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, 39 insertions, 0 deletions

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;