vfs_rootmountalloc(9) on FreeBSD
The text 248f43caeb22, as FreeBSD 7.0-RELEASE, FreeBSD 6.3-RELEASE, FreeBSD 6.2-RELEASE, FreeBSD 6.1-RELEASE, FreeBSD 6.0-RELEASE, FreeBSD 5.5-RELEASE, FreeBSD 5.4-RELEASE, FreeBSD 5.3-RELEASE, FreeBSD 5.2.1-RELEASE, FreeBSD 5.2-RELEASE shipped it.
allocate a root mount structure