[PATCH] drm/i915/dpt: Make DPT object unshrinkable

[Tvrtko Ursulin]

On 23/05/2024 12:19, Ville Syrjälä wrote:
> On Thu, May 23, 2024 at 09:25:45AM +0100, Tvrtko Ursulin wrote:
>>
>> On 22/05/2024 16:29, Vidya Srinivas wrote:
>>> In some scenarios, the DPT object gets shrunk but
>>> the actual framebuffer did not and thus its still
>>> there on the DPT's vm->bound_list. Then it tries to
>>> rewrite the PTEs via a stale CPU mapping. This causes panic.
>>>
>>> Suggested-by: Ville Syrjala <ville.syrjala at linux.intel.com>
>>> Cc: stable at vger.kernel.org
>>> Fixes: 0dc987b699ce ("drm/i915/display: Add smem fallback allocation for dpt")
>>> Signed-off-by: Vidya Srinivas <vidya.srinivas at intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/gem/i915_gem_object.h | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>> index 3560a062d287..e6b485fc54d4 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>> @@ -284,7 +284,8 @@ bool i915_gem_object_has_iomem(const struct drm_i915_gem_object *obj);
>>>    static inline bool
>>>    i915_gem_object_is_shrinkable(const struct drm_i915_gem_object *obj)
>>>    {
>>> -	return i915_gem_object_type_has(obj, I915_GEM_OBJECT_IS_SHRINKABLE);
>>> +	return i915_gem_object_type_has(obj, I915_GEM_OBJECT_IS_SHRINKABLE) &&
>>> +		!obj->is_dpt;
>>
>> Is there a reason i915_gem_object_make_unshrinkable() cannot be used to
>> mark the object at a suitable place?
> 
> Do you have a suitable place in mind?
> i915_gem_object_make_unshrinkable() contains some magic
> ingredients so doesn't look like it can be called willy
> nilly.

After it is created in intel_dpt_create?

I don't see that helper couldn't be called. It is called from madvise 
and tiling for instance without any apparent special considerations.

Also, there is no mention of this angle in the commit message so I 
assumed it wasn't considered. If it was, then it should have been 
mentioned why hacky solution was chosen instead...

> Anyways, looks like I forgot to reply that I already pushed this
> with this extra comment added:
> /* TODO: make DPT shrinkable when it has no bound vmas */

... becuase IMO the special case is quite ugly and out of place. :(

I don't remember from the top of my head how DPT magic works but if 
shrinker protection needs to be tied with VMAs there is also 
i915_make_make(un)shrinkable to try.

Regards,

Tvrtko