Re: [PATCH v1 07/26] mm: memcontrol: prevent memory cgroup release in get_mem_cgroup_from_folio()

[Qi Zheng]

On 11/19/25 4:06 PM, Harry Yoo wrote:
> On Tue, Oct 28, 2025 at 09:58:20PM +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 get_mem_cgroup_from_folio().
> >
> > 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>
> > ---
>
> This patch looks correct to me, so:
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
>
> with a few side question on current implementation rather than
> this change...
>
> >   mm/memcontrol.c | 11 ++++++++---
> >   1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index d484b632c790f..1da3ad77054d3 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -973,14 +973,19 @@ struct mem_cgroup *get_mem_cgroup_from_current(void)
> >    */
> >   struct mem_cgroup *get_mem_cgroup_from_folio(struct folio *folio)
> >   {
> > -	struct mem_cgroup *memcg = folio_memcg(folio);
> > +	struct mem_cgroup *memcg;
> >   
> >   	if (mem_cgroup_disabled())
> >   		return NULL;
> >   
> > +	if (!folio_memcg_charged(folio))
> > +		return root_mem_cgroup;
> > +
> >   	rcu_read_lock();
> > -	if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
>
> I have no idea why we're already calling rcu_read_lock() when a folio pins
> the memcg :)

+1, this was introduced by 
https://github.com/torvalds/linux/commit/f77f0c7514789577125c1b2df145703161736359,
may be some defensive programming.

> > -		memcg = root_mem_cgroup;
> > +retry:
> > +	memcg = folio_memcg(folio);
> > +	if (unlikely(!css_tryget(&memcg->css)))
> > +		goto retry;
> >   	rcu_read_unlock();
> >   	return memcg;
> >   }
> > --
> > 2.20.1