On Sat, 16 Dec 2023, David Laight wrote: > ... > > > PS: put it that way - I can buy "nfsd is doing that only to regular > > > files and not on an arbitrary filesystem, at that; having the thread > > > wait on that sucker is not going to cause too much trouble"; I do *not* > > > buy turning it into a thing usable outside of a very narrow set of > > > circumstances. > > > > > > > Can you say more about "not on an arbitrary filesystem" ? > > I guess you means that procfs and/or sysfs might be problematic as may > > similar virtual filesystems (nfsd maybe). > > Can nfs export an ext4 fs that is on a loopback mount on a file > that is remotely nfs (or other) mounted? Sure. There is no reason this would cause a problem. If the nfs mount were also a loopback mount, that might be interesting. i.e. You have a local filesystem, containing a file with a filesystem image. You nfs-export that local filesystem, and nfs mount it on the same host. Then you loop-mount that file in the nfs-mounted filesystem. Now you are getting into uncharted waters. I've testing loop-back NFS mounting and assure that it works. I haven't tried the double-loop. But if that caused problem, I though it would be fput. It would be fsync or writeback which causes the problem. > > As soon as you get loops like that you might find that fput() starts > being problematic. When calling fput on a regular file there are, I think, only two problem areas. One is that the fput might lead to a lazy-filesystem unmount completing. That only applies to MNT_INTERNAL filesystems, and they are unlikely to be exported (probably it's impossible, but I haven't checked). The other is synchronous (or even async) IO in the filesystem code, maybe completing an unlink or a truncate. This is no worse than any other synchronous IO that nfsd does. Thanks, NeilBrown > > I'm also sure I remember that nfs wasn't supposed to respond to a write > until it had issued the actual disk write - but maybe no one do that > any more because it really is too slow. > (Especially if the 'disk' is a USB stick.) > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
