fs: move mntput_no_expire() slowpath into a dedicated routine

In the stock variant the compiler spills several registers on the stack
and employs stack smashing protection, adding even more code + a branch
on exit..

The actual fast path is small enough that the compiler inlines it for
all callers -- the symbol is no longer emitted.

Forcing noinline on it just for code-measurement purposes shows the fast
path dropping from 111 to 39 bytes.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251114201803.2183505-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

fs/namespace.c

@@ -1345,26 +1345,12 @@ static void delayed_mntput(struct work_struct *unused)
 }
 static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);
 
-static void mntput_no_expire(struct mount *mnt)
+static void noinline mntput_no_expire_slowpath(struct mount *mnt)
 {
 	LIST_HEAD(list);
 	int count;
 
-	rcu_read_lock();
-	if (likely(READ_ONCE(mnt->mnt_ns))) {
-		/*
-		 * Since we don't do lock_mount_hash() here,
-		 * ->mnt_ns can change under us.  However, if it's
-		 * non-NULL, then there's a reference that won't
-		 * be dropped until after an RCU delay done after
-		 * turning ->mnt_ns NULL.  So if we observe it
-		 * non-NULL under rcu_read_lock(), the reference
-		 * we are dropping is not the final one.
-		 */
-		mnt_add_count(mnt, -1);
-		rcu_read_unlock();
-		return;
-	}
+	VFS_BUG_ON(mnt->mnt_ns);
 	lock_mount_hash();
 	/*
 	 * make sure that if __legitimize_mnt() has not seen us grab
@@ -1415,6 +1401,26 @@ static void mntput_no_expire(struct mount *mnt)
 	cleanup_mnt(mnt);
 }
 
+static void mntput_no_expire(struct mount *mnt)
+{
+	rcu_read_lock();
+	if (likely(READ_ONCE(mnt->mnt_ns))) {
+		/*
+		 * Since we don't do lock_mount_hash() here,
+		 * ->mnt_ns can change under us.  However, if it's
+		 * non-NULL, then there's a reference that won't
+		 * be dropped until after an RCU delay done after
+		 * turning ->mnt_ns NULL.  So if we observe it
+		 * non-NULL under rcu_read_lock(), the reference
+		 * we are dropping is not the final one.
+		 */
+		mnt_add_count(mnt, -1);
+		rcu_read_unlock();
+		return;
+	}
+	mntput_no_expire_slowpath(mnt);
+}
+
 void mntput(struct vfsmount *mnt)
 {
 	if (mnt) {