[$] VFS write barriers

In the filesystem track at the 2025 Linux Storage, Filesystem, Memory
Management, and BPF Summit (LSFMM+BPF), Amir Goldstein wanted to resume
discussing
a feature that he had briefly introduced at the end of a 2023 summit session: filesystem 'write
barriers'. The idea is to have an operation that would wait for any
in-flight write()
system calls, but not block any new write() calls as bigger
hammers, such as freezing the filesystem,
would do. His prototype implementation is used by a hierarchical
storage management (HSM) system to create a crash-consistent
change log, but there may be other use cases to consider. He wanted
to discuss implementation options and the possibility of providing an
API for user-space applications.