netdev - Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for sk_error

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Date:   Thu, 18 Jan 2018 08:53:15 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>, rds-devel@....oracle.com,
        santosh.shilimkar@...cle.com
Subject: Re: [PATCH RFC net-next 1/6] sock: MSG_PEEK support for
 sk_error_queue

On Thu, 2018-01-18 at 11:10 -0500, Sowmini Varadhan wrote:
> On (01/18/18 07:54), Eric Dumazet wrote:
> > 
> > Some applications out there would break horribly, trust me.
> > 
> 
> so I'm not particularly attached to that solution, and I appreciate
> the wisdom (and the NACK), but lets try to find a useful alternative
> 
> The current zcopy completion notification mechanism involves syscall
> overhead already, and is also inadequate for threaded applications sharing
> an fd.  Plus it wont work for datagram sockets.
> 
> I'm fine with Willem's suggestion of passing a fixed number of cookies
> as ancillary data (with CTRUNC to denote inadequate buffer) but if we
> are really so thrifty about syscall overhead, we should not be using
> sk_error_queue in the first place- perhaps we can pass up the completion
> notification as ancillary data with recvmsg() on the POLLIN channel
> itself (which is weird if there is no data to recv, and only ancillary
> info to pass up, but hey, we are "performant"!).

The thing is : MSG_PEEK 'support' will also need SO_PEEK_OFF support.

And begins the crazy stuff.

So lets properly design things, and not re-use legacy stuff that is
proven to be not multi-thread ready and too complex.

If you want to design a new channel of communication, do it, and
maintain it.