mm, page_alloc: simplify list handling in rmqueue_bulk()

rmqueue_bulk() fills an empty pcplist with pages from the free list.  It
tries to preserve increasing order by pfn to the caller, because it
leads to better performance with some I/O controllers, as explained in
commit e084b2d95e48 ("page-allocator: preserve PFN ordering when
__GFP_COLD is set").

To preserve the order, it's sufficient to add pages to the tail of the
list as they are retrieved.  The current code instead adds to the head
of the list, but then updates the list head pointer to the last added
page, in each step.  This does result in the same order, but is
needlessly confusing and potentially wasteful, with no apparent benefit.
This patch simplifies the code and adjusts comment accordingly.

Link: http://lkml.kernel.org/r/f6505442-98a9-12e4-b2cd-0fa83874c159@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Jan Kara <jack@suse.cz>
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/page_alloc.c

@@ -2350,16 +2350,16 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 			continue;
 
 		/*
-		 * Split buddy pages returned by expand() are received here
-		 * in physical page order. The page is added to the callers and
-		 * list and the list head then moves forward. From the callers
-		 * perspective, the linked list is ordered by page number in
-		 * some conditions. This is useful for IO devices that can
-		 * merge IO requests if the physical pages are ordered
-		 * properly.
+		 * Split buddy pages returned by expand() are received here in
+		 * physical page order. The page is added to the tail of
+		 * caller's list. From the callers perspective, the linked list
+		 * is ordered by page number under some conditions. This is
+		 * useful for IO devices that can forward direction from the
+		 * head, thus also in the physical page order. This is useful
+		 * for IO devices that can merge IO requests if the physical
+		 * pages are ordered properly.
 		 */
-		list_add(&page->lru, list);
-		list = &page->lru;
+		list_add_tail(&page->lru, list);
 		alloced++;
 		if (is_migrate_cma(get_pcppage_migratetype(page)))
 			__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,