Re: [PATCH v2 2/3] 9p: Introduce option for negative dentry cache retention time

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Wednesday, 21 January 2026 20:56:09 CET Remi Pommarel wrote:
> Add support for a new mount option in v9fs that allows users to specify
> the duration for which negative dentries are retained in the cache. The
> retention time can be set in milliseconds using the ndentrytmo option.
> 
> For the same consistency reasons, this option should only be used in
> exclusive or read-only mount scenarios, aligning with the cache=loose
> usage.
> 
> Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> ---
>  fs/9p/v9fs.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
> index 1da7ab186478..f58a2718e412 100644
> --- a/fs/9p/v9fs.c
> +++ b/fs/9p/v9fs.c
> @@ -39,7 +39,7 @@ enum {
>  	 * source if we rejected it as EINVAL */
>  	Opt_source,
>  	/* Options that take integer arguments */
> -	Opt_debug, Opt_dfltuid, Opt_dfltgid, Opt_afid,
> +	Opt_debug, Opt_dfltuid, Opt_dfltgid, Opt_afid, Opt_ndentrytmo,
>  	/* String options */
>  	Opt_uname, Opt_remotename, Opt_cache, Opt_cachetag,
>  	/* Options that take no arguments */
> @@ -93,6 +93,7 @@ const struct fs_parameter_spec v9fs_param_spec[] = {
>  	fsparam_string	("access",	Opt_access),
>  	fsparam_flag	("posixacl",	Opt_posixacl),
>  	fsparam_u32	("locktimeout",	Opt_locktimeout),
> +	fsparam_s32	("ndentrytmo",	Opt_ndentrytmo),

Not better "ndentrytimeout" ?

My first thought was whether it was really worth introducing a dedicated
timeout option exactly for negative dentries (instead of a general cache
timeout option). But on a 2nd thought it actually needs separate handling, as
negative dentries have the potential to pollute with a ridiculous amount of
bogus entries.

Wouldn't it make sense to enable this option with some meaningful value for
say cache=loose by default? 24 hours maybe?

> 
>  	/* client options */
>  	fsparam_u32	("msize",	Opt_msize),
> @@ -159,6 +160,8 @@ int v9fs_show_options(struct seq_file *m, struct dentry
> *root) from_kgid_munged(&init_user_ns, v9ses->dfltgid));
>  	if (v9ses->afid != ~0)
>  		seq_printf(m, ",afid=%u", v9ses->afid);
> +	if (v9ses->ndentry_timeout != 0)
> +		seq_printf(m, ",ndentrytmo=%d", v9ses->ndentry_timeout);
>  	if (strcmp(v9ses->uname, V9FS_DEFUSER) != 0)
>  		seq_printf(m, ",uname=%s", v9ses->uname);
>  	if (strcmp(v9ses->aname, V9FS_DEFANAME) != 0)
> @@ -337,6 +340,10 @@ int v9fs_parse_param(struct fs_context *fc, struct
> fs_parameter *param) session_opts->session_lock_timeout =
> (long)result.uint_32 * HZ; break;
> 
> +	case Opt_ndentrytmo:
> +		session_opts->ndentry_timeout = result.int_32;
> +		break;
> +
>  	/* Options for client */
>  	case Opt_msize:
>  		if (result.uint_32 < 4096) {