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.) The initial file systems that had case-insensitivty implemented was ext4 and f2fs. The initial use cases was Android devices (which had used this horible wrapfs stacking file system thing which was trivial to deadlock under stress, and its original reason for existing was bug-for-bug compatibility with FAT), and for Steam so that Windows games could have their expected case insensitivity. (Collabora's work was underwritten by Steam.) 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 Anyway, case sensitivity is one of those "interesting" problems which has caused many headaches, including a potential security issue, and a botched attempt to fix that security issue interacting poorly with some of the more subtle design requirements so that file systems can use tree-indexed directory lookups, even with case-insensitivty file names and encrypted directory entries. So in general, unless you have strong financial backing where someone is willing to pay $$$ to address a business-critical use case, my personal advice is to stay far, far, away. And I say this as a someone (with apologies to Linus) who was partially responsible for Linux having case insensitivty lookups in the first place. :-) Cheers, - Ted
