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

[Shakeel Butt <shakeel.butt@linux.dev>]

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>

This looks good to me but would love to have Harry's comment on it.