mbuf: provide mc_uiotomc() a function to copy from uio(9) to mchain

[glebius]

Summary

Implement m_uiotombuf() as a wrapper around mc_uiotomc(). The M_EXTPG is
left untouched. The m_uiotombuf() is left as a compat KPI. New code
should use either mc_uiotomc() or m_uiotombuf_nomap().

[markj]

sys/kern/uipc_mbuf.c

1985

It would be nice to explain what "len" and "lspace" are.

2007

What's the purpose of trying to handle the overflow case? We are silently truncating the uio and creating a 4GB chain, which presumably isn't what the caller wants...?

2020

Can you get rid of progress and just use mc->mc_len?

[glebius]

Review from Mark.

[markj]

sys/kern/uipc_mbuf.c

1998

I'd also assert that uio->uio_segflg == UIO_WRITE.

[glebius]

Utilize mc_first/etc.
Add assertion uio->uio_segflg == UIO_WRITE.

[glebius]

Correct UIO assertion.

[markj]

sys/kern/uipc_mbuf.c

1998

Sorry, I meant uio_rw of course. But why do you assert segflg == UIO_USERSPACE? It should be valid for kernel UIOs as well.

[glebius]

Don't assert uio is from userspace.