From: Josh Hunt <redacted>
Date: Mon, 13 Mar 2017 18:34:41 -0500

In this particular case they really do want to know total # of bytes
in the receive queue, not the data bytes they can consume from an
application pov. The kernel currently only exposes this value through
netlink or /proc/net/udp from what I saw.

Can you explain in what way this is useful?

The difference between skb->len and skb->truesize is really kernel
internal implementation detail, and I'm trying to figure out why
this would be useful to an application.