[Bug 256936] Buggy filesystem detected - message wrongly triggered by FUSE filesystems

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256936

--- Comment #2 from Alan Somers <asomers@FreeBSD.org> ---
If the FUSE daemon tells the kernel that it may cache dirty data, but then the
daemon tells the kernel that the file has changed while it was dirty, then data
has been lost.  Data corruption is a bug.

MooseFS's hands are somewhat tied as it still uses fusefs-libs2.  At that
protocol level there really aren't sufficient operations for the server to
manage the kernel's cache.  But sshfs uses fusefs-libs3, so it ought to be able
to fix the problem.

If it's possible to prove that no data corruption occurred and yet the kernel
still prints this error message, then that would constitute a bug in the
kernel, and I would fix it.  The most obvious way for that to happen is if you
set vfs.fusefs.data_cache_mode=0, completely disabling the cache.  But even if
data_cache_mode=1, it wouldn't surprise me if there are cases where the warning
is printed inappropriately; it's complicated.  Especially because file
attributes can be cached independently of their data.

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