Re: [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count drm_gpuvm structures

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2023-11-02 at 18:32 +0100, Danilo Krummrich wrote:
> Hi Thomas,
> 
> thanks for your timely response on that!
> 
> On 11/2/23 18:09, Thomas Hellström wrote:
> > On Thu, 2023-11-02 at 00:31 +0100, Danilo Krummrich wrote:
> > > Implement reference counting for struct drm_gpuvm.
> > > 
> > > Signed-off-by: Danilo Krummrich <dakr@redhat.com>
> > > ---
> > >   drivers/gpu/drm/drm_gpuvm.c            | 44
> > > +++++++++++++++++++-----
> > > --
> > >   drivers/gpu/drm/nouveau/nouveau_uvmm.c | 20 +++++++++---
> > >   include/drm/drm_gpuvm.h                | 31 +++++++++++++++++-
> > >   3 files changed, 78 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_gpuvm.c
> > > b/drivers/gpu/drm/drm_gpuvm.c
> > > index 53e2c406fb04..6a88eafc5229 100644
> > > --- a/drivers/gpu/drm/drm_gpuvm.c
> > > +++ b/drivers/gpu/drm/drm_gpuvm.c
> > > @@ -746,6 +746,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm, const
> > > char *name,
> > >          gpuvm->rb.tree = RB_ROOT_CACHED;
> > >          INIT_LIST_HEAD(&gpuvm->rb.list);
> > >   
> > > +       kref_init(&gpuvm->kref);
> > > +
> > >          gpuvm->name = name ? name : "unknown";
> > >          gpuvm->flags = flags;
> > >          gpuvm->ops = ops;
> > > @@ -770,15 +772,8 @@ drm_gpuvm_init(struct drm_gpuvm *gpuvm,
> > > const
> > > char *name,
> > >   }
> > >   EXPORT_SYMBOL_GPL(drm_gpuvm_init);
> > >   
> > > -/**
> > > - * drm_gpuvm_destroy() - cleanup a &drm_gpuvm
> > > - * @gpuvm: pointer to the &drm_gpuvm to clean up
> > > - *
> > > - * Note that it is a bug to call this function on a manager that
> > > still
> > > - * holds GPU VA mappings.
> > > - */
> > > -void
> > > -drm_gpuvm_destroy(struct drm_gpuvm *gpuvm)
> > > +static void
> > > +drm_gpuvm_fini(struct drm_gpuvm *gpuvm)
> > >   {
> > >          gpuvm->name = NULL;
> > >   
> > > @@ -790,7 +785,33 @@ drm_gpuvm_destroy(struct drm_gpuvm *gpuvm)
> > >   
> > >          drm_gem_object_put(gpuvm->r_obj);
> > >   }
> > > -EXPORT_SYMBOL_GPL(drm_gpuvm_destroy);
> > > +
> > > +static void
> > > +drm_gpuvm_free(struct kref *kref)
> > > +{
> > > +       struct drm_gpuvm *gpuvm = container_of(kref, struct
> > > drm_gpuvm, kref);
> > > +
> > > +       if (drm_WARN_ON(gpuvm->drm, !gpuvm->ops->vm_free))
> > > +               return;
> > > +
> > > +       drm_gpuvm_fini(gpuvm);
> > > +
> > > +       gpuvm->ops->vm_free(gpuvm);
> > > +}
> > > +
> > > +/**
> > > + * drm_gpuvm_bo_put() - drop a struct drm_gpuvm reference
> > copy-paste error in function name.
> > 
> > Also it appears like xe might put a vm from irq context so we
> > should
> > document the context where this function call is allowable, and if
> > applicable add a might_sleep().
> 
>  From GPUVM PoV I don't see why we can't call this from an IRQ
> context.
> It depends on the driver callbacks of GPUVM (->vm_free) and the resv
> GEM's
> free callback. Both are controlled by the driver. Hence, I don't see
> the
> need for a restriction here.

OK. we should keep in mind though, that if such a restriction is needed
in the future, it might be some work to fix the drivers.

> 
> > 
> > If this function needs to sleep we can work around that in Xe by
> > keeping an xe-private refcount for the xe vm container, but I'd
> > like to
> > avoid that if possible and piggy-back on the refcount introduced
> > here.
> > 
> > > + * @gpuvm: the &drm_gpuvm to release the reference of
> > > + *
> > > + * This releases a reference to @gpuvm.
> > > + */
> > > +void
> > > +drm_gpuvm_put(struct drm_gpuvm *gpuvm)
> > > +{
> > > +       if (gpuvm)
> > > +               kref_put(&gpuvm->kref, drm_gpuvm_free);
> > > +}
> > > +EXPORT_SYMBOL_GPL(drm_gpuvm_put);
> > >   
> > >   static int
> > >   __drm_gpuva_insert(struct drm_gpuvm *gpuvm,
> > > @@ -843,7 +864,7 @@ drm_gpuva_insert(struct drm_gpuvm *gpuvm,
> > >          if (unlikely(!drm_gpuvm_range_valid(gpuvm, addr,
> > > range)))
> > >                  return -EINVAL;
> > >   
> > > -       return __drm_gpuva_insert(gpuvm, va);
> > > +       return __drm_gpuva_insert(drm_gpuvm_get(gpuvm), va);
> > 
> > Here we leak a reference if __drm_gpuva_insert() fails, and IMO the
> > reference should be taken where the pointer holding the reference
> > is
> > assigned (in this case in __drm_gpuva_insert()), or document the
> > reference transfer from the argument close to the assignment.
> 
> Ah, good catch. I had it in __drm_gpuva_insert() originally, but that
> doesn't work, because __drm_gpuva_insert() is used to insert the
> kernel_alloc_node. And we need to __drm_gpuva_remove() the
> kernel_alloc_node
> from drm_gpuvm_fini(), which is called when the reference count is at
> zero
> already. In fact, the __* variants are only there to handle the
> kernel_alloc_node and this one clearly doesn't need reference
> counting.
> 
> > 
> > But since a va itself is not refcounted it clearly can't outlive
> > the
> > vm, so is a reference really needed here?
> 
> Well, technically, it can. It just doesn't make any sense and would
> be
> considered to be a bug. The reference count comes in handy to prevent
> that in the first place.

> 
> I'd like to keep the reference count and just fix up the code.

OK. That's probably being a bit overly cautious IMHO, but I can't see
any major drawbacks either.

> 
> > 
> > I'd suggest using an accessor that instead of using va->vm uses va-
> > > vm_bo->vm, to avoid needing to worry about the vm->vm refcount
> > altoghether.
> 
> No, I want to keep that optional. Drivers should be able to use GPUVM
> to
> track mappings without being required to implement everything else.
> 
> I think PowerVR, for instance, currently uses GPUVM only to track
> mappings
> without everything else.

Yeah, I also realized that userptr is another potential user.
A badly though-trough suggestion..

Thanks,
Thomas


> 
> - Danilo
> 
> > 
> > Thanks,
> > Thomas
> > 
>