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

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

[Hao Li]

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.

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

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

[Michal Hocko]

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.

AFAICS the inconsistency is possible and it would lead some wastage but
no functional problems but the changelog should be more descriptive.

> 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

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

[Hao Li]

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.

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

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

[Michal Hocko]

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