Re: [PATCH] mm: memcg: fix slab over-accounting for in-object objcg metadata

[Harry Yoo <harry@kernel.org>]

On Wed, Aug 12, 2026 at 08:48:58PM +0800, Guopeng Zhang wrote:
> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
> 
> obj_full_size() adds the size of an obj_cgroup pointer to s->size to
> account for slabobj_ext storage, which normally resides outside the
> object.
> 
> Since commit a77d6d338685 ("mm/slab: place slabobj_ext metadata in
> unused space within s->size"), slabobj_ext can reside in object padding
> already covered by s->size. The extra charge then accounts for the same
> memory twice.
> 
> The per-slab obj_exts_in_object bit introduced by commit b5bc35ace2c5
> ("mm/slab: replace slab.stride with obj_exts_in_object") identifies this
> layout. Use it to omit the extra charge only for in-object metadata.
> Unlike in-object metadata, slab-leftover and separately allocated metadata
> are not covered by the per-object s->size charge, so retain the existing
> charge for those cases.
> 
> Reported-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> Link: https://lore.kernel.org/all/20260310113804.245647-1-ranxiaokai627@163.com/
> Fixes: a77d6d338685 ("mm/slab: place slabobj_ext metadata in unused space within s->size")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>  mm/memcontrol.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d1312441a02b..aef480f46891 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3526,12 +3526,16 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>  	refill_obj_stock(objcg, size, true);
>  }
>  
> -static inline size_t obj_full_size(struct kmem_cache *s)
> +static inline size_t obj_full_size(struct kmem_cache *s, struct slab *slab)
>  {
>  	/*
>  	 * For each accounted object there is an extra space which is used
> -	 * to store obj_cgroup membership. Charge it too.
> +	 * to store obj_cgroup membership. Charge it too, unless it is stored
> +	 * in object padding already covered by s->size.
>  	 */
> +	if (obj_exts_in_object(slab))
> +		return s->size;
> +
>  	return s->size + sizeof(struct obj_cgroup *);

From correctness terms of view, even this is not 100% correct because
it doesn't account for the metadata for memory allocation profiling.

But as Vlastimil questioned 5 months ago [1]: is it worth the
complexity? Will simply charging s->size always make difference in
practice?

[1] https://lore.kernel.org/linux-mm/e7ccb3b0-791e-4e01-86c3-65b613e3cc5a@kernel.org