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>

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 9f05837a86a64414ef8ee445c976a9c7a2f1c6e1..738579191fd22cbff71592c4a5e5840ab9574bf7 100644 (file)
--- 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: