Re: [PATCH v1 13/26] mm: mglru: prevent memory cgroup release in mglru

[Qi Zheng]

On 11/19/25 6:13 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:26PM +0800, Qi Zheng wrote:
> > From: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >
> > In the near future, a folio will no longer pin its corresponding
> > memory cgroup. To ensure safety, it will only be appropriate to
> > hold the rcu read lock or acquire a reference to the memory cgroup
> > returned by folio_memcg(), thereby preventing it from being released.
> >
> > In the current patch, the rcu read lock is employed to safeguard
> > against the release of the memory cgroup in mglru.
> >
> > This serves as a preparatory measure for the reparenting of the
> > LRU pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > ---
> >   mm/vmscan.c | 23 +++++++++++++++++------
> >   1 file changed, 17 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 660cd40cfddd4..676e6270e5b45 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -4279,6 +4288,8 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
> >   
> >   	arch_leave_lazy_mmu_mode();
> >   
> > +	rcu_read_unlock();
> > +
> >   	/* feedback from rmap walkers to page table walkers */
> >   	if (mm_state && suitable_to_scan(i, young))
> >   		update_bloom_filter(mm_state, max_seq, pvmw->pmd);
>
> mm_state has the same life cycle as mem_cgroup. So it should be
> protected by rcu read lock?

You are right. The mm_state is defined as follows:

struct lruvec {
        struct lru_gen_mm_state		mm_state;
};

will fix it in the next version.

Thanks,
Qi

>