netdev - Re: [PATCH net-next 2/2] docs: netlink: basic introduction to Netlink

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Message-ID: <84601312213cfd8568be221038c94ab72c9eeccc.camel@sipsolutions.net>
Date:   Fri, 19 Aug 2022 21:20:30 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, corbet@....net,
        stephen@...workplumber.org, sdf@...gle.com, ecree.xilinx@...il.com,
        benjamin.poirier@...il.com, idosch@...sch.org,
        f.fainelli@...il.com, jiri@...nulli.us, dsahern@...nel.org,
        fw@...len.de, linux-doc@...r.kernel.org, jhs@...atatu.com,
        tgraf@...g.ch, jacob.e.keller@...el.com, svinota.saveliev@...il.com
Subject: Re: [PATCH net-next 2/2] docs: netlink: basic introduction to
 Netlink


On Fri, 2022-08-19 at 12:16 -0700, Jakub Kicinski wrote:
> On Fri, 19 Aug 2022 10:54:51 -0700 Jakub Kicinski wrote:
> > > Ugh, I repressed all those memories ... I don't remember now, I guess
> > > I'd have to try it. Also it doesn't just apply to normal stuff but also
> > > multicast, and that can be even trickier.  
> > 
> > No worries, let me try myself. Annoyingly I have this doc on a different
> > branch than my netlink code, that's why I was being lazy :)
> 
> Buffer sizing
> -------------
> 
> Netlink sockets are datagram sockets rather than stream sockets,
> meaning that each message must be received in its entirety by a single
> recv()/recvmsg() system call. If the buffer provided by the user is too
> short, the message will be truncated and the ``MSG_TRUNC`` flag set
> in struct msghdr (struct msghdr is the second argument
> of the recvmsg() system call, *not* a Netlink header).
> 
> Upon truncation the remaining part of the message is discarded.
> 
> Netlink expects that the user buffer will be at least 8kB
> 

I guess technically 8 KiB ;-)

>  or a page
> size of the CPU architecture, whichever is bigger. Particular Netlink
> families may, however, require a larger buffer. 32kB buffer is recommended
> for most efficient handling of dumps (larger buffer fits more dumped
> objects and therefore fewer recvmsg() calls are needed).

Seems reasonable, thanks :)

Honestly most of our problems came from ever-growing message sizes, and
userspace having defaulted to 4k buffers ... annoyingly. Even 8k may not
always be enough for future - so for the kernel guide maybe say we
should mostly not even have GET operations but have a way to restrict
DUMP operations to a certain (set of) object(s), and have the ability to
split objects in the middle when they have a lot of properties ...

johannes