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 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().

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.

But since a va itself is not refcounted it clearly can't outlive the
vm, so is a reference really needed here?

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.

Thanks,
Thomas