[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 <***@cmpxchg.org>
---
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;

[Michal Hocko]

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