VFS: New /proc file /proc/self/mountstats

Create a new file under /proc/self, called mountstats, where mounted file
systems can export information (configuration options, performance counters,
and so on).  Use a mechanism similar to /proc/mounts and s_ops->show_options.

This mechanism does not violate namespace security, and is safe to use while
other processes are unmounting file systems.

Thanks to Mike Waychison for his review and comments.

Test-plan:
Test concurrent mount/unmount operations while cat'ing /proc/self/mountstats.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

3 files changed, 78 insertions, 0 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 39c81a8d6316..71e75bcf4d28 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -399,6 +399,44 @@ struct seq_operations mounts_op = {
         .show   = show_vfsmnt
 };
 
+static int show_vfsstat(struct seq_file *m, void *v)
+{
+        struct vfsmount *mnt = v;
+        int err = 0;
+
+        /* device */
+        if (mnt->mnt_devname) {
+                seq_puts(m, "device ");
+                mangle(m, mnt->mnt_devname);
+        } else
+                seq_puts(m, "no device");
+
+        /* mount point */
+        seq_puts(m, " mounted on ");
+        seq_path(m, mnt, mnt->mnt_root, " \t\n\\");
+        seq_putc(m, ' ');
+
+        /* file system type */
+        seq_puts(m, "with fstype ");
+        mangle(m, mnt->mnt_sb->s_type->name);
+
+        /* optional statistics */
+        if (mnt->mnt_sb->s_op->show_stats) {
+                seq_putc(m, ' ');
+                err = mnt->mnt_sb->s_op->show_stats(m, mnt);
+        }
+
+        seq_putc(m, '\n');
+        return err;
+}
+
+struct seq_operations mountstats_op = {
+        .start  = m_start,
+        .next   = m_next,
+        .stop   = m_stop,
+        .show   = show_vfsstat,
+};
+
 /**
  * may_umount_tree - check if a mount tree is busy
  * @mnt: root of mount tree
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 20feb7568deb..8f1f49ceebec 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -104,6 +104,7 @@ enum pid_directory_inos {
         PROC_TGID_MAPS,
         PROC_TGID_NUMA_MAPS,
         PROC_TGID_MOUNTS,
+        PROC_TGID_MOUNTSTATS,
         PROC_TGID_WCHAN,
 #ifdef CONFIG_MMU
         PROC_TGID_SMAPS,
@@ -144,6 +145,7 @@ enum pid_directory_inos {
         PROC_TID_MAPS,
         PROC_TID_NUMA_MAPS,
         PROC_TID_MOUNTS,
+        PROC_TID_MOUNTSTATS,
         PROC_TID_WCHAN,
 #ifdef CONFIG_MMU
         PROC_TID_SMAPS,
@@ -201,6 +203,7 @@ static struct pid_entry tgid_base_stuff[] = {
         E(PROC_TGID_ROOT,      "root",    S_IFLNK|S_IRWXUGO),
         E(PROC_TGID_EXE,       "exe",     S_IFLNK|S_IRWXUGO),
         E(PROC_TGID_MOUNTS,    "mounts",  S_IFREG|S_IRUGO),
+        E(PROC_TGID_MOUNTSTATS, "mountstats", S_IFREG|S_IRUSR),
 #ifdef CONFIG_MMU
         E(PROC_TGID_SMAPS,     "smaps",   S_IFREG|S_IRUGO),
 #endif
@@ -732,6 +735,38 @@ static struct file_operations proc_mounts_operations = {
         .poll           = mounts_poll,
 };
 
+extern struct seq_operations mountstats_op;
+static int mountstats_open(struct inode *inode, struct file *file)
+{
+        struct task_struct *task = proc_task(inode);
+        int ret = seq_open(file, &mountstats_op);
+
+        if (!ret) {
+                struct seq_file *m = file->private_data;
+                struct namespace *namespace;
+                task_lock(task);
+                namespace = task->namespace;
+                if (namespace)
+                        get_namespace(namespace);
+                task_unlock(task);
+
+                if (namespace)
+                        m->private = namespace;
+                else {
+                        seq_release(inode, file);
+                        ret = -EINVAL;
+                }
+        }
+        return ret;
+}
+
+static struct file_operations proc_mountstats_operations = {
+        .open           = mountstats_open,
+        .read           = seq_read,
+        .llseek         = seq_lseek,
+        .release        = mounts_release,
+};
+
 #define PROC_BLOCK_SIZE (3*1024)                /* 4K page size but our output routines use some slack for overruns */
 
 static ssize_t proc_info_read(struct file * file, char __user * buf,
@@ -1730,6 +1765,10 @@ static struct dentry *proc_pident_lookup(struct inode *dir,
                         inode->i_fop = &proc_smaps_operations;
                         break;
 #endif
+                case PROC_TID_MOUNTSTATS:
+                case PROC_TGID_MOUNTSTATS:
+                        inode->i_fop = &proc_mountstats_operations;
+                        break;
 #ifdef CONFIG_SECURITY
                 case PROC_TID_ATTR:
                         inode->i_nlink = 2;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 128d0082522c..be21e860a9f2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1086,6 +1086,7 @@ struct super_operations {
         void (*umount_begin) (struct super_block *);
 
         int (*show_options)(struct seq_file *, struct vfsmount *);
+        int (*show_stats)(struct seq_file *, struct vfsmount *);
 
         ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
         ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);