Re: NFSD automatically releases all states when underlying file system is unmounted

[Dai Ngo <dai.ngo@oracle.com>]

On 3/19/25 2:46 PM, NeilBrown wrote:
> On Thu, 20 Mar 2025, Dai Ngo wrote:
>> Hi,
>>
>> Currently when the local file system needs to be unmounted for maintenance
>> the admin needs to make sure all the NFS clients have stopped using any files
>> on the NFS shares before the umount(8) can succeed.
> This is easily achieved with
>    echo /path/to/filesystem > /proc/fs/nfsd/unlock_filesystem
>
> Do this after unexporting and before unmounting.

Yes, this works!

>
> All state for NFSv4 exports, and all NLM locks for NFSv2/3 exports, will
> be invalidated and files closed.  NFSv4 clients will get
> NFS4ERR_ADMIN_REVOKED when they attempt to use any state that was on
> that filesystem.

In my test, client gets NFS4ERR_STALE for the PUTFH in the GETATTR compound
which is expected.

>
> (I don't think this flushes the NFSv3 file cache, so a short delay might
>   be needed before the unmount when v3 is used.  That should be fixed)

Thank you very much Neil!

-Dai

>
> NeilBrown
>
>
>> In an environment where there are thousands of clients this manual process
>> seems almost impossible or impractical. The only option available now is to
>> restart the NFS server which would works since the NFS client can recover its
>> state but it seems like this is a big hammer approach.
>>
>> Ideally, when the umount command is run there is a callback from the VFS layer
>> to notify the upper protocols; NFS and SMB, to release its states on this file
>> system for the umount to complete.
>>
>> Is there any existing mechanism to allow NFSD to release its states automatically
>> on unmount?
>>
>> Unmount is not a frequent operation. Is it justifiable to add a bunch of complex
>> code for something is not frequently needed?
>>
>> I appreciate any opinions on this issue.
>>
>> Thanks,
>> -Dai
>>
>>
>>
>>
>>     
>>
>>