Re: [PATCH RFC 04/12] mm/slab: make slab_obj_ext() determine object index

[Suren Baghdasaryan]

On Wed, Jul 15, 2026 at 3:11 AM Vlastimil Babka (SUSE)
<vbabka@xxxxxxxxxx> wrote:
>
> All callers perform the same obj_to_index() calculation to pass the
> index. Simplify by passing object pointer instead and determining the
> index by slab_obj_ext().
>
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  mm/memcontrol.c | 12 +++---------
>  mm/slab.h       | 14 ++++++++------
>  mm/slub.c       | 22 +++++++---------------
>  3 files changed, 18 insertions(+), 30 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6dc4888a90f3..4e427286a88a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2865,15 +2865,13 @@ struct mem_cgroup *mem_cgroup_from_obj_slab(struct slab *slab, void *p)
>          */
>         unsigned long obj_exts;
>         struct slabobj_ext *obj_ext;
> -       unsigned int off;
>
>         obj_exts = slab_obj_exts(slab);
>         if (!obj_exts)
>                 return NULL;
>
>         get_slab_obj_exts(obj_exts);
> -       off = obj_to_index(slab->slab_cache, slab, p);
> -       obj_ext = slab_obj_ext(slab, obj_exts, off);
> +       obj_ext = slab_obj_ext(slab->slab_cache, slab, obj_exts, p);
>         if (obj_ext->objcg) {
>                 struct obj_cgroup *objcg = obj_ext->objcg;
>
> @@ -3541,7 +3539,6 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>         size_t obj_size = obj_full_size(s);
>         struct obj_cgroup *objcg;
>         struct slab *slab;
> -       unsigned long off;
>         size_t i;
>
>         /*
> @@ -3616,8 +3613,7 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>
>                 obj_exts = slab_obj_exts(slab);
>                 get_slab_obj_exts(obj_exts);
> -               off = obj_to_index(s, slab, p[i]);
> -               obj_ext = slab_obj_ext(slab, obj_exts, off);
> +               obj_ext = slab_obj_ext(s, slab, obj_exts, p[i]);
>                 obj_cgroup_get(objcg);
>                 obj_ext->objcg = objcg;
>                 put_slab_obj_exts(obj_exts);
> @@ -3635,10 +3631,8 @@ void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
>                 struct obj_cgroup *objcg;
>                 struct slabobj_ext *obj_ext;
>                 struct obj_stock_pcp *stock;
> -               unsigned int off;
>
> -               off = obj_to_index(s, slab, p[i]);
> -               obj_ext = slab_obj_ext(slab, obj_exts, off);
> +               obj_ext = slab_obj_ext(s, slab, obj_exts, p[i]);
>                 objcg = obj_ext->objcg;
>                 if (!objcg)
>                         continue;
> diff --git a/mm/slab.h b/mm/slab.h
> index 7bd361447c54..36d067d6e7c0 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -646,14 +646,16 @@ static inline unsigned int slab_get_stride(struct slab *slab)
>   * Returns a pointer to the object extension associated with the object.
>   * Must be called within a section covered by get/put_slab_obj_exts().
>   */
> -static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
> -                                              unsigned long obj_exts,
> -                                              unsigned int index)
> +static inline struct slabobj_ext *
> +slab_obj_ext(struct kmem_cache *s, struct slab *slab, unsigned long obj_exts,
> +            const void *obj)
>  {
>         struct slabobj_ext *obj_ext;
> +       unsigned int index;
>
>         VM_WARN_ON_ONCE(obj_exts != slab_obj_exts(slab));
>
> +       index = obj_to_index(s, slab, obj);
>         obj_ext = (struct slabobj_ext *)(obj_exts +
>                                          slab_get_stride(slab) * index);
>         return kasan_reset_tag(obj_ext);
> @@ -669,9 +671,9 @@ static inline unsigned long slab_obj_exts(struct slab *slab)
>         return 0;
>  }
>
> -static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
> -                                              unsigned long obj_exts,
> -                                              unsigned int index)
> +static inline struct slabobj_ext *
> +slab_obj_ext(struct kmem_cache *s, struct slab *slab, unsigned long obj_exts,
> +            const void *obj)
>  {
>         return NULL;
>  }
> diff --git a/mm/slub.c b/mm/slub.c
> index 9e25f2dce7a6..5e3f53bcd0d3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2070,11 +2070,10 @@ static inline void mark_obj_codetag_empty(const void *obj)
>         obj_slab = virt_to_slab(obj);
>         slab_exts = slab_obj_exts(obj_slab);
>         if (slab_exts) {
> +               struct slabobj_ext *ext;
> +
>                 get_slab_obj_exts(slab_exts);
> -               unsigned int offs = obj_to_index(obj_slab->slab_cache,
> -                                                obj_slab, obj);
> -               struct slabobj_ext *ext = slab_obj_ext(obj_slab,
> -                                                      slab_exts, offs);
> +               ext = slab_obj_ext(obj_slab->slab_cache, obj_slab, slab_exts, obj);
>
>                 if (unlikely(is_codetag_empty(&ext->ref))) {
>                         put_slab_obj_exts(slab_exts);
> @@ -2362,10 +2361,8 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags,
>          * check should be added before alloc_tag_add().
>          */
>         if (obj_exts) {
> -               unsigned int obj_idx = obj_to_index(s, slab, object);
> -
>                 get_slab_obj_exts(obj_exts);
> -               obj_ext = slab_obj_ext(slab, obj_exts, obj_idx);
> +               obj_ext = slab_obj_ext(s, slab, obj_exts, object);
>                 alloc_tag_add(&obj_ext->ref, current->alloc_tag, s->size);
>                 put_slab_obj_exts(obj_exts);
>         } else {
> @@ -2386,7 +2383,6 @@ static noinline void
>  __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
>                                int objects)
>  {
> -       int i;
>         unsigned long obj_exts;
>
>         /* slab->obj_exts might not be NULL if it was created for MEMCG accounting. */
> @@ -2398,13 +2394,11 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p
>                 return;
>
>         get_slab_obj_exts(obj_exts);
> -       for (i = 0; i < objects; i++) {
> -               unsigned int off = obj_to_index(s, slab, p[i]);
> -
> +       for (int i = 0; i < objects; i++) {
>                 if (is_kfence_address(p[i]))
>                         continue;
>
> -               alloc_tag_sub(&slab_obj_ext(slab, obj_exts, off)->ref, s->size);
> +               alloc_tag_sub(&slab_obj_ext(s, slab, obj_exts, p[i])->ref, s->size);
>         }
>         put_slab_obj_exts(obj_exts);
>  }
> @@ -2489,7 +2483,6 @@ bool memcg_slab_post_charge(void *p, gfp_t flags)
>         struct kmem_cache *s;
>         struct page *page;
>         struct slab *slab;
> -       unsigned long off;
>
>         page = virt_to_page(p);
>         if (PageLargeKmalloc(page)) {
> @@ -2529,8 +2522,7 @@ bool memcg_slab_post_charge(void *p, gfp_t flags)
>         obj_exts = slab_obj_exts(slab);
>         if (obj_exts) {
>                 get_slab_obj_exts(obj_exts);
> -               off = obj_to_index(s, slab, p);
> -               obj_ext = slab_obj_ext(slab, obj_exts, off);
> +               obj_ext = slab_obj_ext(s, slab, obj_exts, p);
>                 if (unlikely(obj_ext->objcg)) {
>                         put_slab_obj_exts(obj_exts);
>                         return true;
>
> --
> 2.55.0
>