[PATCH] mm, page_alloc: simplify list handling in rmqueue_bulk()

[Vlastimil Babka]

The rmqueue_bulk() function 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
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.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3e60d8a37e3f..4e1b3c686cc2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2330,16 +2330,16 @@ 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,
-- 
2.14.2