Re: [PATCH 1/3] xfs: stabilize the tolower function used for ascii-ci dir hash computation

[Christoph Hellwig <hch@infradead.org>]

On Tue, Apr 04, 2023 at 10:07:06AM -0700, Darrick J. Wong wrote:
> Which means that the kernel and userspace do not agree on the hash value
> for a directory filename that contains those higher values.  The hash
> values are written into the leaf index block of directories that are
> larger than two blocks in size, which means that xfs_repair will flag
> these directories as having corrupted hash indexes and rewrite the index
> with hash values that the kernel now will not recognize.
> 
> Because the ascii-ci feature is not frequently enabled and the kernel
> touches filesystems far more frequently than xfs_repair does, fix this
> by encoding the kernel's toupper predicate and tolower functions into
> libxfs.  This makes userspace's behavior consistent with the kernel.

I agree with making the userspace behavior consistent with the actual
kernel behavior.  Sadly the documented behavior differs from both
of them, so I think we need to also document the actual tables used
in the mkfs.xfs manpage, as it isn't actually just ASCII.

Does the kernel twolower behavior map to an actual documented charset?
In that case we can just point to it, which would be way either than
documenting all the details.