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

[Chris Wilson]

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.
-Chris