[PATCH] mm: memcontrol: fix root_mem_cgroup charging

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

CPU0:                                   CPU1:

objcg = get_obj_cgroup_from_current();
obj_cgroup_charge(objcg);
                                        memcg_reparent_objcgs();
                                            xchg(&objcg->memcg, root_mem_cgroup);
    // memcg == root_mem_cgroup
    memcg = obj_cgroup_memcg(objcg);
    __memcg_kmem_charge(memcg);
        // Do not charge to the root memcg
        try_charge(memcg);

If the objcg->memcg is reparented to the root_mem_cgroup,
obj_cgroup_charge() can pass root_mem_cgroup as the first
parameter to here. The root_mem_cgroup is skipped in the
try_charge(). So the page counters of it do not update.

When we uncharge this, we will decrease the page counters
(e.g. memory and memsw) of the root_mem_cgroup. This will
cause the page counters of the root_mem_cgroup to be out
of balance. Fix it by charging the page to the
root_mem_cgroup unconditional.

Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2db2aeac8a9e..edf604824d63 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3078,6 +3078,19 @@ static int __memcg_kmem_charge(struct mem_cgroup *memcg, gfp_t gfp,
 	if (ret)
 		return ret;
 
+	/*
+	 * If the objcg->memcg is reparented to the root_mem_cgroup,
+	 * obj_cgroup_charge() can pass root_mem_cgroup as the first
+	 * parameter to here. We should charge the page to the
+	 * root_mem_cgroup unconditional to keep it's page counters
+	 * balance.
+	 */
+	if (unlikely(mem_cgroup_is_root(memcg))) {
+		page_counter_charge(&memcg->memory, nr_pages);
+		if (do_memsw_account())
+			page_counter_charge(&memcg->memsw, nr_pages);
+	}
+
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) &&
 	    !page_counter_try_charge(&memcg->kmem, nr_pages, &counter)) {
 
-- 
2.11.0