[PATCH v2] mm: memcontrol: fix root_mem_cgroup charging

[Muchun Song]

The below scenario can cause the page counters of the root_mem_cgroup
to be out of balance.

CPU0:                                   CPU1:

objcg = get_obj_cgroup_from_current()
obj_cgroup_charge_pages(objcg)
                                        memcg_reparent_objcgs()
                                            // reparent to root_mem_cgroup
                                            WRITE_ONCE(iter->memcg, parent)
    // memcg == root_mem_cgroup
    memcg = get_mem_cgroup_from_objcg(objcg)
    // do not charge to the root_mem_cgroup
    try_charge(memcg)

obj_cgroup_uncharge_pages(objcg)
    memcg = get_mem_cgroup_from_objcg(objcg)
    // uncharge from the root_mem_cgroup
    refill_stock(memcg)
        drain_stock(memcg)
            page_counter_uncharge(&memcg->memory)

get_obj_cgroup_from_current() never returns a root_mem_cgroup's objcg,
so we never explicitly charge the root_mem_cgroup. And it's not
going to change. It's all about a race when we got an obj_cgroup
pointing at some non-root memcg, but before we were able to charge it,
the cgroup was gone, objcg was reparented to the root and so we're
skipping the charging. Then we store the objcg pointer and later use
to uncharge the root_mem_cgroup.

This can cause the page counter to be less than the actual value.
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@xxxxxxxxxxxxx>
---
Changeslog in v2:
 - Update commit log.
 - Rename __try_charge to try_charge_memcg.

 mm/memcontrol.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 64ada9e650a5..42dee9798ab8 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2504,8 +2504,8 @@ void mem_cgroup_handle_over_high(void)
 	css_put(&memcg->css);
 }
 
-static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
-		      unsigned int nr_pages)
+static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
+			unsigned int nr_pages)
 {
 	unsigned int batch = max(MEMCG_CHARGE_BATCH, nr_pages);
 	int nr_retries = MAX_RECLAIM_RETRIES;
@@ -2517,8 +2517,6 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	bool drained = false;
 	unsigned long pflags;
 
-	if (mem_cgroup_is_root(memcg))
-		return 0;
 retry:
 	if (consume_stock(memcg, nr_pages))
 		return 0;
@@ -2698,6 +2696,15 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	return 0;
 }
 
+static inline int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
+			     unsigned int nr_pages)
+{
+	if (mem_cgroup_is_root(memcg))
+		return 0;
+
+	return try_charge_memcg(memcg, gfp_mask, nr_pages);
+}
+
 #if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MMU)
 static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
@@ -2925,7 +2932,7 @@ static int obj_cgroup_charge_pages(struct obj_cgroup *objcg, gfp_t gfp,
 
 	memcg = get_mem_cgroup_from_objcg(objcg);
 
-	ret = try_charge(memcg, gfp, nr_pages);
+	ret = try_charge_memcg(memcg, gfp, nr_pages);
 	if (ret)
 		goto out;
 
-- 
2.11.0