Negative dentries, 20 years later

Controlling the size of dentry cache without memory pressure can help us in one of the problems we face with virtiofs. As dentry cache is growing, it keeps inodes around, that means it keeps corresponding data structures around in virtiofs daemon. And it keeps O_PATH fd per inode on server. And soon we run out of number of open fd a process can have. If guest has good amount of RAM, it can cache lot of dentries/inodes without memory pressure and create problems for virtiofs daemon. Some notion/knob of controlling the size of dentry cache without memory pressure can possibly.