drm/i915: Document our internal limit on object size

In many places, we try to count pages using a 32 bit integer. That
implies if we are asked to create an object larger than 43bits, we will
subtly crash much later. Catch this on the boundary, and add a warning
to remind ourselves later on our exabyte systems.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161018120251.25043-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3107,7 +3107,7 @@ void i915_gem_object_free(struct drm_i915_gem_object *obj);
 void i915_gem_object_init(struct drm_i915_gem_object *obj,
 			 const struct drm_i915_gem_object_ops *ops);
 struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
-						  size_t size);
+						   u64 size);
 struct drm_i915_gem_object *i915_gem_object_create_from_data(
 		struct drm_device *dev, const void *data, size_t size);
 void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file);

drivers/gpu/drm/i915/i915_gem.c

@@ -4131,14 +4131,29 @@ static const struct drm_i915_gem_object_ops i915_gem_object_ops = {
 	.put_pages = i915_gem_object_put_pages_gtt,
 };
 
-struct drm_i915_gem_object *i915_gem_object_create(struct drm_device *dev,
-						  size_t size)
+/* Note we don't consider signbits :| */
+#define overflows_type(x, T) \
+	(sizeof(x) > sizeof(T) && (x) >> (sizeof(T) * BITS_PER_BYTE))
+
+struct drm_i915_gem_object *
+i915_gem_object_create(struct drm_device *dev, u64 size)
 {
 	struct drm_i915_gem_object *obj;
 	struct address_space *mapping;
 	gfp_t mask;
 	int ret;
 
+	/* There is a prevalence of the assumption that we fit the object's
+	 * page count inside a 32bit _signed_ variable. Let's document this and
+	 * catch if we ever need to fix it. In the meantime, if you do spot
+	 * such a local variable, please consider fixing!
+	 */
+	if (WARN_ON(size >> PAGE_SHIFT > INT_MAX))
+		return ERR_PTR(-E2BIG);
+
+	if (overflows_type(size, obj->base.size))
+		return ERR_PTR(-E2BIG);
+
 	obj = i915_gem_object_alloc(dev);
 	if (obj == NULL)
 		return ERR_PTR(-ENOMEM);