[Intel-gfx] [BrownBag] drm/i915/gtt: Neuter the deferred unbind callback from gen6_ppgtt_cleanup

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-05-24 09:57:42)
> 
> On 24/05/2019 09:51, Tvrtko Ursulin wrote:
> > 
> > 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 at chris-wilson.co.uk>
> >>>>>>>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>>>>>>> Cc: Tomi Sarvela <tomi.p.sarvela at 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? :)
> 
> Okay ppgtt is gone so can't do it.. annoying.. Cleanup seems to support 
> your claims but I think we need a BFC (big fat comment) above the vma 
> ops override to explains this. With that:

It has FIXME! I really do hope this is short term...
-Chris