netdev - Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if size is not enough

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Message-ID: <20171026153340.GX32305@orbyte.nwl.cc>
Date:   Thu, 26 Oct 2017 17:33:40 +0200
From:   Phil Sutter <phil@....cc>
To:     David Ahern <dsahern@...il.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff
 if size is not enough

On Thu, Oct 26, 2017 at 09:28:00AM -0600, David Ahern wrote:
> On 10/26/17 4:24 AM, Stephen Hemminger wrote:
> >>
> >> The kernel needs a flag that says "give me the message of the buffer is
> >> large enough; if not just PEEK and tell me the length." That would avoid
> >> the double call in most cases.
> > 
> > Actually this has little impact because old code was doing implicit zero
> > of whole buffer, new code does not.
> > 
> 
> The patch calls recvmsg twice; libnl does the same thing. It would be
> better performance wise to have a flag that allows retrieval of the
> message if the supplied buffer is large enough and PEEK semantics if
> not. It was really a comment on how we could do better with proper
> kernel support.

Doesn't MSG_TRUNC without MSG_PEEK do just that?

Cheers, Phil