netdev - Re: [PATCH net] virtio-net: fix page refcnt leaking when fail to allocate frag skb

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Date:	Wed, 20 Nov 2013 10:58:35 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jason Wang <jasowang@...hat.com>, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Michael Dalton <mwdalton@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] virtio-net: fix page refcnt leaking when fail to
 allocate frag skb

On Tue, Nov 19, 2013 at 02:00:11PM -0800, Eric Dumazet wrote:
> On Tue, 2013-11-19 at 23:53 +0200, Michael S. Tsirkin wrote:
> 
> > Which NIC? Virtio? Prior to 2613af0ed18a11d5c566a81f9a6510b73180660a
> > it didn't drop packets received from host as far as I can tell.
> > virtio is more like a pipe than a real NIC in this respect.
> 
> Prior/after to this patch, you were not posting buffers, so if packets
> were received on a physical NIC, you were dropping the packets anyway.
>
> It makes no difference at all, adding a cushion might make you feel
> better, but its really not worth it.
> 
> Under memory stress, it makes better sense to drop a super big GRO
> packet (The one needing frag_list extension ...)
> 
> It gives a better signal to the sender to reduce its pressure, and gives
> opportunity to free more of your memory.
> 

OK, but in that case one wonders whether we should do more to free memory?

E.g. imagine that we dropped a packet of a specific TCP flow
because we couldn't allocate a new packet.

What happens now is that the old packet is freed as well.

So quite likely the next packet in queue will get processed
since it will reuse the memory we have just freed.

The next packet and the next after it etc all will have to go through
the net stack until they get at the socket and are dropped then
because we missed a segment.  Even worse, GRO gets disabled so the load
on receiver goes up instead of down.

Sounds like a problem doesn't it?

GRO actually detects it's the same flow and can see packet is
out of sequence. Why doesn't it drop the packet then?
Alternatively, we could (for example using the pre-allocated skb
like I suggested) notify GRO that it should start dropping packets
of this flow.

What do you think?

-- 
MST
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