Re: [PATCH RFC 11/12] mm/slab: add slab_needs_objcg() helper

On Wed, Jul 15, 2026 at 3:11 AM Vlastimil Babka (SUSE)
<[email protected]> wrote:
>
> Slabs of some caches never need the objcg part of struct slabobj_ext.
> Introduce a helper to query this for a slab.
>
> Currently only is_kmalloc_normal() caches are considered as not needing
> objcg's. We could also consider all kmem caches without SLAB_ACCOUNT,
> however some might be used with and without __GFP_ACCOUNT concurrently
> and we currently don't restrict that. This can be improved later.
>
> To make the evaluation of slab_needs_objcg() faster in the allocation
> and free fast paths, add a obj_exts_needs_objcg flag into slab itself.
> This optimization is only available on 64bit architectures where free
> bits are available for the flag.
>
> Signed-off-by: Vlastimil Babka (SUSE) <[email protected]>

Reviewed-by: Suren Baghdasaryan <[email protected]>

> ---
>  mm/kfence/core.c |  3 +++
>  mm/slab.h        | 31 +++++++++++++++++++++++++++++--
>  mm/slub.c        |  4 ++++
>  3 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 717e8baf7e5d..afeaf80484ad 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -640,6 +640,9 @@ static unsigned long kfence_init_pool(void)
>                 struct slab *slab = page_slab(page);
>                 slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
>                                  MEMCG_DATA_OBJEXTS;
> +#ifdef CONFIG_64BIT
> +               slab->obj_exts_needs_objcg = 1;
> +#endif
>  #endif
>         }
>
> diff --git a/mm/slab.h b/mm/slab.h
> index a50347c9dbe3..948d075cdbef 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -81,10 +81,11 @@ struct freelist_counters {
>  #ifdef CONFIG_64BIT
>                                         /*
>                                          * Some optimizations use free bits in 'counters' field
> -                                        * to save memory. If these free bits are not available,
> -                                        * such optimizations are disabled.
> +                                        * to save memory or CPU. If these free bits are not
> +                                        * available, such optimizations are disabled.
>                                          */
>                                         unsigned obj_exts_in_object:1;
> +                                       unsigned obj_exts_needs_objcg:1;
>  #endif
>                                 };
>                         };
> @@ -580,6 +581,32 @@ static inline bool slab_obj_ext_has_codetag(void)
>  }
>  #endif
>
> +#ifdef CONFIG_MEMCG
> +static inline bool cache_needs_objcg(struct kmem_cache *cache)
> +{
> +       return !is_kmalloc_normal(cache);
> +}
> +
> +static inline bool slab_needs_objcg(struct slab *slab)
> +{
> +#ifdef CONFIG_64BIT
> +       return slab->obj_exts_needs_objcg;
> +#else
> +       return cache_needs_objcg(slab->slab_cache);
> +#endif
> +}
> +#else
> +static inline bool cache_needs_objcg(struct kmem_cache *cache)
> +{
> +       return false;
> +}
> +
> +static inline bool slab_needs_objcg(struct slab *slab)
> +{
> +       return false;
> +}
> +#endif
> +
>  static inline size_t static_obj_ext_size(void)
>  {
>         size_t sz = 0;
> diff --git a/mm/slub.c b/mm/slub.c
> index 4200e7105b30..771d73abacb6 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3426,6 +3426,10 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags,
>
>         slab->objects = oo_objects(oo);
>
> +#ifdef CONFIG_64BIT
> +       if (cache_needs_objcg(s))
> +               slab->obj_exts_needs_objcg = 1;
> +#endif
>         slab->slab_cache = s;
>
>         kasan_poison_slab(slab);
>
> --
> 2.55.0
>