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>

1 files changed, 38 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