fs/proc/task_nommu.c: shift mm_access() from m_start() to proc_maps_open()

Copy-and-paste the changes from "fs/proc/task_mmu.c: shift mm_access()
from m_start() to proc_maps_open()" into task_nommu.c.

Change maps_open() to initialize priv->mm using proc_mem_open(), m_start()
can rely on atomic_inc_not_zero(mm_users) like task_mmu.c does.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_nommu.c

@@ -216,11 +216,11 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	if (!priv->task)
 		return ERR_PTR(-ESRCH);
 
-	mm = mm_access(priv->task, PTRACE_MODE_READ);
-	if (!mm || IS_ERR(mm)) {
+	mm = priv->mm;
+	if (!mm || !atomic_inc_not_zero(&mm->mm_users)) {
 		put_task_struct(priv->task);
 		priv->task = NULL;
-		return mm;
+		return NULL;
 	}
 	down_read(&mm->mmap_sem);
 
@@ -270,14 +270,34 @@ static int maps_open(struct inode *inode, struct file *file,
 {
 	struct proc_maps_private *priv;
 
-	priv = __seq_open_private(file, ops, sizeof(struct proc_maps_private));
+	priv = __seq_open_private(file, ops, sizeof(*priv));
 	if (!priv)
 		return -ENOMEM;
 
 	priv->pid = proc_pid(inode);
+	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
+	if (IS_ERR(priv->mm)) {
+		int err = PTR_ERR(priv->mm);
+
+		seq_release_private(inode, file);
+		return err;
+	}
+
 	return 0;
 }
 
+
+static int map_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *seq = file->private_data;
+	struct proc_maps_private *priv = seq->private;
+
+	if (priv->mm)
+		mmdrop(priv->mm);
+
+	return seq_release_private(inode, file);
+}
+
 static int pid_maps_open(struct inode *inode, struct file *file)
 {
 	return maps_open(inode, file, &proc_pid_maps_ops);
@@ -292,13 +312,13 @@ const struct file_operations proc_pid_maps_operations = {
 	.open		= pid_maps_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
-	.release	= seq_release_private,
+	.release	= map_release,
 };
 
 const struct file_operations proc_tid_maps_operations = {
 	.open		= tid_maps_open,
 	.read		= seq_read,
 	.llseek		= seq_lseek,
-	.release	= seq_release_private,
+	.release	= map_release,
 };