Re: NFSv4.x export options to mark export as case-insensitive, case-preserving? Re: LInux NFSv4.1 client and server- case insensitive filesystems supported?

Chuck Lever
2025-09-10T14:14:22Z

On 9/10/25 7:10 AM, Jeff Layton wrote:
> On Tue, 2025-09-09 at 18:06 +0200, Cedric Blancher wrote:
>> On Tue, 10 Jun 2025 at 07:34, Christoph Hellwig <hch@infradead.org> wrote:
>>>
>>> On Mon, Jun 09, 2025 at 10:16:24AM -0400, Chuck Lever wrote:
>>>>> Date: Wed May 21 16:50:46 2008 +1000
>>>>>
>>>>> dcache: Add case-insensitive support d_ci_add() routine
>>>>
>>>> My memory must be quite faulty then. I remember there being significant
>>>> controversy at the Park City LSF around some patches adding support for
>>>> case insensitivity. But so be it -- I must not have paid terribly close
>>>> attention due to lack of oxygen.
>>>
>>> Well, that is when the ext4 CI code landed, which added the unicode
>>> normalization, and with that another whole bunch of issues.
>>
>> Well, no one likes the Han unification, and the mess the Unicode
>> consortium made from that,
>> But the Chinese are working on a replacement standard for Unicode, so
>> that will be a lot of FUN =:-)
>>
>>>>> That being said no one ever intended any of these to be exported over
>>>>> NFS, and I also question the sanity of anyone wanting to use case
>>>>> insensitive file systems over NFS.
>>>>
>>>> My sense is that case insensitivity for NFS exports is for Windows-based
>>>> clients
>>>
>>> I still question the sanity of anyone using a Windows NFS client in
>>> general, but even more so on a case insensitive file system :)
>>
>> Well, if you want one and the same homedir on both Linux and Windows,
>> then you have the option between the SMB/CIFS and the Windows NFSv4.2
>> driver (I'm not counting the Windows NFSv3 driver due lack of ACL
>> support).
>> Both, as of September 2025, work fine for us for production usage.
>>
>>>> Does it, for example, make sense for NFSD to query the file system
>>>> on its case sensitivity when it prepares an NFSv3 PATHCONF response?
>>>> Or perhaps only for NFSv4, since NFSv4 pretends to have some recognition
>>>> of internationalized file names?
>>>
>>> Linus hates pathconf any anything like it with passion. Altough we
>>> basically got it now with statx by tacking it onto a fast path
>>> interface instead, which he now obviously also hates. But yes, nfsd
>>> not beeing able to query lots of attributes, including actual important
>>> ones is largely due to the lack of proper VFS interfaces.
>>
>> What does Linus recommend as an alternative to pathconf()?
>>
>> Also, AGAIN the question:
>> Due lack of a VFS interface and the urgend use case of needing to
>> export a case-insensitive filesystem via NFSv4.x, could we please get
>> two /etc/exports options, one setting the case-insensitive boolean
>> (true, false, get-default-from-fs) and one for case-preserving (true,
>> false, get-default-from-fs)?
>>
>> So far LInux nfsd does the WRONG thing here, and exports even
>> case-insensitive filesystems as case-sensitive. The Windows NFSv4.1
>> server does it correctly.
>>
>> Ced
>
> I think you don't want an export option for this.
>
> It sounds like what we really need is a mechanism to determine whether
> the inode the client is doing a GETATTR against lies on a case-
> insensitive mount.
>
> Is there a way to detect that in the kernel?

Agreed, I would prefer something automatic rather than an explicit
export option. The best approach is to set this behavior on the
underlying file system via its mount options or on-disk settings.
That way, remote and local users see the exact same CS behavior.

An export option would enable NFSD to lie about case sensitivity.
Maybe that's what is needed? I don't really know. It seems like
a potential interoperability disaster.

Moreover, as we determined the last time this thread was active,
ext4 has a per-directory case insensitivity setting. The NFS
protocol's CS attribute is per file system. That's a giant mismatch
in semantics, and I don't know what to do about that. An export
option would basically override all of that -- as a hack -- but
would get us moving forward. Again, perhaps there are some
significant risks to this approach.

--
Chuck Lever