Did I mention that all this loop blkcg integration is complete and utter
garbage and should have never been merged?

 	struct list_head *cmd_list;
+	struct cgroup_subsys_state *blkcg_css = NULL;
+#ifdef CONFIG_BLK_CGROUP
+	struct request *rq = blk_mq_rq_from_pdu(cmd);
+
+	if (rq->bio && rq->bio->bi_blkg)
+		blkcg_css = &bio_blkcg(rq->bio)->css;
+#endif

This kind of junk has no business in a driver.  The blkcg code
need to provide a helper for retreiving the blkcg_css from a request,
including a stub for the the !CONFIG_BLK_CGROUP case.

 		cur_worker = container_of(*node, struct loop_worker, rb_node);
-		if (cur_worker->blkcg_css == cmd->blkcg_css) {
+		if (cur_worker->blkcg_css == blkcg_css) {
 			worker = cur_worker;
 			break;
-		} else if ((long)cur_worker->blkcg_css < (long)cmd->blkcg_css) {
+		} else if ((long)cur_worker->blkcg_css < (long)blkcg_css) {

No need for an else after a break.  And more importantly no need to cast
a pointer to compare it to another pointer of the same type.

+	struct mem_cgroup *old_memcg = NULL;
+	struct cgroup_subsys_state *memcg_css = NULL;
+
+	kthread_associate_blkcg(worker->blkcg_css);
+#ifdef CONFIG_MEMCG
+	memcg_css = cgroup_get_e_css(worker->blkcg_css->cgroup,
+			&memory_cgrp_subsys);
+#endif
+	if (memcg_css)
+		old_memcg = set_active_memcg(
+				mem_cgroup_from_css(memcg_css));
+

This kind of crap also has absolutely no business in a block driver
and the memcg code should provide a proper helper.