diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b495f29d4746..a0ae64ca55bf 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6157,7 +6157,12 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
 	if (PageCgroupUsed(pc))
 		return;
 
-	/* Re-entrant migration: old page already uncharged? */
+	/*
+	 * Swapcache readahead pages can get migrated before being
+	 * charged, and migration from compaction can happen to an
+	 * uncharged page when the PFN walker finds a page that
+	 * reclaim just put back on the LRU but has not released yet.
+	 */
 	pc = lookup_page_cgroup(oldpage);
 	if (!PageCgroupUsed(pc))
 		return;