[RFC PATCH] ipc, mqueue: lazy call kern_mount_data in new namespaces

[Giuseppe Scrivano]

kern_mount_data is a relatively expensive operation when creating a
new IPC namespace, so delay the mount until its first usage when not
creating the the global namespace.

On my machine, the time for creating 1000 new IPC namespaces dropped
from ~8s to ~2s.

Signed-off-by: Giuseppe Scrivano <gscrivan@xxxxxxxxxx>
---
 include/linux/ipc_namespace.h |  2 +-
 ipc/mqueue.c                  | 47 ++++++++++++++++++++++++++++++++++---------
 ipc/namespace.c               |  2 +-
 3 files changed, 39 insertions(+), 12 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index b5630c8eb2f3..a06617c113f1 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -81,7 +81,7 @@ static inline void shm_destroy_orphaned(struct ipc_namespace *ns) {}
 #endif /* CONFIG_SYSVIPC */
 
 #ifdef CONFIG_POSIX_MQUEUE
-extern int mq_init_ns(struct ipc_namespace *ns);
+extern int mq_init_ns(struct ipc_namespace *ns, bool mount);
 /*
  * POSIX Message Queue default values:
  *
diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index d24025626310..46d2795bbf93 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -87,6 +87,7 @@ struct mqueue_inode_info {
 	unsigned long qsize; /* size of queue in memory (sum of all msgs) */
 };
 
+static struct file_system_type mqueue_fs_type;
 static const struct inode_operations mqueue_dir_inode_operations;
 static const struct file_operations mqueue_file_operations;
 static const struct super_operations mqueue_super_ops;
@@ -776,10 +777,24 @@ static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
 	struct filename *name;
 	int fd, error;
 	struct ipc_namespace *ipc_ns = current->nsproxy->ipc_ns;
-	struct vfsmount *mnt = ipc_ns->mq_mnt;
-	struct dentry *root = mnt->mnt_root;
+	struct vfsmount *mnt;
+	struct dentry *root;
 	int ro;
 
+	spin_lock(&mq_lock);
+	mnt = ipc_ns->mq_mnt;
+	if (unlikely(!mnt)) {
+		mnt = kern_mount_data(&mqueue_fs_type, ipc_ns);
+		if (IS_ERR(mnt)) {
+			spin_unlock(&mq_lock);
+			return PTR_ERR(mnt);
+		}
+		ipc_ns->mq_mnt = mnt;
+	}
+	spin_unlock(&mq_lock);
+
+	root = mnt->mnt_root;
+
 	audit_mq_open(oflag, mode, attr);
 
 	if (IS_ERR(name = getname(u_name)))
@@ -863,6 +878,9 @@ SYSCALL_DEFINE1(mq_unlink, const char __user *, u_name)
 	struct ipc_namespace *ipc_ns = current->nsproxy->ipc_ns;
 	struct vfsmount *mnt = ipc_ns->mq_mnt;
 
+	if (!mnt)
+		return -ENOENT;
+
 	name = getname(u_name);
 	if (IS_ERR(name))
 		return PTR_ERR(name);
@@ -1581,7 +1599,8 @@ static struct file_system_type mqueue_fs_type = {
 	.fs_flags = FS_USERNS_MOUNT,
 };
 
-int mq_init_ns(struct ipc_namespace *ns)
+
+int mq_init_ns(struct ipc_namespace *ns, bool mount)
 {
 	ns->mq_queues_count  = 0;
 	ns->mq_queues_max    = DFLT_QUEUESMAX;
@@ -1590,23 +1609,31 @@ int mq_init_ns(struct ipc_namespace *ns)
 	ns->mq_msg_default   = DFLT_MSG;
 	ns->mq_msgsize_default  = DFLT_MSGSIZE;
 
-	ns->mq_mnt = kern_mount_data(&mqueue_fs_type, ns);
-	if (IS_ERR(ns->mq_mnt)) {
-		int err = PTR_ERR(ns->mq_mnt);
+	if (!mount)
 		ns->mq_mnt = NULL;
-		return err;
+	else {
+		int err;
+
+		ns->mq_mnt = kern_mount_data(&mqueue_fs_type, ns);
+		if (IS_ERR(ns->mq_mnt)) {
+			err = PTR_ERR(ns->mq_mnt);
+			ns->mq_mnt = NULL;
+			return err;
+		}
 	}
 	return 0;
 }
 
 void mq_clear_sbinfo(struct ipc_namespace *ns)
 {
-	ns->mq_mnt->mnt_sb->s_fs_info = NULL;
+	if (ns->mq_mnt)
+		ns->mq_mnt->mnt_sb->s_fs_info = NULL;
 }
 
 void mq_put_mnt(struct ipc_namespace *ns)
 {
-	kern_unmount(ns->mq_mnt);
+	if (ns->mq_mnt)
+		kern_unmount(ns->mq_mnt);
 }
 
 static int __init init_mqueue_fs(void)
@@ -1628,7 +1655,7 @@ static int __init init_mqueue_fs(void)
 
 	spin_lock_init(&mq_lock);
 
-	error = mq_init_ns(&init_ipc_ns);
+	error = mq_init_ns(&init_ipc_ns, true);
 	if (error)
 		goto out_filesystem;
 
diff --git a/ipc/namespace.c b/ipc/namespace.c
index f59a89966f92..9d3689577f66 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -65,7 +65,7 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
 	if (err)
 		goto fail_destroy_msg;
 
-	err = mq_init_ns(ns);
+	err = mq_init_ns(ns, false);
 	if (err)
 		goto fail_destroy_shm;
 
-- 
2.14.3