mm: fix page allocation for larger I/O segments

In some cases the IO subsystem is able to merge requests if the pages are
adjacent in physical memory. This was achieved in the allocator by having
expand() return pages in physically contiguous order in situations were a
large buddy was split. However, list-based anti-fragmentation changed the
order pages were returned in to avoid searching in buffered_rmqueue() for a
page of the appropriate migrate type.

This patch restores behaviour of rmqueue_bulk() preserving the physical
order of pages returned by the allocator without incurring increased search
costs for anti-fragmentation.

Signed-off-by: Mel Gorman <[e-mail]>
Cc: James Bottomley <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Mark Lord <[e-mail]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/page_alloc.c

···
 		struct page *page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
+
+		/*
+		 * 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.
+		 */
 		list_add(&page->lru, list);
 		set_page_private(page, migratetype);
+		list = &page->lru;
 	}
 	spin_unlock(&zone->lock);
 	return i;