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1/29/26 10:10 AM, Harry Yoo wrote: > > On Mon, Jan 19, 2026 at 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 > > Good catch, I think you are right. > > The rcu lock should be added to mod_memcg_state() and > mod_memcg_lruvec_state(). Thanks for confirming! Because it's quite confusing, let me ask few more questions... Q1. Yosry mentioned [1] [2] that stat updates should be done in the same RCU section that is used to grab a refcount of the cgroup. But I don't think your work is relying on that. Instead, I guess, it's relying on the CSS_DYING check from reader side to determine whether it should update stats of the child or parent memcg, right? -> That being said, when rcu_read_lock() is called _after_ stats are reparented, the reader must observe that the CSS_DYING flag is set. [1] https://lore.kernel.org/all/utl6esq7jz5e4f7kwgrpwdjc2rm3yi33ljb6xkm5nxzufa4o7s@hblq2piu3vnz [2] https://lore.kernel.org/all/ebdhvcwygvnfejai5azhg3sjudsjorwmlcvmzadpkhexoeq3tb@5gj5y2exdhpn Q2. When a reader checks CSS_DYING flag, how is the flag change guaranteed to be visible to the reader without any lock, memory barrier, or atomic ops involved? As Shakeel mentioned elsewhere, I hope some explanations for correctness to be included in the commit message :) > I will update to v4 as soon as possible. Thanks a lot! I'll wait for that and will review carefully to make sure it's correct ;) > Thanks, > Qi > > > > > 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