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

[Baolu Lu]

On 2022/4/30 06:19, Fenghua Yu wrote:
> Hi, Jean and Baolu,
>
> On Fri, Apr 29, 2022 at 03:34:36PM +0100, Jean-Philippe Brucker wrote:
> > On Fri, Apr 29, 2022 at 06:51:17AM -0700, Fenghua Yu wrote:
> > > 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().
> >
> > Yes, although for Arm I realized the mmu_notifier grab wasn't sufficient
> > so I sent a separate fix that should go in 5.18 as well
> > https://lore.kernel.org/linux-iommu/20220426130444.300556-1-jean-philippe@xxxxxxxxxx/
> > The Arm driver still touches the arch mm context after mmu_notifier_put().
> > I don't think X86 has that problem.
>
> I think so too. On X86, the mm is not used after mmu_notifier_unregister().
>
> In case of supervisor mode SVM (i.e. svm is bound to init_mm), the code
> is right too because init_mm and its PASID cannot be dropped and
> mmu_notifier_register()/mmu_notifier_unregister() are not called.
>
> So I think no extra fix patch is needed on X86.

Thanks, Fenghua and Jean. It's clear to me now.

Jean, another quick question about the iommu_sva_bind_device()

/**
 * iommu_sva_bind_device() - Bind a process address space to a device
 * @dev: the device
 * @mm: the mm to bind, caller must hold a reference to it
 * @drvdata: opaque data pointer to pass to bind callback

This interface requires the caller to take a reference to mm. Which
reference should it take, mm->mm_count or mm->mm_users? It's better to
make it explicit in this comment.

Best regards,
baolu