proc: fix vma display mismatch between /proc/pid/{maps,smaps}

Commit 4752c369789250eafcd7813e11c8fb689235b0d2 aka
"maps4: simplify interdependence of maps and smaps" broke /proc/pid/smaps,
causing it to display some vmas twice and other vmas not at all. For example:

grep .- /proc/1/smaps >/tmp/smaps; diff /proc/1/maps /tmp/smaps

1 25d24
2 < 7fd7e23aa000-7fd7e23ac000 rw-p 7fd7e23aa000 00:00 0
3 28a28
4 > ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]

The bug has something to do with setting m->version before all the
seq_printf's have been performed. show_map was doing this correctly,
but show_smap was doing this in the middle of its seq_printf sequence.
This patch arranges things so that the setting of m->version in show_smap
is also done at the end of its seq_printf sequence.

Testing: in addition to the above grep test, for each process I summed
up the 'Rss' fields of /proc/pid/smaps and compared that to the 'VmRSS'
field of /proc/pid/status. All matched except for Xorg (which has a
/dev/mem mapping which Rss accounts for but VmRSS does not). This result
gives us some confidence that neither /proc/pid/maps nor /proc/pid/smaps
are any longer skipping or double-counting vmas.

Signed-off-by: Joe Korty <[e-mail]>
Cc: Matt Mackall <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Alexey Dobriyan <[e-mail]>

fs/proc/task_mmu.c

···
 	return ret;
 }
 
-static int show_map(struct seq_file *m, void *v)
+static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
 {
-	struct proc_maps_private *priv = m->private;
-	struct task_struct *task = priv->task;
-	struct vm_area_struct *vma = v;
 	struct mm_struct *mm = vma->vm_mm;
 	struct file *file = vma->vm_file;
 	int flags = vma->vm_flags;
···
 		}
 	}
 	seq_putc(m, '\n');
+}
+
+static int show_map(struct seq_file *m, void *v)
+{
+	struct vm_area_struct *vma = v;
+	struct proc_maps_private *priv = m->private;
+	struct task_struct *task = priv->task;
+
+	show_map_vma(m, vma);
 
 	if (m->count < m->size)  /* vma is copied successfully */
 		m->version = (vma != get_gate_vma(task))? vma->vm_start: 0;
···
 
 static int show_smap(struct seq_file *m, void *v)
 {
+	struct proc_maps_private *priv = m->private;
+	struct task_struct *task = priv->task;
 	struct vm_area_struct *vma = v;
 	struct mem_size_stats mss;
-	int ret;
 	struct mm_walk smaps_walk = {
 		.pmd_entry = smaps_pte_range,
 		.mm = vma->vm_mm,
···
 	if (vma->vm_mm && !is_vm_hugetlb_page(vma))
 		walk_page_range(vma->vm_start, vma->vm_end, &smaps_walk);
 
-	ret = show_map(m, v);
-	if (ret)
-		return ret;
+	show_map_vma(m, vma);
 
 	seq_printf(m,
 		   "Size:           %8lu kB\n"
···
 		   mss.referenced >> 10,
 		   mss.swap >> 10);
 
-	return ret;
+	if (m->count < m->size)  /* vma is copied successfully */
+		m->version = (vma != get_gate_vma(task)) ? vma->vm_start : 0;
+	return 0;
 }
 
 static const struct seq_operations proc_pid_smaps_op = {