Re: [PATCH 2/2] mm/memcg: migrate per-node hugetlb lruvec stat together with hugetlb folio

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Tue, 22 Sep 2026 06:15:07 +0200 "Oscar Salvador (SUSE)" <osalvador@kernel.org> wrote:

> On Mon, Sep 21, 2026 at 05:12:35PM +0800, Hongfu Li wrote:
> > From: Hongfu Li <lihongfu@kylinos.cn>
> > 
> > memory.numa_stat exposes per-node hugetlb counters from per-node lruvec
> > stats. These stats are accounted against folio_nid(): incremented on
> > the folio's node when handed to a user, decremented when the folio is
> > returned to the pool.
> > 
> > During hugetlb folio migration, mem_cgroup_migrate() moves the charge
> > to the new folio and drops the memcg data of the old one, so the free
> > of the old folio right after the migration skips the memcg per-node
> > lruvec decrement. The hugetlb count stays attributed to the old node
> > for the rest of the life of the charge, while the target folio gets no
> > increment on the new node; its later free decrements a counter that
> > was never incremented.
> > 
> > Migrate the per-node lruvec accounting alongside migration. Global
> > memcg totals remain balanced because they track resource consumption,
> > not node placement.
> > 
> > Fixes: 05d4532b60e3 ("memcg/hugetlb: add hugeTLB counters to memcg")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
> 
> For the fix itself:
> 
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> 
> question below:
> 
> > ---
> >  mm/memcontrol.c | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 1460cba53588..9c96ebd5436f 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5598,6 +5598,34 @@ void mem_cgroup_replace_folio(struct folio *old, struct folio *new)
> >  	rcu_read_unlock();
> >  }
> >  
> > +#ifdef CONFIG_HUGETLB_PAGE
> > +static void move_hugetlb_lruvec_stat(struct obj_cgroup *objcg,
> > +				     struct folio *old, struct folio *new)
> > +{
> > +	long nr_pages = folio_nr_pages(old);
> > +	struct mem_cgroup *memcg;
> > +	int old_nid = folio_nid(old);
> > +	int new_nid = folio_nid(new);
> > +
> > +	if (old_nid == new_nid)
> > +		return;
> > +
> > +	rcu_read_lock();
> > +	memcg = obj_cgroup_memcg(objcg);
> > +	mod_memcg_lruvec_state(mem_cgroup_lruvec(memcg, NODE_DATA(old_nid)),
> > +			       NR_HUGETLB, -nr_pages);
> > +	mod_memcg_lruvec_state(mem_cgroup_lruvec(memcg, NODE_DATA(new_nid)),
> > +			       NR_HUGETLB, nr_pages);
> > +	rcu_read_unlock();
> 
> Why do we need the whole thing to be embraced by rcu?

Hi Oscar,

I believe it's because now getting the memcg from the objcg requires
an RCU lock to make sure it doesn't get removed while we work on the
memcg. I think this is since Qi Zheng's "Eliminate Dying Memory
Cgroup" series.

Joshua