Re: [PATCH] mm: memcontrol: treat disabled memcg as kmem accounting disabled

[Michal Hocko <mhocko@suse.com>]

On Fri 28-08-26 08:47:54, Hao Li wrote:
> On Thu, Aug 27, 2026 at 02:04:13PM +0200, Michal Hocko wrote:
> > On Thu 27-08-26 17:17:50, Hao Li wrote:
> > > mem_cgroup_kmem_disabled() currently only checks whether the
> > > "cgroup.memory=nokmem" option is specified. However, kmem accounting is
> > > also unavailable when memcg itself is disabled.
> > > 
> > > Check both conditions to ensure the function accurately reflects the
> > > kmem accounting state.
> > 
> > It would be really great if you could describe how we could end up with
> > the inconsistent memcg enabled but kmem enabled and what kind of effect
> > does this have.
> 
> Yes, thanks for point out this.
> 
> > 
> > AFAICS the inconsistency is possible and it would lead some wastage but
> > no functional problems but the changelog should be more descriptive.
> 
> Exactly! The most direct benefit is that when memcg is disabled,
> new_kmalloc_cache() will not need to create a separate `KMALLOC_CGROUP` slub
> cache, but can simply alias it to `KMALLOC_NORMAL`. This avoids wastage.

This is definitely important detail to mention in the chagelog. Same as
the effect on the __list_lru_init and other callers. TBH I am no longer
100% sure this is correct. You need to explain more why this is just
wastage rathe than a subtle side effect that is desirable.

> If this sounds reasonable, I would be happy to explain it in more detail
> in v2.
> 
> > 
> > > Signed-off-by: Hao Li <hao.li@linux.dev>
> > > ---
> > >  mm/memcontrol.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 1ebceade4021..b28f6165c354 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -132,7 +132,7 @@ static DEFINE_SPINLOCK(objcg_lock);
> > >  
> > >  bool mem_cgroup_kmem_disabled(void)
> > >  {
> > > -	return cgroup_memory_nokmem;
> > > +	return cgroup_memory_nokmem || mem_cgroup_disabled();
> > >  }
> > >  
> > >  static void memcg_uncharge(struct mem_cgroup *memcg, unsigned int nr_pages);
> > > -- 
> > > 2.54.0
> > 
> > -- 
> > Michal Hocko
> > SUSE Labs
> 
> -- 
> Thanks,
> Hao

-- 
Michal Hocko
SUSE Labs