NFS hang during nfstest_interop LOCALIO test

[tonyhutter]

System information

Type	Version/Name
Distribution Name	Fedora
Distribution Version	40
Kernel Version	6.13.4-100.fc40.x86_64
Architecture	x86_64
OpenZFS Version	master (f65fc98)

Describe the problem you're observing

I've been running the nfstest suite against ZFS+NFS on Fedora 40. I've noticed that "test23" hangs in the nfstest_delegation test suite when run against a ZFS pool exported though NFS. It does not hang if I run the same test against a NFS-exported btrfs dir.

I'm running the test with both the client and the server on the same VM:

nfstest_delegation --createlog --client <my IP> --server <my IP> --export /

When the test hangs, I see a nfslocaliod kernel thread running at 100% CPU. I assume this has to do with NFS LOCALIO since both the client and server are running on the same VM. LOCALIO is a relatively new NFS feature that arrived in the 6.12 kernel.

I have not yet tried running "test23" using a non-local client or server.

Describe how to reproduce the problem

nfstest_delegation --createlog --client <my IP> --server <my IP> --export /

Include any warning/errors/backtraces from the system logs

The stack was blank for the nfslocaliod kthread. The userspace test had this stack:

[<0>] folio_wait_bit_common+0x12a/0x310
[<0>] folio_wait_writeback+0x2b/0x80
[<0>] __filemap_fdatawait_range+0x86/0xf0
[<0>] filemap_write_and_wait_range+0x85/0xb0
[<0>] nfs_wb_all+0x25/0x120 [nfs]
[<0>] nfs4_file_flush+0x6d/0x90 [nfsv4]
[<0>] filp_flush+0x3a/0x70
[<0>] __x64_sys_close+0x32/0x80
[<0>] do_syscall_64+0x82/0x160
[<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

[tonyhutter]

These nfstest_interop test all seem to fail with the same hang: test23,test24,test26,test27

    test23:
        Verify appending data with NFSv4 and NFSv4.1 is correctly read using NFSv4

    test24:
        Verify appending data with NFSv4 and NFSv4.1 is correctly read using NFSv4.1
...
    test26:
        Verify appending data with NFSv4.1 and NFSv4 is correctly read using NFSv4

    test27:
        Verify appending data with NFSv4.1 and NFSv4 is correctly read using NFSv4.1