mbuf: provide new type for mbuf manipulation - mbuf chain

[glebius]

Summary

It tracks both the first mbuf and last mbuf, making it handy to use inside
functions that are interested in both. It also tracks length of data and
memory usage. It can be allocated on stack and passed to an mbuf
allocation or another mbuf manipulation function. It can be embedded into
some kernel facility internal structure representing most simple data
buffer. It uses modern queue(3) based linkage, but is also compatible with
old style m_next linkage. Transitioning older code to new type can be done
gradually - a code that doesn't understand the chain yet, can be supplied
with STAILQ_FIRST(&mc.mc_q). So you can have a mix of old style and new
style code in one function as a temporary solution.

[markj]

sys/sys/mbuf.h

1698

I know this is painful to change at this point, but I don't like the mc_ prefix. It seems too terse. mchain_ is only four characters longer, and is consistent with mbufq_.

1730

Assert that the chain is empty?

[glebius]

sys/sys/mbuf.h

1698

I was thinking of mc_foo() functions to be substitute for m_foo() functions in many places, with local variable named mc instead if two variables named m and tail, seeing mchain as an alternative to mbuf itself, rather than to mbufq. Also, to be fully consistent with mbufq the prefix should be mbufc_.

Let me work on other things you noticed and let's come back to that later.

[glebius]

sys/sys/mbuf.h

1730

Why? The supposed usage of this KPI is that mc has just been allocated on stack. A prologue of a function that makes use of mchain, but was called by old code:

foo(struct mbuf *m)
{
      struct mchain mc;

      mc_init_m(&mc, m);

[markj]

sys/sys/mbuf.h

1698

Ok. I think if the mbuf_* KPIs were added today, they'd be more likely to have an mbuf_ prefix rather than m_. Though, now I also see some mb_ functions (e.g., mb_dupcl()) and I wonder why the prefix is different. Anyway, I will not push further on the name.

1730

I see now, ok.

[glebius]

Provide mc_first(), mc_last(), mc_empty(), mc_freem().