Introduce a new mount option, ndentrycache, for v9fs that allows users
to specify how long negative dentries are retained in the cache. The
retention time can be set in milliseconds (e.g. ndentrycache=10000 for
a 10secs retention time), or negative entries can be kept until the
buffer cache management removes them by using the option without a
value (i.e. ndentrycache).

For 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 | 73 ++++++++++++++++++++++++++++++++--------------------
 fs/9p/v9fs.h | 23 ++++++++++-------
 2 files changed, 58 insertions(+), 38 deletions(-)

diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index c5dca81a553e..a26bd9070786 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -39,11 +39,12 @@ 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_ndentrycachetmo,
 	/* String options */
 	Opt_uname, Opt_remotename, Opt_cache, Opt_cachetag,
 	/* Options that take no arguments */
 	Opt_nodevmap, Opt_noxattr, Opt_directio, Opt_ignoreqv,
+	Opt_ndentrycache,
 	/* Access options */
 	Opt_access, Opt_posixacl,
 	/* Lock timeout option */
@@ -77,41 +78,43 @@ static const struct constant_table p9_versions[] = {
  * the client, and all the transports.
  */
 const struct fs_parameter_spec v9fs_param_spec[] = {
-	fsparam_string	("source",	Opt_source),
-	fsparam_u32hex	("debug",	Opt_debug),
-	fsparam_uid	("dfltuid",	Opt_dfltuid),
-	fsparam_gid	("dfltgid",	Opt_dfltgid),
-	fsparam_u32	("afid",	Opt_afid),
-	fsparam_string	("uname",	Opt_uname),
-	fsparam_string	("aname",	Opt_remotename),
-	fsparam_flag	("nodevmap",	Opt_nodevmap),
-	fsparam_flag	("noxattr",	Opt_noxattr),
-	fsparam_flag	("directio",	Opt_directio),
-	fsparam_flag	("ignoreqv",	Opt_ignoreqv),
-	fsparam_string	("cache",	Opt_cache),
-	fsparam_string	("cachetag",	Opt_cachetag),
-	fsparam_string	("access",	Opt_access),
-	fsparam_flag	("posixacl",	Opt_posixacl),
-	fsparam_u32	("locktimeout",	Opt_locktimeout),
+	fsparam_string	("source",		Opt_source),
+	fsparam_u32hex	("debug",		Opt_debug),
+	fsparam_uid	("dfltuid",		Opt_dfltuid),
+	fsparam_gid	("dfltgid",		Opt_dfltgid),
+	fsparam_u32	("afid",		Opt_afid),
+	fsparam_string	("uname",		Opt_uname),
+	fsparam_string	("aname",		Opt_remotename),
+	fsparam_flag	("nodevmap",		Opt_nodevmap),
+	fsparam_flag	("noxattr",		Opt_noxattr),
+	fsparam_flag	("directio",		Opt_directio),
+	fsparam_flag	("ignoreqv",		Opt_ignoreqv),
+	fsparam_string	("cache",		Opt_cache),
+	fsparam_string	("cachetag",		Opt_cachetag),
+	fsparam_string	("access",		Opt_access),
+	fsparam_flag	("posixacl",		Opt_posixacl),
+	fsparam_u32	("locktimeout",		Opt_locktimeout),
+	fsparam_flag	("ndentrycache",	Opt_ndentrycache),
+	fsparam_u32	("ndentrycache",	Opt_ndentrycachetmo),
 
 	/* client options */
-	fsparam_u32	("msize",	Opt_msize),
-	fsparam_flag	("noextend",	Opt_legacy),
-	fsparam_string	("trans",	Opt_trans),
-	fsparam_enum	("version",	Opt_version, p9_versions),
+	fsparam_u32	("msize",		Opt_msize),
+	fsparam_flag	("noextend",		Opt_legacy),
+	fsparam_string	("trans",		Opt_trans),
+	fsparam_enum	("version",		Opt_version, p9_versions),
 
 	/* fd transport options */
-	fsparam_u32	("rfdno",	Opt_rfdno),
-	fsparam_u32	("wfdno",	Opt_wfdno),
+	fsparam_u32	("rfdno",		Opt_rfdno),
+	fsparam_u32	("wfdno",		Opt_wfdno),
 
