On Wed, 21 Feb 2024 at 22:08, Josef Bacik <[email protected]> wrote: > > On Wed, Feb 21, 2024 at 04:06:34PM +0100, Miklos Szeredi wrote: > > On Wed, 21 Feb 2024 at 01:51, Kent Overstreet <[email protected]> > > wrote: > > > > > > Recently we had a pretty long discussion on statx extensions, which > > > eventually got a bit offtopic but nevertheless hashed out all the major > > > issues. > > > > > > To summarize: > > > - guaranteeing inode number uniqueness is becoming increasingly > > > infeasible, we need a bit to tell userspace "inode number is not > > > unique, use filehandle instead" > > > > This is a tough one. POSIX says "The st_ino and st_dev fields taken > > together uniquely identify the file within the system." > > > > Which is what btrfs has done forever, and we've gotten yelled at forever for > doing it. We have a compromise and a way forward, but it's not a widely held > view that changing st_dev to give uniqueness is an acceptable solution. It > may > have been for overlayfs because you guys are already doing something special, > but it's not an option that is afforded the rest of us.
Overlayfs tries hard not to use st_dev to give uniqueness and instead partitions the 64bit st_ino space within the same st_dev. There are various fallback cases, some involve switching st_dev and some using non-persistent st_ino. What overlayfs does may or may not be applicable to btrfs/bcachefs, but that's not my point. My point is that adding a flag to statx does not solve anything. You can't just say that from now on btrfs doesn't have use unique st_ino/st_dev because we've just indicated that in statx and everything is fine. That will trigger the no-regressions rule and then it's game over. At least I would expect that to happen. What we can do instead is introduce a new API that is better, and thankfully we already have one in the form of file handles. The problem I see is that you think you can get away with then reverting back st_dev to be uniform across subvolumes. But you can't. I see two options: a) do some hacks, like overlayfs does b) introduce a new "st_dev_v2" that will do the right thing and applications can move over. Thanks, Miklos
