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

[Johannes Weiner]

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.