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11:34:53AM +0800, Qi Zheng wrote: > > > On 1/18/26 11:20 AM, Shakeel Butt wrote: > > On Wed, Jan 14, 2026 at 07:32:55PM +0800, Qi Zheng wrote: > > > From: Qi Zheng > > > > > > To resolve the dying memcg issue, we need to reparent LRU folios of child > > > memcg to its parent memcg. The following counts are all non-hierarchical > > > and need to be reparented to prevent the counts of parent memcg overflow. > > > > > > 1. memcg->vmstats->state_local[i] > > > 2. pn->lruvec_stats->state_local[i] > > > > > > This commit implements the specific function, which will be used during > > > the reparenting process. > > > > Please add more explanation which was discussed in the email chain at > > https://lore.kernel.org/all/5dsb6q2r4xsi24kk5gcnckljuvgvvp6nwifwvc4wuho5hsifeg@5ukg2dq6ini5/ > > OK, will do. > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 70583394f421f..7aa32b97c9f17 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -225,6 +225,28 @@ static inline struct obj_cgroup *__memcg_reparent_objcgs(struct mem_cgroup *memc > > > return objcg; > > > } > > > +#ifdef CONFIG_MEMCG_V1 > > > +static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force); > > > + > > > +static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgroup *parent) > > > +{ > > > + if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) > > > + return; > > > + > > > + synchronize_rcu(); > > > > Hmm synchrinuze_rcu() is a heavy hammer here. Also you would need rcu > > read lock in mod_memcg_state() & mod_memcg_lruvec_state() for this > > synchronize_rcu(). > > Since these two functions require memcg or lruvec, they are already > within the critical section of the RCU lock. What happens if someone grabbed a refcount and then release the rcu read lock before percpu refkill and then call mod_memcg[_lruvec]_state()? In this case, can we end up reparenting in the middle of non-hierarchical stat update because they don't have RCU grace period? Something like T1 T2 - rcu_read_lock() - get memcg refcnt - rcu_read_unlock() - call mod_memcg_state() - CSS_IS_DYING is not set - Set CSS_IS_DYING - Trigger percpu refkill - Trigger offline_css() -> reparent non-hierarchical - update non-hierarchical stats stats - put memcg refcount > > Hmm instead of synchronize_rcu() here, we can use queue_rcu_work() in > > css_killed_ref_fn(). It would be as simple as the following: > > It does look much simpler, will do. > > Thanks, > Qi -- Cheers, Harry / Hyeonggon