mm, mmzone: clarify the usage of zone padding

Zone padding separates write-intensive fields used by page allocation,
compaction and vmstats but the comments are a little misleading and need
clarification.

Link: http://lkml.kernel.org/r/1467970510-21195-5-git-send-email-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -477,20 +477,21 @@ struct zone {
 	unsigned long		wait_table_hash_nr_entries;
 	unsigned long		wait_table_bits;
 
+	/* Write-intensive fields used from the page allocator */
 	ZONE_PADDING(_pad1_)
+
 	/* free areas of different sizes */
 	struct free_area	free_area[MAX_ORDER];
 
 	/* zone flags, see below */
 	unsigned long		flags;
 
-	/* Write-intensive fields used from the page allocator */
+	/* Primarily protects free_area */
 	spinlock_t		lock;
 
+	/* Write-intensive fields used by compaction and vmstats. */
 	ZONE_PADDING(_pad2_)
 
-	/* Write-intensive fields used by page reclaim */
-
 	/*
 	 * When free pages are below this point, additional steps are taken
 	 * when reading the number of free pages to avoid per-cpu counter