Re: [PATCH 2/3] fat: add the msdos_format_name() filename cache

[Al Viro <viro@zeniv.linux.org.uk>]

On Sun, Aug 29, 2021 at 02:25:29PM +0000, Caleb D.S. Brzezinski wrote:
> Implement the main msdos_format_name() filename cache. If used as a
> module, all memory allocated for the cache is freed when the module is
> de-registered.
> 
> Signed-off-by: Caleb D.S. Brzezinski <calebdsb@protonmail.com>
> ---
>  fs/fat/namei_msdos.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/fs/fat/namei_msdos.c b/fs/fat/namei_msdos.c
> index 7561674b1..f9d4f63c3 100644
> --- a/fs/fat/namei_msdos.c
> +++ b/fs/fat/namei_msdos.c
> @@ -124,6 +124,16 @@ static int msdos_format_name(const unsigned char *name, int len,
>  	unsigned char *walk;
>  	unsigned char c;
>  	int space;
> +	u64 hash;
> +	struct msdos_name_node *node;
> +
> +	/* check if the name is already in the cache */
> +
> +	hash = msdos_fname_hash(name);
> +	if (find_fname_in_cache(res, hash))
> +		return 0;

Huh?  How could that possibly work, seeing that
	* your hash function only looks at the first 8 characters
	* your find_fname_in_cache() assumes that hash collisions
are impossible, which is... unlikely, considering the nature of
that hash function
	* find_fname_in_cache(res, hash) copies at most 8 characters
into res in case of match.  Where does the extension come from?

Out of curiosity, how have you tested that thing?