drm/i915/gtt: Relax assertion for pt_used

When inserting the final level PTE, we check that we are not overflowing
the page table (checking that pt_used does not exceed the size of the
table). However, we have to allow for every other PTE to be pinned by a
simultaneous removal thread (as on remove we bump the pt_used counter
before adjusting the table).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821042044.7354-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -1110,7 +1110,8 @@ static int __gen8_ppgtt_alloc(struct i915_address_space * const vm,
 			    atomic_read(&pt->used));
 
 			atomic_add(count, &pt->used);
-			GEM_BUG_ON(atomic_read(&pt->used) > I915_PDES);
+			/* All other pdes may be simultaneously removed */
+			GEM_BUG_ON(atomic_read(&pt->used) > 2 * I915_PDES);
 			*start += count;
 		}
 	} while (idx++, --len);