Introduce a new mount option, negtimeout, 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. negtimeout=10000 for
a 10secs retention time) or a negative value (e.g. negtimeout=-1) to
keep negative entries until the buffer cache management removes them.

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>
---
 Documentation/filesystems/9p.rst |  5 +++++
 fs/9p/v9fs.c                     | 16 +++++++++++++++-
 fs/9p/v9fs.h                     | 23 +++++++++++++----------
 3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/Documentation/filesystems/9p.rst b/Documentation/filesystems/9p.rst
index be3504ca034a..b014c7aabba4 100644
--- a/Documentation/filesystems/9p.rst
+++ b/Documentation/filesystems/9p.rst
@@ -238,6 +238,11 @@ Options
   cachetag	cache tag to use the specified persistent cache.
 		cache tags for existing cache sessions can be listed at
 		/sys/fs/9p/caches. (applies only to cache=fscache)
+
+  negtimeout    the duration (in milliseconds) that negative dentries (paths
+                that do not actually exist) are retained in the cache. If
+                set to a negative value, those entries are kept indefinitely
+                until evicted by the buffer cache management system
   ============= ===============================================================
 
 Behavior
diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index c5dca81a553e..f16517be7fc2 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_negtimeout,
 	/* 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	("negtimeout",	Opt_negtimeout),
 
 	/* client options */
 	fsparam_u32	("msize",	Opt_msize),
@@ -159,6 +160,9 @@ 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->flags & V9FS_NDENTRY_TIMEOUT_SET)
+		seq_printf(m, ",negtimeout=%d",
+			   (int)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 +341,16 @@ 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_negtimeout:
+		session_opts->flags |= V9FS_NDENTRY_TIMEOUT_SET;
+		if (result.int_32 < 0) {
+			session_opts->ndentry_timeout_ms =
+				NDENTRY_TIMEOUT_NEVER;
+		} else {
+			session_opts->ndentry_timeout_ms = result.int_32;
+		}
+		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 e630c5111d74..32179f64a603 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_TIMEOUT_SET: Has negative dentry timeout retention time been
+ *                            overridden 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_TIMEOUT_SET = 0x400,
 };
 
 /**
-- 
2.52.0