Re: Hitting WARN_ON_ONCE on skb_try_coalesce

If we pack the struct, we have enough space to save the original
truesize and restore it when dequeing it.
This way the skb won't be charging sender buf too much while sitting on
netem queue and will allow proper accouting on rx buf if it's
reinjected.

The check on skb->sk in the last chunk is for handling duplicated
packets.

This patch makes it not trigger the Warn for this test. Wdyt?

---8<---

--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c

@@ -147,7 +147,8 @@ struct netem_sched_data {
 struct netem_skb_cb {
 	psched_time_t	time_to_send;
 	ktime_t		tstamp_save;
-};
+	unsigned int	truesize;
+} __attribute__ ((packed));

 

 static struct sk_buff *netem_rb_to_skb(struct rb_node *rb)

@@ -428,6 +429,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	struct sk_buff *skb2;
 	struct sk_buff *segs = NULL;
 	unsigned int len = 0, last_len, prev_len = qdisc_pkt_len(skb);
+	unsigned int truesize;
 	int nb = 0;
 	int count = 1;
 	int rc = NET_XMIT_SUCCESS;

@@ -452,8 +454,10 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	/* If a delay is expected, orphan the skb. (orphaning usually takes
 	 * place at TX completion time, so _before_ the link transit delay)
 	 */
-	if (q->latency || q->jitter)
+	if (q->latency || q->jitter) {
+		truesize = skb->truesize;
 		skb_orphan_partial(skb);
+	}
 
 	/*
 	 * If we need to duplicate packet, then re-insert at top of the

@@ -508,6 +512,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	qdisc_qstats_backlog_inc(sch, skb);
 
 	cb = netem_skb_cb(skb);
+	cb->truesize = truesize;
 	if (q->gap == 0 ||		/* not doing reordering */
 	    q->counter < q->gap - 1 ||	/* inside last reordering gap */
 	    q->reorder < get_crandom(&q->reorder_cor)) {

@@ -591,6 +596,10 @@ tfifo_dequeue:
 	if (skb) {
 		qdisc_qstats_backlog_dec(sch, skb);
 deliver:
+		if (skb->sk && skb->truesize == 1) {
+			skb->truesize = netem_skb_cb(skb)->truesize;
+			atomic_add(skb->truesize - 1, &skb->sk->sk_wmem_alloc);
+		}
 		qdisc_bstats_update(sch, skb);
 		return skb;
 	}