Re: [PATCH v6 30/33] mm: memcontrol: prepare for reparenting non-hierarchical stats

[Qi Zheng]

Hi Harry,

On 3/23/26 3:53 PM, Harry Yoo (Oracle) wrote:
> On Thu, Mar 05, 2026 at 07:52:48PM +0800, Qi Zheng wrote:
> > From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> >
> > To resolve the dying memcg issue, we need to reparent LRU folios of child
> > memcg to its parent memcg. This could cause problems for non-hierarchical
> > stats.
> >
> > As Yosry Ahmed pointed out:
> >
> > ```
> > In short, if memory is charged to a dying cgroup at the time of
> > reparenting, when the memory gets uncharged the stats updates will occur
> > at the parent. This will update both hierarchical and non-hierarchical
> > stats of the parent, which would corrupt the parent's non-hierarchical
> > stats (because those counters were never incremented when the memory was
> > charged).
> > ```
> >
> > Now we have the following two types of non-hierarchical stats, and they
> > are only used in CONFIG_MEMCG_V1:
> >
> > a. memcg->vmstats->state_local[i]
> > b. pn->lruvec_stats->state_local[i]
> >
> > To ensure that these non-hierarchical stats work properly, we need to
> > reparent these non-hierarchical stats after reparenting LRU folios. To
> > this end, this commit makes the following preparations:
> >
> > 1. implement reparent_state_local() to reparent non-hierarchical stats
> > 2. make css_killed_work_fn() to be called in rcu work, and implement
> >     get_non_dying_memcg_start() and get_non_dying_memcg_end() to avoid race
> >     between mod_memcg_state()/mod_memcg_lruvec_state()
> >     and reparent_state_local()
> >
> > Co-developed-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> > Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > ---
> >   kernel/cgroup/cgroup.c |  9 ++--
> >   mm/memcontrol-v1.c     | 16 +++++++
> >   mm/memcontrol-v1.h     |  7 +++
> >   mm/memcontrol.c        | 97 ++++++++++++++++++++++++++++++++++++++++++
> >   4 files changed, 125 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 23b70bd80ddc9..b0519a16f5684 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -473,6 +501,30 @@ unsigned long lruvec_page_state_local(struct lruvec *lruvec,
> >   	return x;
> >   }
> >   
> > +#ifdef CONFIG_MEMCG_V1
> > +static void __mod_memcg_lruvec_state(struct mem_cgroup_per_node *pn,
> > +				     enum node_stat_item idx, int val);
> > +
> > +void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg,
> > +				       struct mem_cgroup *parent, int idx)
> > +{
> > +	int nid;
> > +
> > +	for_each_node(nid) {
> > +		struct lruvec *child_lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
> > +		struct lruvec *parent_lruvec = mem_cgroup_lruvec(parent, NODE_DATA(nid));
> > +		unsigned long value = lruvec_page_state_local(child_lruvec, idx);
> > +		struct mem_cgroup_per_node *child_pn, *parent_pn;
> > +
> > +		child_pn = container_of(child_lruvec, struct mem_cgroup_per_node, lruvec);
> > +		parent_pn = container_of(parent_lruvec, struct mem_cgroup_per_node, lruvec);
> > +
> > +		__mod_memcg_lruvec_state(child_pn, idx, -value);
> > +		__mod_memcg_lruvec_state(parent_pn, idx, value);
>
> We should probably change the type of `@val` from int to val to avoid
> losing non hierarchical stats during reparenting?

The parameter and return value of memcg_state_val_in_pages() are both
of type int, so perhaps we need a cleanup patch to do this?

I will send a cleanup patchset to do this, which includes the following:

https://lore.kernel.org/all/5e178b4e-a9e0-44dc-a18d-8c014365ee2f@xxxxxxxxx/

Thanks,
Qi


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