LinuxKPI: mm.h: implement virt_to_head_page()

[bz]

Summary

Implement virt_to_head_page(). We will have to improve this if we
start to allocate sub-page fragments.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

[markj]

This seems ok, but I don't understand the reference to sub-page fragments. The purpose of this function is to return the "head" of a "compound" page. A compound page is an aligned, power-of-2-sized run of contiguous physical pages, and the head page is the first page of that run. So for instance the first 4KB page of a superpage. We don't maintain any notion of compound pages in the LinuxKPI, so your implementation is right IMO, but I can't see the connection to page_frag_cache etc.

sys/compat/linuxkpi/common/include/linux/mm.h

162

Or just return (virt_to_page(p));

[bz]

Address comments from @markj