driver, that reference
> > > should be gone by the time __mmdrop() is actually freeing the PASID.
> >
> > I agree with all that. The concern was about tearing down the PASID in the
> > IOMMU and device from the release() MMU notifier, which would happen in
> > exit_mmap(). But doing the teardown at or before __mmdrop() is fine. And
> > since the IOMMU drivers need to hold mm->mm_count anyway between bind()
> > and unbind(), I think Fenghua's fix works.
> But I didn't find mmgrab()/mmdrop() get called in both arm and intel
> IOMMU drivers.
> $ git grep mmgrab drivers/iommu/
> [no output]
> Do we need to add these in a separated fix patch, or I missed anything
> here?

On both ARM and X86, sva_bind() calls mmu_notifier_register()->mmgrab() and
sva_unbind() calls mmu_notifier_unregister()/mmu_notifier_put()->mmdrop().

So mm->mm_count are already counted in existing ARM and X86 binding and
unbinding. The fix patch just frees the PASID in __mmdrop() after
no more mm->mm_count.

There is no need to add extra mmgrab() and mmdrop() pair.