drm/i915/selftests: Constrain mock_gtt tests to fit within RAM

Be pessimistic and presume that we actually allocate every page we
exercise via the mock_gtt (e.g. for gvt). In which case we have to keep
our working set under the available physical memory to prevent oom.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180710080424.7821-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -1244,6 +1244,7 @@ static int exercise_mock(struct drm_i915_private *i915,
 				     u64 hole_start, u64 hole_end,
 				     unsigned long end_time))
 {
+	const u64 limit = totalram_pages << PAGE_SHIFT;
 	struct i915_gem_context *ctx;
 	struct i915_hw_ppgtt *ppgtt;
 	IGT_TIMEOUT(end_time);
@@ -1256,7 +1257,7 @@ static int exercise_mock(struct drm_i915_private *i915,
 	ppgtt = ctx->ppgtt;
 	GEM_BUG_ON(!ppgtt);
 
-	err = func(i915, &ppgtt->vm, 0, ppgtt->vm.total, end_time);
+	err = func(i915, &ppgtt->vm, 0, min(ppgtt->vm.total, limit), end_time);
 
 	mock_context_close(ctx);
 	return err;