linux-kernel - [PATCH 10/17] fs: use boolean to indicate anonymous mount namespace

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Message-Id: <20251110-work-namespace-nstree-fixes-v1-10-e8a9264e0fb9@kernel.org>
Date: Mon, 10 Nov 2025 16:08:22 +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 10/17] fs: use boolean to indicate anonymous mount
 namespace

Stop playing games with the namespace id and use a boolean instead:

* This will remove the special-casing we need to do everywhere for mount
  namespaces.

* It will allow us to use asserts on the namespace id for initial
  namespaces everywhere.

* It will allow us to put anonymous mount namespaces on the namespaces
  trees in the future and thus make them available to statmount() and
  listmount().

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 fs/mount.h     | 3 ++-
 fs/namespace.c | 5 +++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/mount.h b/fs/mount.h
index f13a28752d0b..2d28ef2a3aed 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -27,6 +27,7 @@ struct mnt_namespace {
 	unsigned int		nr_mounts; /* # of mounts in the namespace */
 	unsigned int		pending_mounts;
 	refcount_t		passive; /* number references not pinning @mounts */
+	bool			is_anon;
 } __randomize_layout;
 
 struct mnt_pcp {
@@ -175,7 +176,7 @@ static inline bool is_local_mountpoint(const struct dentry *dentry)
 
 static inline bool is_anon_ns(struct mnt_namespace *ns)
 {
-	return ns->ns.ns_id == 0;
+	return ns->is_anon;
 }
 
 static inline bool anon_ns_root(const struct mount *m)
diff --git a/fs/namespace.c b/fs/namespace.c
index ad19530a13b2..efaff8680eaf 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4093,8 +4093,9 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool a
 		dec_mnt_namespaces(ucounts);
 		return ERR_PTR(ret);
 	}
-	if (!anon)
-		ns_tree_gen_id(new_ns);
+	ns_tree_gen_id(new_ns);
+
+	new_ns->is_anon = anon;
 	refcount_set(&new_ns->passive, 1);
 	new_ns->mounts = RB_ROOT;
 	init_waitqueue_head(&new_ns->poll);

-- 
2.47.3