Created attachment 2156010 [details]
tcpdump from server side, vers3=n, reads/writes hang

Description of problem:
Disabling NFSv3 (vers3=n) on the server causes reads and writes on the client to hang in uninterruptible (D+) state.  Re-enabling NFSv3 (vers3=y) and restarting nfs-server will un-hang the client and the read/write will then succeed.

Version-Release number of selected component (if applicable):
Kernel 7.2.1-300.fc45.x86_64
nfs-utils 2.9.2-1.rc1.fc45

How reproducible:
Every time.

Steps to Reproduce:
1.  Install a minimal Fedora 45 into two VMs on the same network; disable firewalld on both to ensure no firewall configuration issues.
2.  Start nfs-server on one of them, and export a filesystem.
3.  On the other VM, mount the filesystem (default mount options).  At this point, reads and writes work fine.
4.  Now on the server, edit /etc/nfs.conf and add/change vers3=n to disable NFSv3.  Restart nfs-server.
5.  Attempt reads/writes on the client; they will hang in D+ state.
6.  Change vers3 back to y on the server to re-enable NFSv3.
7.  The client reads and writes will now resume.  Usually instantly, but I've seen it take some seconds.  At this point all reads/writes work as normal again.

Actual results:
Reads/writes hang in D+ when NFSv3 is disabled.

Expected results:
Reads/writes succeed when NFSv3 is disabled.

Additional info:
I've tested it with both btrfs and ext4 as the underlying filesystem, with no change in behavior.