Subject: Re: Question about ip_defrag

From: Florian Westphal <[email-masked]>
Date: Mon, 28 Aug 2017 16:00:32 +0200

liujian (CE) <[email-masked]> wrote:
> Hi
> 
> I checked our 3.10 kernel, we had backported all percpu_counter bug fix in lib/percpu_counter.c and include/linux/percpu_counter.h.
> And I check 4.13-rc6, also has the issue if NIC's rx cpu num big enough.
> 
> > > > > the issue:
> > > > > Ip_defrag fail caused by frag_mem_limit reached 4M(frags.high_thresh).
> > > > > At this moment,sum_frag_mem_limit is about 10K.
> 
> So should we change ipfrag high/low thresh to a reasonable value ? 
> And if it is, is there a standard to change the value?

Each cpu can have frag_percpu_counter_batch bytes rest doesn't know
about so with 64 cpus that is ~8 mbyte.

possible solutions:
1. reduce frag_percpu_counter_batch to 16k or so
2. make both low and high thresh depend on NR_CPUS

liujian, does this change help in any way?

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -123,6 +123,17 @@ static bool inet_fragq_should_evict(const struct inet_frag_queue *q)
 	       frag_mem_limit(q->net) >= q->net->low_thresh;
 }
 
+/* ->mem batch size is huge, this can cause severe discrepancies
+ * between actual value (sum of pcpu values) and the global estimate.
+ *
+ * Use a smaller batch to give an opportunity for the global estimate
+ * to more accurately reflect current state.
+ */
+static void update_frag_mem_limit(struct netns_frags *nf, unsigned int batch)
+{
+	 percpu_counter_add_batch(&nf->mem, 0, batch);
+}
+
 static unsigned int
 inet_evict_bucket(struct inet_frags *f, struct inet_frag_bucket *hb)
 {
@@ -146,8 +157,12 @@ inet_evict_bucket(struct inet_frags *f, struct inet_frag_bucket *hb)
 
 	spin_unlock(&hb->chain_lock);
 
-	hlist_for_each_entry_safe(fq, n, &expired, list_evictor)
+	hlist_for_each_entry_safe(fq, n, &expired, list_evictor) {
+		struct netns_frags *nf = fq->net;
+
 		f->frag_expire((unsigned long) fq);
+		update_frag_mem_limit(nf, 1);
+	}
 
 	return evicted;
 }
@@ -396,8 +411,10 @@ struct inet_frag_queue *inet_frag_find(struct netns_frags *nf,
 	struct inet_frag_queue *q;
 	int depth = 0;
 
-	if (frag_mem_limit(nf) > nf->low_thresh)
+	if (frag_mem_limit(nf) > nf->low_thresh) {
 		inet_frag_schedule_worker(f);
+		update_frag_mem_limit(nf, SKB_TRUESIZE(1500) * 16);
+	}
 
 	hash &= (INETFRAGS_HASHSZ - 1);
 	hb = &f->hash[hash];
@@ -416,6 +433,8 @@ struct inet_frag_queue *inet_frag_find(struct netns_frags *nf,
 	if (depth <= INETFRAGS_MAXDEPTH)
 		return inet_frag_create(nf, f, key);
 
+	update_frag_mem_limit(nf, 1);
+
 	if (inet_frag_may_rebuild(f)) {
 		if (!f->rebuild)
 			f->rebuild = true;