On Mon, May 27, 2024 at 09:29:48AM -0700, hch@xxxxxxxxxxxxx wrote: > On Mon, May 27, 2024 at 03:38:40PM +0000, Trond Myklebust wrote: > > > It > > > does not matter what mount you use to access it. > > > > Sure. However if you are providing a path argument, then presumably you > > need to know which file system (aka super_block) it eventually resolves > > to. > > Except that you can't, at least not without running into potential > races. The only way to fix a race vs unmount/remount is to include > the fsid part in the kernel generated file handle. > > > > > If your use case isn't NFS servers, then what use case are you > > targeting, and how do you expect those applications to use this API? > > The main user of the open by handle syscalls seems to be fanotify > magic. It's also used by userspace for uniquely identifying cgroups via handles as cgroups and - even without open_by_handle_at() - to check whether a file is still valid. And again a 64bit mount is is a simple way to race-free go to whatever superblock uuid you want. They cannot be recycled and are unique for the lifetime of the system.
