drm/i915: Store a permanent error in obj->mm.pages

Once the object has been truncated, it is unrecoverable. To facilitate
detection of this state store the error in obj->mm.pages.

This is required for the next patch which should be applied to v4.10
(via stable), so we also need to mark this patch for backporting. In
that regard, let's consider this to be a fix/improvement too.

v2: Avoid dereferencing the ERR_PTR when freeing the object.

Fixes: 1233e2db199d ("drm/i915: Move object backing storage manipulation to its own locking")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <stable@vger.kernel.org> # v4.10+
Link: http://patchwork.freedesktop.org/patch/msgid/20170307132031.32461-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -2130,6 +2130,7 @@ i915_gem_object_truncate(struct drm_i915_gem_object *obj)
 	 */
 	shmem_truncate_range(file_inode(obj->base.filp), 0, (loff_t)-1);
 	obj->mm.madv = __I915_MADV_PURGED;
+	obj->mm.pages = ERR_PTR(-EFAULT);
 }
 
 /* Try to discard unwanted pages */
@@ -2229,7 +2230,9 @@ void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
 
 	__i915_gem_object_reset_page_iter(obj);
 
-	obj->ops->put_pages(obj, pages);
+	if (!IS_ERR(pages))
+		obj->ops->put_pages(obj, pages);
+
 unlock:
 	mutex_unlock(&obj->mm.lock);
 }
@@ -2449,7 +2452,7 @@ int __i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 	if (err)
 		return err;
 
-	if (unlikely(!obj->mm.pages)) {
+	if (unlikely(IS_ERR_OR_NULL(obj->mm.pages))) {
 		err = ____i915_gem_object_get_pages(obj);
 		if (err)
 			goto unlock;
@@ -2527,7 +2530,7 @@ void *i915_gem_object_pin_map(struct drm_i915_gem_object *obj,
 
 	pinned = true;
 	if (!atomic_inc_not_zero(&obj->mm.pages_pin_count)) {
-		if (unlikely(!obj->mm.pages)) {
+		if (unlikely(IS_ERR_OR_NULL(obj->mm.pages))) {
 			ret = ____i915_gem_object_get_pages(obj);
 			if (ret)
 				goto err_unlock;