Re: [PATCH v3 1/1] mm/slab: mark alloc tags empty for sheaves allocated with __GFP_NO_OBJ_EXT

[Vlastimil Babka]

On 2/25/26 8:08 PM, Suren Baghdasaryan wrote:
> On Wed, Feb 25, 2026 at 4:34 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>>
>> alloc_empty_sheaf() allocates sheaves from SLAB_KMALLOC caches using
>> __GFP_NO_OBJ_EXT to avoid recursion, however it does not mark their
>> allocation tags empty before freeing, which results in a warning when
>> CONFIG_MEM_ALLOC_PROFILING_DEBUG is set. Fix this by marking allocation
>> tags for such sheaves as empty.
>>
> 
> I think this should also have:
> 
> Fixes: 4c0a17e28340 ("slab: prevent recursive kmalloc() in alloc_empty_sheaf()")
> 
> and CC to stable for inclusion into 6.19.
> Andrew, Vlastimil, should I post another version CC'ing stable or you
> can add that line and forward to stable?

I will add it. Note, they don't care about getting an actual email, but
seeing the Cc: line in the mainline commit.

>> Reported-by: David Wang <00107082@xxxxxxx>
>> Closes: https://lore.kernel.org/all/20260223155128.3849-1-00107082@xxxxxxx/
>> Analyzed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
>> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
>> Tested-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
>> Tested-by: David Wang <00107082@xxxxxxx>
>> ---
>>  include/linux/gfp_types.h |  2 ++
>>  mm/slab.h                 |  4 ++--
>>  mm/slub.c                 | 33 +++++++++++++++++++++++----------
>>  3 files changed, 27 insertions(+), 12 deletions(-)
>>
>> diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
>> index 814bb2892f99..6c75df30a281 100644
>> --- a/include/linux/gfp_types.h
>> +++ b/include/linux/gfp_types.h
>> @@ -139,6 +139,8 @@ enum {
>>   * %__GFP_ACCOUNT causes the allocation to be accounted to kmemcg.
>>   *
>>   * %__GFP_NO_OBJ_EXT causes slab allocation to have no object extension.
>> + * mark_obj_codetag_empty() should be called upon freeing for objects allocated
>> + * with this flag to indicate that their NULL tags are expected and normal.
>>   */
>>  #define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE)
>>  #define __GFP_WRITE    ((__force gfp_t)___GFP_WRITE)
>> diff --git a/mm/slab.h b/mm/slab.h
>> index 71c7261bf822..f6ef862b60ef 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -290,14 +290,14 @@ static inline void *nearest_obj(struct kmem_cache *cache,
>>
>>  /* Determine object index from a given position */
>>  static inline unsigned int __obj_to_index(const struct kmem_cache *cache,
>> -                                         void *addr, void *obj)
>> +                                         void *addr, const void *obj)
>>  {
>>         return reciprocal_divide(kasan_reset_tag(obj) - addr,
>>                                  cache->reciprocal_size);
>>  }
>>
>>  static inline unsigned int obj_to_index(const struct kmem_cache *cache,
>> -                                       const struct slab *slab, void *obj)
>> +                                       const struct slab *slab, const void *obj)
>>  {
>>         if (is_kfence_address(obj))
>>                 return 0;
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 862642c165ed..34c32749f091 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2041,18 +2041,18 @@ static inline void dec_slabs_node(struct kmem_cache *s, int node,
>>
>>  #ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
>>
>> -static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
>> +static inline void mark_obj_codetag_empty(const void *obj)
>>  {
>> -       struct slab *obj_exts_slab;
>> +       struct slab *obj_slab;
>>         unsigned long slab_exts;
>>
>> -       obj_exts_slab = virt_to_slab(obj_exts);
>> -       slab_exts = slab_obj_exts(obj_exts_slab);
>> +       obj_slab = virt_to_slab(obj);
>> +       slab_exts = slab_obj_exts(obj_slab);
>>         if (slab_exts) {
>>                 get_slab_obj_exts(slab_exts);
>> -               unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
>> -                                                obj_exts_slab, obj_exts);
>> -               struct slabobj_ext *ext = slab_obj_ext(obj_exts_slab,
>> +               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);
>>
>>                 if (unlikely(is_codetag_empty(&ext->ref))) {
>> @@ -2090,7 +2090,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>>
>>  #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>>
>> -static inline void mark_objexts_empty(struct slabobj_ext *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) {}
>> @@ -2211,7 +2211,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>>                  * assign slabobj_exts in parallel. In this case the existing
>>                  * objcg vector should be reused.
>>                  */
>> -               mark_objexts_empty(vec);
>> +               mark_obj_codetag_empty(vec);
>>                 if (unlikely(!allow_spin))
>>                         kfree_nolock(vec);
>>                 else
>> @@ -2254,7 +2254,7 @@ static inline void free_slab_obj_exts(struct slab *slab, bool allow_spin)
>>          * 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_obj_codetag_empty(obj_exts);
>>         if (allow_spin)
>>                 kfree(obj_exts);
>>         else
>> @@ -2312,6 +2312,10 @@ static void alloc_slab_obj_exts_early(struct kmem_cache *s, struct slab *slab)
>>
>>  #else /* CONFIG_SLAB_OBJ_EXT */
>>
>> +static inline void mark_obj_codetag_empty(const void *obj)
>> +{
>> +}
>> +
>>  static inline void init_slab_obj_exts(struct slab *slab)
>>  {
>>  }
>> @@ -2783,6 +2787,15 @@ static inline struct slab_sheaf *alloc_empty_sheaf(struct kmem_cache *s,
>>
>>  static void free_empty_sheaf(struct kmem_cache *s, struct slab_sheaf *sheaf)
>>  {
>> +       /*
>> +        * If the sheaf was created with __GFP_NO_OBJ_EXT flag then its
>> +        * corresponding extension is NULL and alloc_tag_sub() will throw a
>> +        * warning, therefore replace NULL with CODETAG_EMPTY to indicate
>> +        * that the extension for this sheaf is expected to be NULL.
>> +        */
>> +       if (s->flags & SLAB_KMALLOC)
>> +               mark_obj_codetag_empty(sheaf);
>> +
>>         kfree(sheaf);
>>
>>         stat(s, SHEAF_FREE);
>>
>> base-commit: 7dff99b354601dd01829e1511711846e04340a69
>> --
>> 2.53.0.414.gf7e9f6c205-goog
>>