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

[Chris Wilson]

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).
-Chris