[$] VFS write barriers

In the filesystem track at the 2025 Linux Storage, Filesystem, MemoryManagement, and BPF Summit (LSFMM+BPF), Amir Goldstein wanted to resumediscussinga feature that he had briefly introduced at the end of a 2023 summit session: filesystem "writebarriers". The idea is to have an operation that would wait for anyin-flight write()system calls, but not block any new write() calls as biggerhammers, such as freezing the filesystem,would do. His prototype implementation is used by a hierarchicalstorage management (HSM) system to create a crash-consistentchange log, but there may be other use cases to consider. He wantedto discuss implementation options and the possibility of providing anAPI for user-space applications.