zsmalloc: partial page ordering within a fullness_list

We want to see more ZS_FULL pages and less ZS_ALMOST_{FULL, EMPTY}
pages.  Put a page with higher ->inuse count first within its
->fullness_list, which will give us better chances to fill up this page
with new objects (find_get_zspage() return ->fullness_list head for new
object allocation), so some zspages will become ZS_ALMOST_FULL/ZS_FULL
quicker.

It performs a trivial and cheap ->inuse compare which does not slow down
zsmalloc and in the worst case keeps the list pages in no particular
order.

A more expensive solution could sort fullness_list by ->inuse count.

[minchan@kernel.org: code adjustments]
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -643,13 +643,22 @@ static void insert_zspage(struct page *page, struct size_class *class,
 	if (fullness >= _ZS_NR_FULLNESS_GROUPS)
 		return;
 
-	head = &class->fullness_list[fullness];
-	if (*head)
-		list_add_tail(&page->lru, &(*head)->lru);
-
-	*head = page;
 	zs_stat_inc(class, fullness == ZS_ALMOST_EMPTY ?
 			CLASS_ALMOST_EMPTY : CLASS_ALMOST_FULL, 1);
+
+	head = &class->fullness_list[fullness];
+	if (!*head) {
+		*head = page;
+		return;
+	}
+
+	/*
+	 * We want to see more ZS_FULL pages and less almost
+	 * empty/full. Put pages with higher ->inuse first.
+	 */
+	list_add_tail(&page->lru, &(*head)->lru);
+	if (page->inuse >= (*head)->inuse)
+		*head = page;
 }
 
 /*