fs/proc/task_mmu.c: simplify the vma_stop() logic

m_start() drops ->mmap_sem and does mmput() if it retuns vsyscall
vma. This is because in this case m_stop()->vma_stop() obviously
can't use gate_vma->vm_mm.

Now that we have proc_maps_private->mm we can simplify this logic:

  - Change m_start() to return with ->mmap_sem held unless it returns
    IS_ERR_OR_NULL().

  - Change vma_stop() to use priv->mm and avoid the ugly vma checks,
    this makes "vm_area_struct *vma" unnecessary.

  - This also allows m_start() to use vm_stop().

  - Cleanup m_next() to follow the new locking rule.

    Note: m_stop() looks very ugly, and this temporary uglifies it
    even more. Fixed by the next change.

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

fs/proc/task_mmu.c

@@ -129,14 +129,13 @@ static void release_task_mempolicy(struct proc_maps_private *priv)
 }
 #endif
 
-static void vma_stop(struct proc_maps_private *priv, struct vm_area_struct *vma)
+static void vma_stop(struct proc_maps_private *priv)
 {
-	if (vma && vma != priv->tail_vma) {
-		struct mm_struct *mm = vma->vm_mm;
+	struct mm_struct *mm = priv->mm;
+
 	release_task_mempolicy(priv);
 	up_read(&mm->mmap_sem);
 	mmput(mm);
-	}
 }
 
 static void *m_start(struct seq_file *m, loff_t *pos)
@@ -199,12 +198,13 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	if (vma)
 		return vma;
 
-	release_task_mempolicy(priv);
 	/* End of vmas has been reached */
 	m->version = (tail_vma != NULL)? 0: -1UL;
-	up_read(&mm->mmap_sem);
-	mmput(mm);
+	if (tail_vma)
 		return tail_vma;
+
+	vma_stop(priv);
+	return NULL;
 }
 
 static void *m_next(struct seq_file *m, void *v, loff_t *pos)
@@ -212,21 +212,24 @@ static void *m_next(struct seq_file *m, void *v, loff_t *pos)
 	struct proc_maps_private *priv = m->private;
 	struct vm_area_struct *vma = v;
 	struct vm_area_struct *tail_vma = priv->tail_vma;
+	struct vm_area_struct *next;
 
 	(*pos)++;
 	if (vma && (vma != tail_vma) && vma->vm_next)
 		return vma->vm_next;
-	vma_stop(priv, vma);
-	return (vma != tail_vma)? tail_vma: NULL;
+
+	next = (vma != tail_vma) ? tail_vma : NULL;
+	if (!next)
+		vma_stop(priv);
+	return next;
 }
 
 static void m_stop(struct seq_file *m, void *v)
 {
 	struct proc_maps_private *priv = m->private;
-	struct vm_area_struct *vma = v;
 
-	if (!IS_ERR(vma))
-		vma_stop(priv, vma);
+	if (!IS_ERR_OR_NULL(v))
+		vma_stop(priv);
 	if (priv->task)
 		put_task_struct(priv->task);
 }