Re: [PATCH] docs: netlink: Correct buffer sizing info

[Konstantin Shabanov <mail@etehtsea.me>]

On Tue, 12 May 2026 17:27:57 -0700 Jakub Kicinski wrote:
> On Tue, 12 May 2026 17:30:53 +0700 Konstantin Shabanov wrote:
> > Update the docs to match the code (include/linux/netlink.h):
> >
> >   /*
> >    *	skb should fit one page. This choice is good for headerless malloc.
> >    *	But we should limit to 8K so that userspace does not have to
> >    *	use enormous buffer sizes on recvmsg() calls just to avoid
> >    *	MSG_TRUNC when PAGE_SIZE is very large.
> >   */
> >   #if PAGE_SIZE < 8192UL
> >   #define NLMSG_GOODSIZE	SKB_WITH_OVERHEAD(PAGE_SIZE)
> >   #else
> >   #define NLMSG_GOODSIZE	SKB_WITH_OVERHEAD(8192UL)
> >   #endif
>
> You should explain what you think the problem is in the commit message.
> Maybe if you did you'd realize you're comparing kernel header comment
> to user space guidance which are (obviously?) the inverse of each
> other..

I thought that the comment is self-explaining:

  * But we should limit to 8K so that userspace does not have to
  *	use enormous buffer sizes on recvmsg() calls just to avoid
  *	MSG_TRUNC when PAGE_SIZE is very large.

The problem is that according to the comment, kernel isn't going to send
more than 8K in a single reply and the documentation is currently recommends the opposite:
to create _at least_ 8K buffer what looks excessive.
Also, the logic in the comment is aligned with userspace libraries (libmnl [1]
and wireguard-tools [2]).

[1]: https://git.netfilter.org/libmnl/tree/include/libmnl/libmnl.h?id=54dea548d796653534645c6e3c8577eaf7d77411#n20
[2]: https://git.zx2c4.com/wireguard-tools/tree/src/netlink.h?id=a998407747005ea7e4e0258d96f105c97241e1d3#n70