[$] A new API for tree-in-dcache filesystems

There are a number of kernel filesystems that store their directory entries
directly in the directory-entry cache (dcache) without having any permanent
storage for those objects. It started out as a 'neat hack' for ramfs,
Al Viro said, at the start of his filesystem-track session at
the
2024 Linux Storage,
Filesystem, Memory Management, and BPF Summit. Unfortunately, as the use
of this technique has grown into other filesystems, there has been a lot of
scope creep that has gotten out of control. He wanted to discuss some new
infrastructure that he is working on to try to clean some of that up.