mm/memory_hotplug: document why shuffle_zone() is relevant

It's not completely obvious why we have to shuffle the complete zone -
introduced in commit e900a91 ("mm: shuffle initial free memory to
improve memory-side-cache utilization") - because some sort of shuffling
is already performed when onlining pages via __free_one_page(), placing
MAX_ORDER-1 pages either to the head or the tail of the freelist.  Let's
document why we have to shuffle the complete zone when exposing larger,
contiguous physical memory areas to the buddy.

Signed-off-by: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Dan Williams <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Michal Hocko <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: davidhildenbrand

mm/memory_hotplug.c

@@ -831,6 +831,14 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
 	zone->zone_pgdat->node_present_pages += onlined_pages;
 	pgdat_resize_unlock(zone->zone_pgdat, &flags);
 
+	/*
+	 * When exposing larger, physically contiguous memory areas to the
+	 * buddy, shuffling in the buddy (when freeing onlined pages, putting
+	 * them either to the head or the tail of the freelist) is only helpful
+	 * for maintaining the shuffle, but not for creating the initial
+	 * shuffle. Shuffle the whole zone to make sure the just onlined pages
+	 * are properly distributed across the whole freelist.
+	 */
 	shuffle_zone(zone);
 
 	node_states_set_node(nid, &arg);