+/*
+ * Number of free order-0 pages that should be available above given watermark
+ * to make sure compaction has reasonable chance of not running out of free
+ * pages that it needs to isolate as migration target during its work.
+ */
+static inline unsigned long compact_gap(unsigned int order)
+{
+ /*
+ * Although all the isolations for migration are temporary, compaction
+ * free scanner may have up to 1 << order pages on its list and then
+ * try to split an (order - 1) free page. At that point, a gap of
+ * 1 << order might not be enough, so it's safer to require twice that
+ * amount. Note that the number of pages on the list is also
+ * effectively limited by COMPACT_CLUSTER_MAX, as that's the maximum
+ * that the migrate scanner can have isolated on migrate list, and free
+ * scanner is only invoked when the number of isolated free pages is
+ * lower than that. But it's not worth to complicate the formula here
+ * as a bigger gap for higher orders than strictly necessary can also
+ * improve chances of compaction success.
+ */
+ return 2UL << order;
+}
+