netdev - Bridge + Conntrack + SKB Recycle: Fragment Reassembly Errors

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Date:	Tue, 10 Nov 2009 09:09:47 -0700
From:	<ben@...footnetworks.com>
To:	<netdev@...r.kernel.org>
Subject: Bridge + Conntrack + SKB Recycle: Fragment Reassembly Errors

We have observed significant reassembly errors when combining
routing/bridging with conntrack + nf_defrag_ipv4 loaded, and
skb_recycle_check - enabled interfaces.  For our test, we had a single
linux device with two interfaces (gianfars in this case) with SKB
recycling enabled.  We sent large, continuous pings across the bridge,
like this:
ping -s 64000 -A <dest IP>

Then, we ran netstat -s --raw, and noticed that IPSTATS_MIB_REASMFAILS
were happening for about 40% of the received datagrams.  Tracing the
code in ip_fragment.c, we instrumented each of the
IPSTATS_MIB_REASMFAILS locations, and found the culprit to be
ip_evictor.  Nothing looked unusual here, so we placed tracing in
ip_frag_queue, directly above:
	atomic_add(skb->truesize, &qp->q.net->mem);

We noticed that quite a few of the skb->truesize numbers were in the 67K
range, which quickly overwhelms the default 192K-ish ipfrag_low_thresh.
This means that the next time inet_frag_evictor is run:
 work = atomic_read(&nf->mem) - nf->low_thresh;

Will surely be positive, and it is likely that our huge-frag-containing
queue will be one of those evicted. 

Looking at the source of these huge skbs, it seems that during
re-fragmentation in br_nf_dev_queue_xmit (which calls ip_fragment with
CONFIG_NF_CONNTRACK_IPV4 enabled), the huge datagram that was allocated
to hold a successfully-reassembled skb may be getting reused?  In any
case, when skb_recycle_check(skb, min_rx_size) is called, the huge
(skb->truesize huge, not data huge) skb is recycled for use on RX, and
it eventually gets enqueued for reassembly, causing the
inet_frag_evictor to have a positive work value.

Our solution was to add an upper-bounds check to skb_recycle_check,
which prevents the large-ish SKBs from being used to create future
frags, and overwhelming ipfrag_low_thresh.  This seems quite clunky,
although I would be happy to submit this as a patch...

If this is not the right place...what is the "right" place?

Ben Menchaca
Bigfoot Networks

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