Re: [PATCH v4] slab: clear OBJEXTS_ALLOC_FAIL when freeing a slab

[Vlastimil Babka <vbabka@suse.cz>]

On 10/15/25 14:59, Hao Ge wrote:
> From: Hao Ge <gehao@kylinos.cn>
> 
> If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> But we did not clear it when freeing the slab. Since OBJEXTS_ALLOC_FAIL and
> MEMCG_DATA_OBJEXTS currently share the same bit position, during the
> release of the associated folio, a VM_BUG_ON_FOLIO() check in
> folio_memcg_kmem() is triggered because it was mistakenly assumed that
> a valid folio->memcg_data was not cleared before freeing the folio.
> 
> When freeing a slab, we clear slab->obj_exts if the obj_ext array has been
> successfully allocated. So let's clear OBJEXTS_ALLOC_FAIL when freeing
> a slab if the obj_ext array allocated fail to allow them to be returned
> to the buddy system more smoothly.

Thanks!

> Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL")
> Suggested-by: Harry Yoo <harry.yoo@oracle.com>
> Signed-off-by: Hao Ge <gehao@kylinos.cn>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Since we changed the approach completely, I think we should not carry over
the previously added review tags in this case.

> ---
> v4: Based on the discussion between Vlastimil and Harry,
>     modify the solution to clear OBJEXTS_ALLOC_FAIL when freeing a slab.
>     This does seem more reasonable. Thank you both.
> ---
>  mm/slab.h | 26 ++++++++++++++++++++++++++
>  mm/slub.c |  6 ++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 078daecc7cf5..52424d6871bd 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -547,6 +547,28 @@ static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
>  	return (struct slabobj_ext *)(obj_exts & ~OBJEXTS_FLAGS_MASK);
>  }
>  
> +/*
> + * objexts_clear_alloc_fail -  Clear the OBJEXTS_ALLOC_FAIL for
> + * the slab object extension vector associated with a slab.
> + * @slab: a pointer to the slab struct
> + */
> +static inline void objexts_clear_alloc_fail(struct slab *slab)
> +{
> +	unsigned long obj_exts = READ_ONCE(slab->obj_exts);
> +
> +#ifdef CONFIG_MEMCG
> +	/*
> +	 * obj_exts should be either NULL, a valid pointer with
> +	 * MEMCG_DATA_OBJEXTS bit set or be equal to OBJEXTS_ALLOC_FAIL.
> +	 */
> +	VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS) &&
> +		       obj_exts != OBJEXTS_ALLOC_FAIL, slab_page(slab));
> +	VM_BUG_ON_PAGE(obj_exts & MEMCG_DATA_KMEM, slab_page(slab));
> +#endif
> +
> +	obj_exts &= ~OBJEXTS_ALLOC_FAIL;
> +	WRITE_ONCE(slab->obj_exts, obj_exts);
> +}

This is much larger than necessary I think. See below.

>  int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>                          gfp_t gfp, bool new_slab);
>  
> @@ -557,6 +579,10 @@ static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
>  	return NULL;
>  }
>  
> +static inline void objexts_clear_alloc_fail(struct slab *slab)
> +{
> +}
> +
>  #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 b1f15598fbfd..80166a4a62f9 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2169,6 +2169,12 @@ static inline void free_slab_obj_exts(struct slab *slab)
>  {
>  	struct slabobj_ext *obj_exts;
>  
> +	/*
> +	 * If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> +	 * Therefore, we should clear the OBJEXTS_ALLOC_FAIL flag first when freeing a slab.
> +	 */
> +	objexts_clear_alloc_fail(slab);
> +
>  	obj_exts = slab_obj_exts(slab);
>  	if (!obj_exts)
>  		return;

It should be enough that this return path does "slab->obj_exts = 0;" no?