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

There are a number of kernel filesystems that store their directory entriesdirectly in the directory-entry cache (dcache) without having any permanentstorage for those objects. It started out as a "neat hack" for ramfs,Al Viro said, at the start of his filesystem-track session atthe2024 Linux Storage,Filesystem, Memory Management, and BPF Summit. Unfortunately, as the useof this technique has grown into other filesystems, there has been a lot ofscope creep that has gotten out of control. He wanted to discuss some newinfrastructure that he is working on to try to clean some of that up.