mm: update the description for vm_total_pages

vm_total_pages is calculated by nr_free_pagecache_pages(), which counts
the number of pages which are beyond the high watermark within all
zones.  So vm_total_pages is not equal to total number of pages which
the VM controls.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -137,7 +137,11 @@ struct scan_control {
  * From 0 .. 100.  Higher means more swappy.
  */
 int vm_swappiness = 60;
-unsigned long vm_total_pages;	/* The total number of pages which the VM controls */
+/*
+ * The total number of pages which are beyond the high watermark within all
+ * zones.
+ */
+unsigned long vm_total_pages;
 
 static LIST_HEAD(shrinker_list);
 static DECLARE_RWSEM(shrinker_rwsem);