Re: [PATCH RFC 06/12] mm/slab: abstract slabobj_ext.ref access

[Suren Baghdasaryan]

On Wed, Jul 15, 2026 at 3:11 AM Vlastimil Babka (SUSE)
<vbabka@xxxxxxxxxx> wrote:
>
> In preparation for changes to the structure, abstract access to the ref
> field with a slab_obj_ext_codetag_ref() function. Rename the field to
> _ctref to make an unexpected direct access a compile error.
>
> No functional change intended.
>
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  mm/slab.h | 10 +++++++++-
>  mm/slub.c | 42 ++++++++++++++++++++++++++++--------------
>  2 files changed, 37 insertions(+), 15 deletions(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 789bd292075f..e3f8e42070f1 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -558,7 +558,7 @@ struct slabobj_ext {
>         struct obj_cgroup *_objcg;
>  #endif
>  #ifdef CONFIG_MEM_ALLOC_PROFILING
> -       union codetag_ref ref;
> +       union codetag_ref _ctref;
>  #endif
>  } __aligned(8);
>
> @@ -668,6 +668,14 @@ static inline struct obj_cgroup **slab_obj_ext_objcgp(struct slabobj_ext *obj_ex
>  }
>  #endif
>
> +#ifdef CONFIG_MEM_ALLOC_PROFILING
> +static inline union codetag_ref *
> +slab_obj_ext_codetag_ref(struct slab *slab, struct slabobj_ext *obj_ext)
> +{
> +       return &obj_ext->_ctref;
> +}
> +#endif
> +
>  int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>                         gfp_t gfp, unsigned int alloc_flags);
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 48e10198a3ce..2bfcabc4c51a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2071,18 +2071,20 @@ static inline void mark_obj_codetag_empty(const void *obj)
>         slab_exts = slab_obj_exts(obj_slab);
>         if (slab_exts) {
>                 struct slabobj_ext *ext;
> +               union codetag_ref *ref;
>
>                 get_slab_obj_exts(slab_exts);
>                 ext = slab_obj_ext(obj_slab->slab_cache, obj_slab, slab_exts, obj);
> +               ref = slab_obj_ext_codetag_ref(obj_slab, ext);
>
> -               if (unlikely(is_codetag_empty(&ext->ref))) {
> +               if (unlikely(is_codetag_empty(ref))) {
>                         put_slab_obj_exts(slab_exts);
>                         return;
>                 }
>
>                 /* codetag should be NULL here */
> -               WARN_ON(ext->ref.ct);
> -               set_codetag_empty(&ext->ref);
> +               WARN_ON(ref->ct);
> +               set_codetag_empty(ref);
>                 put_slab_obj_exts(slab_exts);
>         }
>  }
> @@ -2092,19 +2094,22 @@ static inline bool mark_failed_objexts_alloc(struct slab *slab)
>         return cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL) == 0;
>  }
>
> -static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> -                       struct slabobj_ext *vec, unsigned int objects)
> +static inline void handle_failed_objexts_alloc(struct slab *slab,
> +               unsigned long obj_exts, struct slabobj_ext *vec)
>  {
>         /*
>          * If vector previously failed to allocate then we have live
>          * objects with no tag reference. Mark all references in this
>          * vector as empty to avoid warnings later on.
>          */
> -       if (obj_exts == OBJEXTS_ALLOC_FAIL) {
> -               unsigned int i;
> +       if (obj_exts != OBJEXTS_ALLOC_FAIL)
> +               return;
> +
> +       for (unsigned int i = 0; i < slab->objects; i++) {
> +               union codetag_ref *ref = slab_obj_ext_codetag_ref(slab, vec);
>
> -               for (i = 0; i < objects; i++)
> -                       set_codetag_empty(&vec[i].ref);
> +               set_codetag_empty(ref);
> +               vec++;
>         }
>  }
>
> @@ -2112,8 +2117,8 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>
>  static inline void mark_obj_codetag_empty(const void *obj) {}
>  static inline bool mark_failed_objexts_alloc(struct slab *slab) { return false; }
> -static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> -                       struct slabobj_ext *vec, unsigned int objects) {}
> +static inline void handle_failed_objexts_alloc(struct slab *slab,
> +               unsigned long obj_exts, struct slabobj_ext *vec) {}
>
>  #endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>
> @@ -2181,7 +2186,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  #endif
>  retry:
>         old_exts = READ_ONCE(slab->obj_exts);
> -       handle_failed_objexts_alloc(old_exts, vec, slab->objects);
> +       handle_failed_objexts_alloc(slab, old_exts, vec);
>
>         if (new_slab) {
>                 /*
> @@ -2361,9 +2366,15 @@ __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) {
> +               union codetag_ref *ref;
> +
>                 get_slab_obj_exts(obj_exts);
> +
>                 obj_ext = slab_obj_ext(s, slab, obj_exts, object);
> -               alloc_tag_add(&obj_ext->ref, current->alloc_tag, s->size);
> +               ref = slab_obj_ext_codetag_ref(slab, obj_ext);
> +
> +               alloc_tag_add(ref, current->alloc_tag, s->size);
> +
>                 put_slab_obj_exts(obj_exts);
>         } else {
>                 alloc_tag_set_inaccurate(current->alloc_tag);
> @@ -2395,10 +2406,13 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p
>
>         get_slab_obj_exts(obj_exts);
>         for (int i = 0; i < objects; i++) {
> +               struct slabobj_ext *ext;
> +
>                 if (is_kfence_address(p[i]))
>                         continue;
>
> -               alloc_tag_sub(&slab_obj_ext(s, slab, obj_exts, p[i])->ref, s->size);
> +               ext = slab_obj_ext(s, slab, obj_exts, p[i]);
> +               alloc_tag_sub(slab_obj_ext_codetag_ref(slab, ext), s->size);
>         }
>         put_slab_obj_exts(obj_exts);
>  }
>
> --
> 2.55.0
>