Re: LInux NFSv4.1 client and server- case insensitive filesystems supported?

[Chuck Lever <chuck.lever@oracle.com>]

On 6/7/25 6:39 PM, Theodore Ts'o wrote:
> On Sat, Jun 07, 2025 at 02:30:37PM -0400, Chuck Lever wrote:
>>
>> My impression is that real case-insensitivity has been added to the
>> dentry cache in support of FAT on Android devices (or something like
>> that). That clears the path a bit for NFSD, but it needs to be
>> researched to see if that new support is adequate for NFS to use.
> 
> Case insensitivty was added in Linux in 2019, with the primary coding
> work being done by Gabriel Krisman Bertazi of Collabora, and design
> work being done being done by Gabriel, Michael Halcrow, and myself.
> (Michael Halcrow in particular was responsible for devising how to
> make case-insensitivity work with filename encryption and indexed
> directories.)

For some reason I thought case-insensitivity support was merged more
recently than that. I recall it first appearing as a session at LSF in
Park City, but maybe that one was in 2018.


> There is an interesting write-up about NFS and case-insensitivity in a
> relatively recent Internet-Draft[1], dated 2025-May-16.  In this I-D,
> it points out that one of the primary concerns is that if the client
> caches negative lookups under one case (say, MaDNeSS), and then the
> file is created using a different case (say "madness"), then the
> negative dentry cache indicating that MaDNeSS does not exist needs to
> be removed when "madness" is created.  I'm not sure how Linux's NFS
> client handles negative dentries, since even without
> case-insensitivity, a file name that previously didn't exist could
> have subsequently been created by another client on a different host.
> So does Linux's NFS client simply does not use negative dentries, or
> does it have some kind of cache invalidation scheme when the directory
> has a new mtime, or some such?
> 
> [1] https://www.ietf.org/id/draft-ietf-nfsv4-internationalization-12.html#name-handling-of-string-equivale

nfs(5) describes the lookupcache= mount option. It controls how the
Linux NFS client caches positive and negative lookup results.


-- 
Chuck Lever