From Bill Irwin

Tiny hugetlbpage ram-backed filesystem.

Some way to export hugetlbfs through more standard system call
interfaces was needed, and hugetlbfs already had inodes with ratnodes
etc.  used to track offset -> page translations, so adding the rest of
a filesystem around it was easy and natural.  Most of it is identical
to ramfs, except ->f_op->mmap() is now just a wrapper around the
hugetlb_prefault() to fill in the VMA, and to simplify it,
->readpage(), ->prepare_write(), and ->commit_write() are omitted.

Permissions:

(1) check capable(CAP_IPC_LOCK) in ->f_ops->mmap
        This may be redundant but it errors out with less state to
        clean up and at least clarifies the fact that checks are
        being performed at the relevant entry points.

(2) check capable(CAP_IPC_LOCK) in hugetlbfs_zero_setup()
        This is called at shmget() time and is an actual potential
        security hole. hugetlb_prefault() does not perform this
        check itself, so it must be done here.