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Subject[PATCH] mm: memcontrol: fix root_mem_cgroup charging
Date
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
page_counter_uncharge(&memcg->memory)

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

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1e68a9992b01..81b54bd9b9e0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2686,8 +2686,8 @@ 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(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;
@@ -2699,8 +2699,6 @@ 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;
@@ -2880,6 +2878,15 @@ 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, gfp_mask, nr_pages);
+}
+
#if defined(CONFIG_MEMCG_KMEM) || defined(CONFIG_MMU)
static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
{
@@ -3125,7 +3132,7 @@ 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, gfp, nr_pages);
if (ret)
goto out;

--
2.11.0
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