mqueue: switch to on-demand creation of internal mount

Instead of doing that upon each ipcns creation, we do that the first
time mq_open(2) or mqueue mount is done in an ipcns.  What's more,
doing that allows to get rid of mount_ns() use - we can go with
considerably cheaper mount_nodev(), avoiding the loop over all
mqueue superblock instances; ipcns->mq_mnt is used to locate preexisting
instance in O(1) time instead of O(instances) mount_ns() would've
cost us.

Based upon the version by Giuseppe Scrivano <gscrivan@redhat.com>; I've
added handling of userland mqueue mounts (original had been broken in
that area) and added a switch to mount_nodev().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 55 insertions, 19 deletions

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 9f05837a86a6..738579191fd2 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -325,8 +325,9 @@ err:
 static int mqueue_fill_super(struct super_block *sb, void *data, int silent)
 {
         struct inode *inode;
-        struct ipc_namespace *ns = sb->s_fs_info;
+        struct ipc_namespace *ns = data;
 
+        sb->s_fs_info = ns;
         sb->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
         sb->s_blocksize = PAGE_SIZE;
         sb->s_blocksize_bits = PAGE_SHIFT;
@@ -343,18 +344,44 @@ static int mqueue_fill_super(struct super_block *sb, void *data, int silent)
         return 0;
 }
 
+static struct file_system_type mqueue_fs_type;
+/*
+ * Return value is pinned only by reference in ->mq_mnt; it will
+ * live until ipcns dies.  Caller does not need to drop it.
+ */
+static struct vfsmount *mq_internal_mount(void)
+{
+        struct ipc_namespace *ns = current->nsproxy->ipc_ns;
+        struct vfsmount *m = ns->mq_mnt;
+        if (m)
+                return m;
+        m = kern_mount_data(&mqueue_fs_type, ns);
+        spin_lock(&mq_lock);
+        if (unlikely(ns->mq_mnt)) {
+                spin_unlock(&mq_lock);
+                if (!IS_ERR(m))
+                        kern_unmount(m);
+                return ns->mq_mnt;
+        }
+        if (!IS_ERR(m))
+                ns->mq_mnt = m;
+        spin_unlock(&mq_lock);
+        return m;
+}
+
 static struct dentry *mqueue_mount(struct file_system_type *fs_type,
                          int flags, const char *dev_name,
                          void *data)
 {
-        struct ipc_namespace *ns;
-        if (flags & SB_KERNMOUNT) {
-                ns = data;
-                data = NULL;
-        } else {
-                ns = current->nsproxy->ipc_ns;
-        }
-        return mount_ns(fs_type, flags, data, ns, ns->user_ns, mqueue_fill_super);
+        struct vfsmount *m;
+        if (flags & SB_KERNMOUNT)
+                return mount_nodev(fs_type, flags, data, mqueue_fill_super);
+        m = mq_internal_mount();
+        if (IS_ERR(m))
+                return ERR_CAST(m);
+        atomic_inc(&m->mnt_sb->s_active);
+        down_write(&m->mnt_sb->s_umount);
+        return dget(m->mnt_root);
 }
 
 static void init_once(void *foo)
@@ -743,13 +770,16 @@ static int prepare_open(struct dentry *dentry, int oflag, int ro,
 static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
                       struct mq_attr *attr)
 {
-        struct vfsmount *mnt = current->nsproxy->ipc_ns->mq_mnt;
-        struct dentry *root = mnt->mnt_root;
+        struct vfsmount *mnt = mq_internal_mount();
+        struct dentry *root;
         struct filename *name;
         struct path path;
         int fd, error;
         int ro;
 
+        if (IS_ERR(mnt))
+                return PTR_ERR(mnt);
+
         audit_mq_open(oflag, mode, attr);
 
         if (IS_ERR(name = getname(u_name)))
@@ -760,6 +790,7 @@ static int do_mq_open(const char __user *u_name, int oflag, umode_t mode,
                 goto out_putname;
 
         ro = mnt_want_write(mnt);       /* we'll drop it in any case */
+        root = mnt->mnt_root;
         inode_lock(d_inode(root));
         path.dentry = lookup_one_len(name->name, root, strlen(name->name));
         if (IS_ERR(path.dentry)) {
@@ -808,6 +839,9 @@ 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);
@@ -1534,28 +1568,26 @@ int mq_init_ns(struct ipc_namespace *ns)
         ns->mq_msgsize_max   = DFLT_MSGSIZEMAX;
         ns->mq_msg_default   = DFLT_MSG;
         ns->mq_msgsize_default  = DFLT_MSGSIZE;
+        ns->mq_mnt = NULL;
 
-        ns->mq_mnt = kern_mount_data(&mqueue_fs_type, ns);
-        if (IS_ERR(ns->mq_mnt)) {
-                int 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)
 {
+        struct vfsmount *m;
         int error;
 
         mqueue_inode_cachep = kmem_cache_create("mqueue_inode_cache",
@@ -1577,6 +1609,10 @@ static int __init init_mqueue_fs(void)
         if (error)
                 goto out_filesystem;
 
+        m = kern_mount_data(&mqueue_fs_type, &init_ipc_ns);
+        if (IS_ERR(m))
+                goto out_filesystem;
+        init_ipc_ns.mq_mnt = m;
         return 0;
 
 out_filesystem: