Re: [RFC PATCH V3 4/7] mm/slab: use stride to access slabobj_ext

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Oct 27, 2025 at 5:29 AM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> Use a configurable stride value when accessing slab object extension
> metadata instead of assuming a fixed sizeof(struct slabobj_ext).
>
> Store stride value in free bits of slab->counters field. This allows
> for flexibility in cases where the extension is embedded within
> slab objects.
>
> Since these free bits exist only on 64-bit, any future optimizations
> that need to change stride value cannot be enabled on 32-bit architectures.
>
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>

I hope slab_obj_exts() can be removed in the next revision, but otherwise LGTM.

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/slab.h | 37 +++++++++++++++++++++++++++++++++----
>  mm/slub.c |  2 ++
>  2 files changed, 35 insertions(+), 4 deletions(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index df2c987d950d..22ee28cb55e1 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -83,6 +83,14 @@ struct slab {
>                                                          * that the slab was corrupted
>                                                          */
>                                                         unsigned frozen:1;
> +#ifdef CONFIG_64BIT
> +                                                       /*
> +                                                        * Some optimizations use free bits in 'counters' field
> +                                                        * to save memory. In case ->stride field is not available,
> +                                                        * such optimizations are disabled.
> +                                                        */
> +                                                       unsigned short stride;
> +#endif
>                                                 };
>                                         };
>                                 };
> @@ -550,6 +558,26 @@ static inline unsigned long slab_obj_exts(struct slab *slab)
>         return obj_exts & ~OBJEXTS_FLAGS_MASK;
>  }
>
> +#ifdef CONFIG_64BIT
> +static inline void slab_set_stride(struct slab *slab, unsigned short stride)
> +{
> +       slab->stride = stride;
> +}
> +static inline unsigned short slab_get_stride(struct slab *slab)
> +{
> +       return slab->stride;
> +}
> +#else
> +static inline void slab_set_stride(struct slab *slab, unsigned short stride)
> +{
> +       VM_WARN_ON_ONCE(stride != sizeof(struct slabobj_ext));
> +}
> +static inline unsigned short slab_get_stride(struct slab *slab)
> +{
> +       return sizeof(struct slabobj_ext);
> +}
> +#endif
> +
>  /*
>   * slab_obj_ext - get the pointer to the slab object extension metadata
>   * associated with an object in a slab.
> @@ -563,13 +591,10 @@ static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
>                                                unsigned long obj_exts,
>                                                unsigned int index)
>  {
> -       struct slabobj_ext *obj_ext;
> -
>         VM_WARN_ON_ONCE(!slab_obj_exts(slab));
>         VM_WARN_ON_ONCE(obj_exts != slab_obj_exts(slab));
>
> -       obj_ext = (struct slabobj_ext *)obj_exts;
> -       return &obj_ext[index];
> +       return (struct slabobj_ext *)(obj_exts + slab_get_stride(slab) * index);
>  }
>
>  int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> @@ -588,6 +613,10 @@ static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
>         return NULL;
>  }
>
> +static inline void slab_set_stride(struct slab *slab, unsigned int stride) { }
> +static inline unsigned int slab_get_stride(struct slab *slab) { return 0; }
> +
> +
>  #endif /* CONFIG_SLAB_OBJ_EXT */
>
>  static inline enum node_stat_item cache_vmstat_idx(struct kmem_cache *s)
> diff --git a/mm/slub.c b/mm/slub.c
> index ae73403f8c29..4383740a4d34 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2134,6 +2134,8 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  #endif
>         old_exts = READ_ONCE(slab->obj_exts);
>         handle_failed_objexts_alloc(old_exts, vec, objects);
> +       slab_set_stride(slab, sizeof(struct slabobj_ext));
> +
>         if (new_slab) {
>                 /*
>                  * If the slab is brand new and nobody can yet access its
> --
> 2.43.0
>