mm: memcontrol: clarify migration where old page is uncharged

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@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 1 deletions

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;