mbuf: Add M_WRITABLE_EXTPG

[jhb]

Summary

This can be used instead of M_WRITABLE where the calling code is
prepared to handle writing to M_EXTPG mbufs.

[gallatin]

I'm confused.. if we are marking non-writable M_EXTPG mufs as M_RDONLY, why can't we simply remove the M_EXTPG check from M_WRITABLE? Why do we need a new macro?

[jhb]

In D46787#1066743, @gallatin wrote:

I'm confused.. if we are marking non-writable M_EXTPG mufs as M_RDONLY, why can't we simply remove the M_EXTPG check from M_WRITABLE? Why do we need a new macro?

Because there are many places that call M_WRITABLE in the tree and if it succeeds expect to directly use mtod() to get a writable pointer to the data payload (possibly after using m_pullup which panics if called on an M_EXTPG mbuf). m_copyback() is explicitly aware of M_EXTPG mbufs and handles them correctly, so the new macro works fine the assertion there.

[gallatin]

Ah, OK, I understand now.