drm/i915: Do not drop pagetables when empty

This is the minimal backport for stable of the upstream commit:

commit dd19674b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Feb 15 08:43:46 2017 +0000

    drm/i915: Remove bitmap tracking for used-ptes

Due to a race with the shrinker, when we try to allocate a pagetable, we
may end up shrinking it instead. This comes as a nasty surprise as we
try to dereference it to fill in the pagetable entries for the object.

In linus/master this is fixed by pinning the pagetables prior to
allocation, but that backport is roughly
 drivers/gpu/drm/i915/i915_gem_gtt.c |   10 ----------
 1 file changed, 10 deletions(-)
i.e. unsuitable for stable. Instead we neuter the code that tried to
free the pagetables.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99295
Fixes: 2ce5179f ("drm/i915/gtt: Free unused lower-level page tables")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.10+
Tested-by: Maël Lavault <mael.lavault@protonmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -755,10 +755,6 @@ static bool gen8_ppgtt_clear_pt(struct i915_address_space *vm,
 	GEM_BUG_ON(pte_end > GEN8_PTES);
 
 	bitmap_clear(pt->used_ptes, pte, num_entries);
-	if (USES_FULL_PPGTT(vm->i915)) {
-		if (bitmap_empty(pt->used_ptes, GEN8_PTES))
-			return true;
-	}
 
 	pt_vaddr = kmap_px(pt);
 
@@ -798,9 +794,6 @@ static bool gen8_ppgtt_clear_pd(struct i915_address_space *vm,
 		}
 	}
 
-	if (bitmap_empty(pd->used_pdes, I915_PDES))
-		return true;
-
 	return false;
 }
 
@@ -829,9 +822,6 @@ static bool gen8_ppgtt_clear_pdp(struct i915_address_space *vm,
 
 	mark_tlbs_dirty(ppgtt);
 
-	if (bitmap_empty(pdp->used_pdpes, I915_PDPES_PER_PDP(dev_priv)))
-		return true;
-
 	return false;
 }