Re: [PATCH v2 13/28] mm: migrate: prevent memory cgroup release in folio_migrate_mapping()

[Qi Zheng]

On 12/18/25 10:26 PM, Johannes Weiner wrote:
> On Thu, Dec 18, 2025 at 10:09:21AM +0100, David Hildenbrand (Red Hat) wrote:
> > On 12/17/25 08:27, 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 folio_migrate_mapping().
> >
> > We usually avoid talking about "patches".
> >
> > In __folio_migrate_mapping(), the rcu read lock ...
> >
> > > 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>
> > > Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > > ---
> > >    mm/migrate.c | 2 ++
> > >    1 file changed, 2 insertions(+)
> > >
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 5169f9717f606..8bcd588c083ca 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -671,6 +671,7 @@ static int __folio_migrate_mapping(struct address_space *mapping,
> > >    		struct lruvec *old_lruvec, *new_lruvec;
> > >    		struct mem_cgroup *memcg;
> > >    
> > > +		rcu_read_lock();
> > >    		memcg = folio_memcg(folio);
> >
> > In general, LGTM
> >
> > I wonder, though, whether we should embed that in the ABI.
> >
> > Like "lock RCU and get the memcg" in one operation, to the "return memcg
> > and unock rcu" in another operation.
> >
> > Something like "start / end" semantics.
>
> The advantage of open-coding this particular one is that 1)
> rcu_read_lock() is something the caller could already be
> holding/using, implicitly or explicitly; and 2) it's immediately
> obvious that this is an atomic section (which was already useful in
> spotting a bug in the workingset patch of this series).
>
> "start/end" terminology hides this. "lock" we can't use because it
> would suggest binding stability. The only other idea I'd have would be
> to spell it all out:
>
> memcg = folio_memcg_rcu_read_lock(folio);
> stuff(memcg);
> otherstuff();
> rcu_read_unlock();
>
> But that might not be worth it. Maybe somebody can think of a better
> name. But I'd be hesitant to trade off the obviousness of what's going
> on given how simple the locking + access scheme is.

Agree. I also prefer to keep the open-coding method for now, and if a
better helper is available later, a cleanup patch can be added to
accomplish this.

Thanks,
Qi