On Tue, Dec 12, 2023 at 10:35:40AM +0100, Christian Brauner wrote: > On Tue, Dec 12, 2023 at 10:28:12AM +0100, Miklos Szeredi wrote: > > On Tue, 12 Dec 2023 at 10:23, Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > > > > On Tue, Dec 12, 2023 at 09:10:47AM +0000, David Howells wrote: > > > > Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > > > > > > > > > > I suggest: > > > > > > > > > > > > > > > > STATX_ATTR_INUM_NOT_UNIQUE - it is possible that two files have the > > > > > > > > same inode number > > > > > > > > > > This is just ugly with questionable value. A constant reminder of how > > > > > broken this is. Exposing the subvolume id also makes this somewhat redundant. > > > > > > > > There is a upcoming potential problem where even the 64-bit field I placed in > > > > statx() may be insufficient. The Auristor AFS server, for example, has a > > > > 96-bit vnode ID, but I can't properly represent this in stx_ino. Currently, I > > > > > > Is that vnode ID akin to a volume? Because if so you could just > > > piggy-back on a subvolume id field in statx() and expose it there. > > > > And how would exporting a subvolume ID and expecting userspace to take > > that into account when checking for hard links be meaningfully > > different than expecting userspace to retrieve the file handle and > > compare that? > > > > The latter would at least be a generic solution, including stacking fs > > inodes, instead of tackling just a specific corner of the problem > > space. > > So taking a step back here, please. The original motivation for this > discussion was restricted to handle btrfs - and now bcachefs as well. > Both have a concept of a subvolume so it made sense to go that route. > IOW, it wasn't originally a generic problem or pitched as such. > > Would overlayfs be able to utilize an extended inode field as well? No, the original motivation was not just btrfs and bcachefs; overlayfs fundamentally needs to export a bigger identifier than the host filesystems - pigeonhole principle, if anyone remembers their combinatorics. This applies to any filesystem which is under the hood reexporting from other filesystems; there's a lot of stuff going on in container filesystems right now, and I expect it'll come up there (you can create new identifiers if you're exporting file by file, but not if it's directory trees). And Neal brought up NFS re-exports; I think there's a compelling argument to be made that we ought to be able to round trip NFS filehandles.
