[patch] mm: memcontrol: clarify migration where old page is uncharged

[Johannes Weiner]

Better explain re-entrant migration when compaction races with
reclaim, and also mention swapcache readahead pages as possible
uncharged migration sources.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/memcontrol.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fc1d7ca96b9d..76892eb89d26 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6166,7 +6166,12 @@ 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;
-- 
2.1.2

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