Re: [PATCH v2 17/28] mm: thp: prevent memory cgroup release in folio_split_queue_lock{_irqsave}()

[Shakeel Butt]

On Wed, Dec 17, 2025 at 05:27:17PM -0500, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:41PM +0800, Qi Zheng wrote:
> > From: Qi Zheng <zhengqi.arch@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_split_queue_lock{_irqsave}().
> > 
> > Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> > Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > ---
> >  mm/huge_memory.c | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 12b46215b30c1..b9e6855ec0b6a 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1154,13 +1154,25 @@ split_queue_lock_irqsave(int nid, struct mem_cgroup *memcg, unsigned long *flags
> >  
> >  static struct deferred_split *folio_split_queue_lock(struct folio *folio)
> >  {
> > -	return split_queue_lock(folio_nid(folio), folio_memcg(folio));
> > +	struct deferred_split *queue;
> > +
> > +	rcu_read_lock();
> > +	queue = split_queue_lock(folio_nid(folio), folio_memcg(folio));
> > +	rcu_read_unlock();
> 
> Ah, the memcg destruction path is acquiring the split queue lock for
> reparenting. Once you have it locked, it's safe to drop the rcu lock.

Qi, please add the above explanation in a comment and with that:

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>