Hello NFS list, I'd like to improve the NFS re-export feature, especially wrt. crossmounts. Currently a NFS client will face EIO when crossing a mount point on the re-exporting server. This was discussed here[0]. While in that discussion the assumption was that check_export() in fs/nfsd/export.c emits EIO I did further experiments and realized that EIO actually comes from the NFS client side of the re-exporting server. nfs_encode_fh() in fs/nfs/export.c checks for IS_AUTOMOUNT(inode), if this is the case it refuses to create a new file handle. So while accessing /files/disk2 directly on the re-exporting server triggers an automount, accessing via nfsd the export function of the client side gives up. AFAIU the suggested proxy-only-mode[1] will not address this problem, right? One workaround is manually adding an export for each volume on the re-exporting server. This kinda works but is tedious and error prone. I have a crazy idea how to automate this: Since nfs_encode_fh() in the NFS client side of the re-exporting server can detect crossing mounts, we could install a new export on the sever side as soon the IS_AUTOMOUNT(inode) case arises. We could even use the same fsid. What do you think? Another obstacle is file handle wrapping. When re-exporting, the NFS client side adds inode and file information to each file handle, the server side also adds information. In my test setup this enlarges a 16 bytes file handle to 40 bytes. The proxy-only-mode won't help us either here. Did you consider using the opaque file handle from the server as lookup key in a (persisted) data structure? That way at least the client side of the re-exporting server no longer has to enlarge the file handle with inode and file type information. If the re-exporting server re-exports just one server (proxy-only-mode) we could also skip adding the fsid to the handle. What do you think? I'm looking forward to hear your comments. Thanks, //richard [0] https://marc.info/?l=linux-nfs&m=161670807413876&w=2 [1] https://linux-nfs.org/wiki/index.php/NFS_proxy-only_mode
