more generally make it easier to extend 'struct mbuf in the future', make
a number of changes to the data structure:
- As we anticipate embedding mbufs headers within variable-size regions of
memory in the future, change the definitions of byte arrays embedded in
mbufs to be of size [0] rather than [MLEN] and [MHLEN]. In fact, the
cxgbe driver already uses 'struct mbuf' on the front of other storage
sizes, but we would like the global mbuf allocator do be able to do this
as well.
- Fold 'struct m_hdr' into 'struct mbuf' itself, eliminating a set of
macros that aliased 'mh_foo' field names to 'm_foo' names such as
'm_next'. These present a particular problem as we would like to add
new mbuf-header fields -- e.g., 'm_size' -- that, if similarly named via
macros, would introduce collisions with many other variable names in the
kernel.
- Rename 'struct m_ext' to 'struct struct_m_ext' so that we can add
compile-time assertions without bumping into the still-extant 'm_ext'
macro.
- Remove the MSIZE compile-time assertion for 'struct mbuf', but add new
assertions for alignment of embedded data arrays (64-bit alignment even
on 32-bit platforms), and for the sizes the mbuf header, packet header,
and m_ext structure.
- Document that these assertions exist in comments in mbuf.h.
This change is not intended to cause (non-trivial) behavioural
differences, but is a precursor to further mbuf-allocator work.
Differential Revision: https://reviews.freebsd.org/D1483
Reviewed by: bz, gnn, np, glebius ("go ahead, I trust you")
Sponsored by: EMC / Isilon Storage Division
08b6ceecbb