linux-kernel - [PATCH 2/8] ns: don't increment or decrement initial namespaces

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Message-Id: <20251109-namespace-6-19-fixes-v1-2-ae8a4ad5a3b3@kernel.org>
Date: Sun, 09 Nov 2025 22:11:23 +0100
From: Christian Brauner <brauner@...nel.org>
To: linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>, 
 Jeff Layton <jlayton@...nel.org>
Cc: Jann Horn <jannh@...gle.com>, Mike Yuan <me@...dnzj.com>, 
 Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, 
 Lennart Poettering <mzxreary@...inter.de>, 
 Daan De Meyer <daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, 
 Amir Goldstein <amir73il@...il.com>, Tejun Heo <tj@...nel.org>, 
 Johannes Weiner <hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, bpf@...r.kernel.org, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 netdev@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, 
 Christian Brauner <brauner@...nel.org>
Subject: [PATCH 2/8] ns: don't increment or decrement initial namespaces

There's no need to bump the active reference counts of initial
namespaces as they're always active and can simply remain at 1.

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 include/linux/ns_common.h | 23 ++++++++++++++++++++---
 kernel/nscommon.c         |  6 ++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/include/linux/ns_common.h b/include/linux/ns_common.h
index bd4492ef6ffc..791b18dc77d0 100644
--- a/include/linux/ns_common.h
+++ b/include/linux/ns_common.h
@@ -141,6 +141,12 @@ static __always_inline bool is_initial_namespace(struct ns_common *ns)
 				 IPC_NS_INIT_INO - MNT_NS_INIT_INO + 1));
 }
 
+static __always_inline bool is_ns_init_id(const struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(ns->ns_id == 0);
+	return ns->ns_id <= NS_LAST_INIT_ID;
+}
+
 #define to_ns_common(__ns)                                    \
 	_Generic((__ns),                                      \
 		struct cgroup_namespace *:       &(__ns)->ns, \
@@ -285,14 +291,19 @@ void __ns_ref_active_get_owner(struct ns_common *ns);
 
 static __always_inline void __ns_ref_active_get(struct ns_common *ns)
 {
-	WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
-	VFS_WARN_ON_ONCE(is_initial_namespace(ns) && __ns_ref_active_read(ns) <= 0);
+	/* Initial namespaces are always active. */
+	if (!is_ns_init_id(ns))
+		WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
 }
 #define ns_ref_active_get(__ns) \
 	do { if (__ns) __ns_ref_active_get(to_ns_common(__ns)); } while (0)
 
 static __always_inline bool __ns_ref_active_get_not_zero(struct ns_common *ns)
 {
+	/* Initial namespaces are always active. */
+	if (is_ns_init_id(ns))
+		return true;
+
 	if (atomic_inc_not_zero(&ns->__ns_ref_active)) {
 		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
 		return true;
@@ -307,6 +318,10 @@ void __ns_ref_active_put_owner(struct ns_common *ns);
 
 static __always_inline void __ns_ref_active_put(struct ns_common *ns)
 {
+	/* Initial namespaces are always active. */
+	if (is_ns_init_id(ns))
+		return;
+
 	if (atomic_dec_and_test(&ns->__ns_ref_active)) {
 		VFS_WARN_ON_ONCE(is_initial_namespace(ns));
 		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
@@ -319,8 +334,10 @@ static __always_inline void __ns_ref_active_put(struct ns_common *ns)
 static __always_inline struct ns_common *__must_check ns_get_unless_inactive(struct ns_common *ns)
 {
 	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) && !__ns_ref_read(ns));
-	if (!__ns_ref_active_read(ns))
+	if (!__ns_ref_active_read(ns)) {
+		VFS_WARN_ON_ONCE(is_ns_init_id(ns));
 		return NULL;
+	}
 	if (!__ns_ref_get(ns))
 		return NULL;
 	return ns;
diff --git a/kernel/nscommon.c b/kernel/nscommon.c
index d67ae7ad7759..70cb66232e4c 100644
--- a/kernel/nscommon.c
+++ b/kernel/nscommon.c
@@ -177,6 +177,7 @@ void __ns_ref_active_put_owner(struct ns_common *ns)
 		ns = ns_owner(ns);
 		if (!ns)
 			return;
+		VFS_WARN_ON_ONCE(is_ns_init_id(ns));
 		if (!atomic_dec_and_test(&ns->__ns_ref_active))
 			return;
 	}
@@ -276,6 +277,10 @@ void __ns_ref_active_put_owner(struct ns_common *ns)
  */
 void __ns_ref_active_resurrect(struct ns_common *ns)
 {
+	/* Initial namespaces are always active. */
+	if (is_ns_init_id(ns))
+		return;
+
 	/* If we didn't resurrect the namespace we're done. */
 	if (atomic_fetch_add(1, &ns->__ns_ref_active))
 		return;
@@ -289,6 +294,7 @@ void __ns_ref_active_resurrect(struct ns_common *ns)
 		if (!ns)
 			return;
 
+		VFS_WARN_ON_ONCE(is_ns_init_id(ns));
 		if (atomic_fetch_add(1, &ns->__ns_ref_active))
 			return;
 	}

-- 
2.47.3