fq.h: Port memory limit mechanism from fq_codel

The reusable fairness queueing implementation (fq.h) lacks the memory
usage limit that the fq_codel qdisc has. This means that small
devices (e.g. WiFi routers) can run out of memory when flooded with a
large number of packets. This ports the memory limit feature from
fq_codel to fq.h.

Signed-off-by: Toke Høiland-Jørgensen <[e-mail]>
Signed-off-by: Johannes Berg <[e-mail]>

include/net/fq.h

···
 	u32 flows_cnt;
 	u32 perturbation;
 	u32 limit;
+	u32 memory_limit;
+	u32 memory_usage;
 	u32 quantum;
 	u32 backlog;
 	u32 overlimit;
+	u32 overmemory;
 	u32 collisions;
 };
 

include/net/fq_impl.h

···
 	tin->backlog_packets--;
 	flow->backlog -= skb->len;
 	fq->backlog--;
+	fq->memory_usage -= skb->truesize;
 
 	if (flow->backlog == 0) {
 		list_del_init(&flow->backlogchain);
···
 	flow->backlog += skb->len;
 	tin->backlog_bytes += skb->len;
 	tin->backlog_packets++;
+	fq->memory_usage += skb->truesize;
 	fq->backlog++;
 
 	fq_recalc_backlog(fq, tin, flow);
···
 
 	__skb_queue_tail(&flow->queue, skb);
 
-	if (fq->backlog > fq->limit) {
+	if (fq->backlog > fq->limit || fq->memory_usage > fq->memory_limit) {
 		flow = list_first_entry_or_null(&fq->backlogs,
 						struct fq_flow,
 						backlogchain);
···
 
 		flow->tin->overlimit++;
 		fq->overlimit++;
+		if (fq->memory_usage > fq->memory_limit)
+			fq->overmemory++;
 	}
 }
 
···
 	fq->perturbation = prandom_u32();
 	fq->quantum = 300;
 	fq->limit = 8192;
+	fq->memory_limit = 16 << 20; /* 16 MBytes */
 
 	fq->flows = kcalloc(fq->flows_cnt, sizeof(fq->flows[0]), GFP_KERNEL);
 	if (!fq->flows)