[$] ID-mapped mounts

The ID-mapped mounts feature was added toLinux in 5.12, but the general idea behind it goes back a fair bitfurther. There are a number of different situations where the user andgroup IDs for files on disk do not match the current human (or process) user of thosefiles, so ID-mapped mounts provide a way to resolve that problem-withoutchanging the files on disk. The developer of the feature, ChristianBrauner, led a discussion at the 2022 Linux Storage,Filesystem, Memory-management and BPF Summit (LSFMM) on ID-mapped mounts.