netdev - Re: UDP multicast packet loss not reported if TX ring overrun?

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Message-ID: <alpine.DEB.1.10.0908251258000.26329@gentwo.org>
Date:	Tue, 25 Aug 2009 13:01:05 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	Sridhar Samudrala <sri@...ibm.com>,
	Nivedita Singhvi <niv@...ibm.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: UDP multicast packet loss not reported if TX ring overrun?

On Tue, 25 Aug 2009, Eric Dumazet wrote:

> > The initial point was that the SNMP counters are not updated if IP_RECVERR
> > is not set which is clearly a bug and your and my patch addresses that.
>
> Technically speaking, the send() syscall is in error. Frame is not sent, so
> there is no drop at all. Like trying to send() from a bad user buffer, or write()
> to a too big file...

Frame is submitted to the IP layer which discards it. That is the
definition of an output discard.

> Question is : should we just allow send() to return an error (-ENOBUF) regardless
> of IP_RECVERR being set or not ? I dont think it would be so bad after all.
> Most apps probably dont care, or already handle the error.

Some applications will then start to fail because so far you can send with
impunity without getting errors. AFAICT IP_RECVERR was added to preserve
that behavior. Your patch is changing basic send() semantics.
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