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

[Qi Zheng]

On 12/20/25 9:11 AM, Shakeel Butt wrote:
> 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:

OK, will do.

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

Thanks!