Re: [PATCH V5 4/8] mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Jan 07, 2026 at 03:56:41PM +0100, Vlastimil Babka wrote:
> On 1/5/26 09:02, Harry Yoo wrote:
> > Currently, the slab allocator assumes that slab->obj_exts is a pointer
> > to an array of struct slabobj_ext objects. However, to support storage
> > methods where struct slabobj_ext is embedded within objects, the slab
> > allocator should not make this assumption. Instead of directly
> > dereferencing the slabobj_exts array, abstract access to
> > struct slabobj_ext via helper functions.
> > 
> > Introduce a new API slabobj_ext metadata access:
> > 
> >   slab_obj_ext(slab, obj_exts, index) - returns the pointer to
> >   struct slabobj_ext element at the given index.
> > 
> > Directly dereferencing the return value of slab_obj_exts() is no longer
> > allowed. Instead, slab_obj_ext() must always be used to access
> > individual struct slabobj_ext objects.
> > 
> > Convert all users to use these APIs.
> > No functional changes intended.
> > 
> > +/*
> > + * slab_obj_ext - get the pointer to the slab object extension metadata
> > + * associated with an object in a slab.
> > + * @slab: a pointer to the slab struct
> > + * @obj_exts: a pointer to the object extension vector
> > + * @index: an index of the object
> > + *
> > + * Returns a pointer to the object extension associated with the object.
> > + */
> > +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));
> 
> The first check seems redundant given we have the second one?
> If we get passed obj_ext 0 and slab_obj_exts() is also 0, it will blow up quickly anyway.

Right, will do.

> > +
> > +	obj_ext = (struct slabobj_ext *)obj_exts;
> > +	return &obj_ext[index];
> >  }
> >  
> >  int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> > @@ -533,7 +558,13 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >  
> >  #else /* CONFIG_SLAB_OBJ_EXT */
> >  
> > -static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
> > +static inline unsigned long slab_obj_exts(struct slab *slab)
> > +{
> > +	return 0;
> > +}
> > +
> > +static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
> > +					       unsigned int index)
> 
> Hmm this is missing the obj_exts parameter? Either will not compile
> !CONFIG_SLAB_OBJ_EXT or isn't reachable in that config anyway?

It seems it's not reachable as it didn't break !CONFIG_SLAB_OBJ_EXT
builds and I'll fix the prototype anyway.

> >  {
> >  	return NULL;
> >  }
> > @@ -550,7 +581,7 @@ static inline enum node_stat_item cache_vmstat_idx(struct kmem_cache *s)
> >  bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
> >  				  gfp_t flags, size_t size, void **p);
> >  void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
> > -			    void **p, int objects, struct slabobj_ext *obj_exts);
> > +			    void **p, int objects, unsigned long obj_exts);
> >  #endif
> >  
> >  void kvfree_rcu_cb(struct rcu_head *head);
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 0e32f6420a8a..84bd4f23dc4a 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> 
> <snip>
> 
> > @@ -2176,7 +2178,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >  
> >  static inline void free_slab_obj_exts(struct slab *slab)
> >  {
> > -	struct slabobj_ext *obj_exts;
> > +	unsigned long obj_exts;
> 
> I think in this function we could leave it as pointer.
> 
> >  	obj_exts = slab_obj_exts(slab);
> 
> And do a single cast here.
> 
> >  	if (!obj_exts) {
> > @@ -2196,11 +2198,11 @@ static inline void free_slab_obj_exts(struct slab *slab)
> >  	 * NULL, therefore replace NULL with CODETAG_EMPTY to indicate that
> >  	 * the extension for obj_exts is expected to be NULL.
> >  	 */
> > -	mark_objexts_empty(obj_exts);
> > +	mark_objexts_empty((struct slabobj_ext *)obj_exts);
> >  	if (unlikely(READ_ONCE(slab->obj_exts) & OBJEXTS_NOSPIN_ALLOC))
> > -		kfree_nolock(obj_exts);
> > +		kfree_nolock((void *)obj_exts);
> >  	else
> > -		kfree(obj_exts);
> > +		kfree((void *)obj_exts);
> >  	slab->obj_exts = 0;
> >  }
> 
> And avoid those 3 above.

That works for me, will do.

-- 
Cheers,
Harry / Hyeonggon