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

[Jakub Kicinski <kuba@kernel.org>]

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 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).