netdev - Re: [PATCH RFC] netlink: a flag requesting header w/o data in error ACK

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Message-Id: <20131113.153112.1336285577814329683.davem@davemloft.net>
Date:	Wed, 13 Nov 2013 15:31:12 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	jbenc@...hat.com
Cc:	netdev@...r.kernel.org, tgraf@...g.ch, jhs@...atatu.com,
	pablo@...filter.org
Subject: Re: [PATCH RFC] netlink: a flag requesting header w/o data in
 error ACK

From: Jiri Benc <jbenc@...hat.com>
Date: Wed, 13 Nov 2013 16:46:09 +0100

> Currently, ACK in case of error contains a full copy of the originating
> message, although many applications do not need it. This can cause lost ACKs
> with large netlink messages, especially after commit c05cdb1b864f ("netlink:
> allow large data transfers from user-space").
> 
> Introduce a new message flag, NLM_F_SHORT_NACK, which requests only message
> header to be included in ACK regardless of the error code. This flag has no
> effect if NLM_F_ACK is not set.
> 
> Signed-off-by: Jiri Benc <jbenc@...hat.com>
> ---
> I'm not entirely happy with taking a bit in netlink message header for this.
> Alternative approach would be to make this a socket option (as suggested by
> David) but that would mean a socket lookup in netlink_ack for each and every
> ACK, which doesn't sound nice, either. If this is preferred, I'll rework the
> patch.

It's not really all that difficult, you can either attack the sender socket
to in_skb->sk, or pass a new boolean around in the various code paths that
lead to the ACK.
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