[PATCH] mm, compaction: create compact_gap wrapper-fix

[Vlastimil Babka]

Clarify the comment of compact_gap() wrt COMPACT_CLUSTER_MAX, per Joonsoo.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 include/linux/compaction.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index e7f0d34a90fe..585d55cb0dc0 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -67,9 +67,16 @@ static inline unsigned long compact_gap(unsigned int order)
 {
 	/*
 	 * Although all the isolations for migration are temporary, compaction
-	 * 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.
+	 * 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;
 }
-- 
2.9.2