SUNRPC: Enqueue swapper tagged RPCs at the head of the transmit queue

Avoid memory starvation by giving RPCs that are tagged with the
RPC_TASK_SWAPPER flag the highest priority.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xprt.c

@@ -1112,6 +1112,17 @@ xprt_request_enqueue_transmit(struct rpc_task *task)
 				INIT_LIST_HEAD(&req->rq_xmit2);
 				goto out;
 			}
+		} else if (RPC_IS_SWAPPER(task)) {
+			list_for_each_entry(pos, &xprt->xmit_queue, rq_xmit) {
+				if (pos->rq_cong || pos->rq_bytes_sent)
+					continue;
+				if (RPC_IS_SWAPPER(pos->rq_task))
+					continue;
+				/* Note: req is added _before_ pos */
+				list_add_tail(&req->rq_xmit, &pos->rq_xmit);
+				INIT_LIST_HEAD(&req->rq_xmit2);
+				goto out;
+			}
 		} else {
 			list_for_each_entry(pos, &xprt->xmit_queue, rq_xmit) {
 				if (pos->rq_task->tk_owner != task->tk_owner)