[OpenAFS] AFS-Cache hickups with Ubuntu and Kernel 6.17

[Gaja Sophie Peters]

TLDR: OpenAFS on Ubuntu with kernel 6.17 is in certain circumstances not 
updating the Cache on file-deletion or file-creation happening on a 
different machine, the only fix is: echo 3 >/proc/sys/vm/drop_caches


Long version: We discovered a problem with the recent advent of the 
kernel 6.17 as the newest "hardware enablement kernel" in Ubuntu 24.04. 
Basically the existence or non-existence of a file is not correctly 
reflected by the AFS-Cache, when a file is deleted or created on another 
machine after the file was seen (or not) on the current machine.

### first test: file deletion not propagated
     #this command on a different machine# echo X >test-afs
     $ cat test-afs
     X
     #this command on the other machine# rm test-afs
     $ cat test-afs
     X
     $ fs flushall
     $ cat test-afs
     cat: test-afs: Input/output error
     #as root# echo 3 >/proc/sys/vm/drop_caches
     $ cat test-afs
     cat: test-afs: No such file or directory
### second test: file creation not propagated
     #this command on a different machine# echo Y >test-afs
     $ cat test-afs
     cat: test-afs: No such file or directory
     $ fs flushall
     $ cat test-afs
     cat: test-afs: No such file or directory
     #as root# echo 3 >/proc/sys/vm/drop_caches
     $ cat test-afs
     Y

We haven't yet tried to pinpoint at which exact kernel-version that 
problem got introduced, but the Ubuntu-kernels 6.8 and 6.14 are 
unaffected, the Ubuntu-kernel 6.17 is affected (kAFS on 6.17 is 
unaffected). It doesn't make a difference, if the OpenAFS-version is 
1.8.14 with the Ubuntu-patches for Kernel 6.17 or if it is OpenAFS 1.8.15

The file server hosting the volume was originally 1.8.13.2-1 from Debian 
Trixie, but updating to 1.8.15 does not change anything, either.

Is this known? Any idea how to deal with it?

Greetings,
Gaja Peters