page allocator: move check for disabled anti-fragmentation out of fas…

…tpath

On low-memory systems, anti-fragmentation gets disabled as there is
nothing it can do and it would just incur overhead shuffling pages between
lists constantly.  Currently the check is made in the free page fast path
for every page.  This patch moves it to a slow path.  On machines with low
memory, there will be small amount of additional overhead as pages get
shuffled between lists but it should quickly settle.

Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Christoph Lameter <[email protected]>
Reviewed-by: KOSAKI Motohiro <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/mmzone.h

@@ -50,9 +50,6 @@ extern int page_group_by_mobility_disabled;
 
 static inline int get_pageblock_migratetype(struct page *page)
 {
-	if (unlikely(page_group_by_mobility_disabled))
-		return MIGRATE_UNMOVABLE;
-
 	return get_pageblock_flags_group(page, PB_migrate, PB_migrate_end);
 }
 

mm/page_alloc.c

@@ -168,6 +168,10 @@ int page_group_by_mobility_disabled __read_mostly;
 
 static void set_pageblock_migratetype(struct page *page, int migratetype)
 {
+
+	if (unlikely(page_group_by_mobility_disabled))
+		migratetype = MIGRATE_UNMOVABLE;
+
 	set_pageblock_flags_group(page, (unsigned long)migratetype,
 					PB_migrate, PB_migrate_end);
 }