Re: [BrownBag] drm/i915/gtt: Neuter the deferred unbind callback from gen6_ppgtt_cleanup

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 24/05/2019 09:36, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-05-24 09:31:45)
>>
>> On 24/05/2019 09:29, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2019-05-24 09:23:40)
>>>>
>>>> On 24/05/2019 09:17, Chris Wilson wrote:
>>>>> Quoting Tvrtko Ursulin (2019-05-24 09:13:14)
>>>>>>
>>>>>> On 24/05/2019 07:45, Chris Wilson wrote:
>>>>>>> Having deferred the vma destruction to a worker where we can acquire the
>>>>>>> struct_mutex, we have to avoid chasing back into the now destroyed
>>>>>>> ppgtt. The pd_vma is special in having a custom unbind function to scan
>>>>>>> for unused pages despite the VMA itself being notionally part of the
>>>>>>> GGTT. As such, we need to disable that callback to avoid a
>>>>>>> use-after-free.
>>>>>>>
>>>>>>> This unfortunately blew up so early during boot that CI declared the
>>>>>>> machine unreachable as opposed to being the major failure it was. Oops.
>>>>>>>
>>>>>>> Fixes: d3622099c76f ("drm/i915/gtt: Always acquire struct_mutex for gen6_ppgtt_cleanup")
>>>>>>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>>>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>>>> Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
>>>>>>> ---
>>>>>>>      drivers/gpu/drm/i915/i915_gem_gtt.c | 28 ++++++++++++++++++++++++++++
>>>>>>>      1 file changed, 28 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>>>> index 8d8a4b0ad4d9..266baa11df64 100644
>>>>>>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>>>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
>>>>>>> @@ -1847,6 +1847,33 @@ static void gen6_ppgtt_cleanup_work(struct work_struct *wrk)
>>>>>>>          kfree(work);
>>>>>>>      }
>>>>>>>      
>>>>>>> +static int nop_set_pages(struct i915_vma *vma)
>>>>>>> +{
>>>>>>> +     return -ENODEV;
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void nop_clear_pages(struct i915_vma *vma)
>>>>>>> +{
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int nop_bind(struct i915_vma *vma,
>>>>>>> +                 enum i915_cache_level cache_level,
>>>>>>> +                 u32 unused)
>>>>>>> +{
>>>>>>> +     return -ENODEV;
>>>>>>> +}
>>>>>>> +
>>>>>>> +static void nop_unbind(struct i915_vma *vma)
>>>>>>> +{
>>>>>>> +}
>>>>>>> +
>>>>>>> +static const struct i915_vma_ops nop_vma_ops = {
>>>>>>> +     .set_pages = nop_set_pages,
>>>>>>> +     .clear_pages = nop_clear_pages,
>>>>>>> +     .bind_vma = nop_bind,
>>>>>>> +     .unbind_vma = nop_unbind,
>>>>>>> +};
>>>>>>> +
>>>>>>>      static void gen6_ppgtt_cleanup(struct i915_address_space *vm)
>>>>>>>      {
>>>>>>>          struct gen6_hw_ppgtt *ppgtt = to_gen6_ppgtt(i915_vm_to_ppgtt(vm));
>>>>>>> @@ -1855,6 +1882,7 @@ static void gen6_ppgtt_cleanup(struct i915_address_space *vm)
>>>>>>>          /* FIXME remove the struct_mutex to bring the locking under control */
>>>>>>>          INIT_WORK(&work->base, gen6_ppgtt_cleanup_work);
>>>>>>>          work->vma = ppgtt->vma;
>>>>>>> +     work->vma->ops = &nop_vma_ops;
>>>>>>
>>>>>> Could we use some asserts before overriding the vma ops? Like
>>>>>> GEM_BUG_ON(vma->pages)? And something for still bound?
>>>>>
>>>>> It technically still is bound as it is in the GGTT but currently
>>>>> unpinned -- that will be checked on destroy, it's just we also get an
>>>>> unbind callback. vma->pages doesn't exist for this (set to ERR_PTR).
>>>>
>>>> If we are getting the unbind callback and we nop-ed it, who will
>>>> actually do it's job?
>>>
>>> The callback is just a hook for us to prune within the ppgtt.
>>> It still is removed from GGTT by i915_vma_unbind().
>>
>> So it needs GEM_BUG_ON(ppgtt->scan_for_unused_pt) before overriding the
>> unbind?
> 
> No. They get freed by the cleanup itself. The scan is just an
> opportunistic prune if either the context/mm is evicted but still alive.

Then the same assert in gen6_ppgtt_cleanup_work? :)

Regards,

Tvrtko
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