 	/* rdma transport options */
-	fsparam_u32	("sq",		Opt_sq_depth),
-	fsparam_u32	("rq",		Opt_rq_depth),
-	fsparam_u32	("timeout",	Opt_timeout),
+	fsparam_u32	("sq",			Opt_sq_depth),
+	fsparam_u32	("rq",			Opt_rq_depth),
+	fsparam_u32	("timeout",		Opt_timeout),
 
 	/* fd and rdma transprt options */
-	fsparam_u32	("port",	Opt_port),
-	fsparam_flag	("privport",	Opt_privport),
+	fsparam_u32	("port",		Opt_port),
+	fsparam_flag	("privport",		Opt_privport),
 	{}
 };
 
@@ -159,6 +162,10 @@ 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_ms == NDENTRY_TMOUT_NEVER)
+		seq_printf(m, ",ndentrycache");
+	else if (v9ses->flags & V9FS_NDENTRY_TMOUT_SET)
+		seq_printf(m, ",ndentrycache=%u", v9ses->ndentry_timeout_ms);
 	if (strcmp(v9ses->uname, V9FS_DEFUSER) != 0)
 		seq_printf(m, ",uname=%s", v9ses->uname);
 	if (strcmp(v9ses->aname, V9FS_DEFANAME) != 0)
@@ -337,6 +344,16 @@ 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_ndentrycache:
+		session_opts->flags |= V9FS_NDENTRY_TMOUT_SET;
+		session_opts->ndentry_timeout_ms = NDENTRY_TMOUT_NEVER;
+		break;
+
+	case Opt_ndentrycachetmo:
+		session_opts->flags |= V9FS_NDENTRY_TMOUT_SET;
+		session_opts->ndentry_timeout_ms = result.uint_64;
+		break;
+
 	/* Options for client */
 	case Opt_msize:
 		if (result.uint_32 < 4096) {
diff --git a/fs/9p/v9fs.h b/fs/9p/v9fs.h
index 8410f7883109..2e42729c6c20 100644
--- a/fs/9p/v9fs.h
+++ b/fs/9p/v9fs.h
@@ -24,6 +24,8 @@
  * @V9FS_ACCESS_ANY: use a single attach for all users
  * @V9FS_ACCESS_MASK: bit mask of different ACCESS options
  * @V9FS_POSIX_ACL: POSIX ACLs are enforced
+ * @V9FS_NDENTRY_TMOUT_SET: Has negative dentry timeout retention time been
+ *			    overriden by ndentrycache mount option
  *
  * Session flags reflect options selected by users at mount time
  */
@@ -34,16 +36,17 @@
 #define V9FS_ACL_MASK V9FS_POSIX_ACL
 
 enum p9_session_flags {
-	V9FS_PROTO_2000U    = 0x01,
-	V9FS_PROTO_2000L    = 0x02,
-	V9FS_ACCESS_SINGLE  = 0x04,
-	V9FS_ACCESS_USER    = 0x08,
-	V9FS_ACCESS_CLIENT  = 0x10,
-	V9FS_POSIX_ACL      = 0x20,
-	V9FS_NO_XATTR       = 0x40,
-	V9FS_IGNORE_QV      = 0x80, /* ignore qid.version for cache hints */
-	V9FS_DIRECT_IO      = 0x100,
-	V9FS_SYNC           = 0x200
+	V9FS_PROTO_2000U       = 0x01,
+	V9FS_PROTO_2000L       = 0x02,
+	V9FS_ACCESS_SINGLE     = 0x04,
+	V9FS_ACCESS_USER       = 0x08,
+	V9FS_ACCESS_CLIENT     = 0x10,
+	V9FS_POSIX_ACL         = 0x20,
+	V9FS_NO_XATTR          = 0x40,
+	V9FS_IGNORE_QV         = 0x80, /* ignore qid.version for cache hints */
+	V9FS_DIRECT_IO         = 0x100,
+	V9FS_SYNC              = 0x200,
+	V9FS_NDENTRY_TMOUT_SET = 0x400,
 };
 
 /**
-- 
2.52.0