Re: [PATCH] memcg: skip cgroup_file_notify if spinning is not allowed

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Sep 08, 2025 at 11:28:20AM +0200, Michal Hocko wrote:
> On Fri 05-09-25 13:16:06, Shakeel Butt wrote:
> > Generally memcg charging is allowed from all the contexts including NMI
> > where even spinning on spinlock can cause locking issues. However one
> > call chain was missed during the addition of memcg charging from any
> > context support. That is try_charge_memcg() -> memcg_memory_event() ->
> > cgroup_file_notify().
> > 
> > The possible function call tree under cgroup_file_notify() can acquire
> > many different spin locks in spinning mode. Some of them are
> > cgroup_file_kn_lock, kernfs_notify_lock, pool_workqeue's lock. So, let's
> > just skip cgroup_file_notify() from memcg charging if the context does
> > not allow spinning.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks.

> 
> > ---
> >  include/linux/memcontrol.h | 23 ++++++++++++++++-------
> >  mm/memcontrol.c            |  7 ++++---
> >  2 files changed, 20 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 9dc5b52672a6..054fa34c936a 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -993,22 +993,25 @@ static inline void count_memcg_event_mm(struct mm_struct *mm,
> >  	count_memcg_events_mm(mm, idx, 1);
> >  }
> >  
> > -static inline void memcg_memory_event(struct mem_cgroup *memcg,
> > -				      enum memcg_memory_event event)
> > +static inline void __memcg_memory_event(struct mem_cgroup *memcg,
> > +					enum memcg_memory_event event,
> > +					bool allow_spinning)
> >  {
> >  	bool swap_event = event == MEMCG_SWAP_HIGH || event == MEMCG_SWAP_MAX ||
> >  			  event == MEMCG_SWAP_FAIL;
> >  
> >  	atomic_long_inc(&memcg->memory_events_local[event]);
> 
> Doesn't this involve locking on 32b? I guess we do not care all that
> much but we might want to bail out early on those arches for
> !allow_spinning
> 

I am prototyping irq_work based approach and if that looks good then we
might not need to worry about 32b at all.