netdev - Re: [PATCH net] netlink: fix netlink

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Message-ID: <20131113122504.0ddcbf89@griffin>
Date:	Wed, 13 Nov 2013 12:25:04 +0100
From:	Jiri Benc <jbenc@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	pablo@...filter.org, jhs@...atatu.com, tgraf@...g.ch,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] netlink: fix netlink_ack with large messages

On Tue, 12 Nov 2013 14:35:15 -0500 (EST), David Miller wrote:
> > Do you think capping at NLMSG_GOODSIZE would be too low? The allocation
> > won't fit into one page with NLMSG_GOODSIZE but I doubt we can go lower
> > than that. Alternatively, we can do some math to fully use the two
> > pages, like
> > 	NLMSG_GOODSIZE + min(PAGE_SIZE, 8192UL) - NLMSG_HDRLEN - NLMSG_ALIGN(sizeof(struct nlmsgerr))
> > (which I'm not sure is worth it).
> 
> I don't think this is the way to go.
> 
> I think since existing apps expect the whole message, we have to
> provide it.
> 
> We should add a new socket option so that applications can ask that
> messages not be quoted in ACKs, as I've stated a few times already in
> this thread.

I completely agree with this, sorry for not being clear. I just
understood from the thread that the way to go is to do both, in order
to not generate too large ACKs for the _new_ code (i.e. for the
messages that were not plausible before "netlink: allow large data
transfers from user-space"). I don't know what the "too large" should
be, though, hence the question.

But then, if we don't do any capping, the only outcome of a failed
allocation is the ACK won't be sent and it's clearly stated that
netlink does not provide reliability. Works for me.

 Jiri

-- 
Jiri Benc
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