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From:   Muchun Song <songmuchun@bytedance.com>
To:     guro@fb.com, hannes@cmpxchg.org, mhocko@kernel.org,
        akpm@linux-foundation.org, shakeelb@google.com,
        vdavydov.dev@gmail.com
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, Muchun Song <songmuchun@bytedance.com>
Subject: [RFC PATCH 01/15] mm: memcontrol: fix page charging in page replacement
Date:   Tue, 30 Mar 2021 18:15:17 +0800
Message-Id: <20210330101531.82752-2-songmuchun@bytedance.com>
In-Reply-To: <20210330101531.82752-1-songmuchun@bytedance.com>
References: <20210330101531.82752-1-songmuchun@bytedance.com>
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The pages aren't accounted at the root level, so do not charge the page
to the root memcg in page replacement. Although we do not display the
value (mem_cgroup_usage) so there shouldn't be any actual problem, but
there is a WARN_ON_ONCE in the page_counter_cancel(). Who knows if it
will trigger? So it is better to fix it.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memcontrol.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 249bf6b4d94c..d0c4f6e91e17 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6936,9 +6936,11 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
 	/* Force-charge the new page. The old one will be freed soon */
 	nr_pages = thp_nr_pages(newpage);
 
-	page_counter_charge(&memcg->memory, nr_pages);
-	if (do_memsw_account())
-		page_counter_charge(&memcg->memsw, nr_pages);
+	if (!mem_cgroup_is_root(memcg)) {
+		page_counter_charge(&memcg->memory, nr_pages);
+		if (do_memsw_account())
+			page_counter_charge(&memcg->memsw, nr_pages);
+	}
 
 	css_get(&memcg->css);
 	commit_charge(newpage, memcg);
-- 
2.11.0