Re: [PATCH 0/2] fix kmem over-charging for embedded obj_exts array

[Harry Yoo]

On Tue, Mar 10, 2026 at 11:38:02AM +0000, ranxiaokai627@xxxxxxx wrote:
> From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> 
> Since commit a77d6d338685 ("mm/slab: place slabobj_ext metadata
> in unused space within s->size"), the struct slabobj_ext array can
> use slab leftover space or be embedded into the slub object to save
> memory. In these cases, no extra kmalloc space is allocated for the
> obj_exts array.
> 
> However, obj_full_size() always returns extra sizeof(struct obj_cgroup *)
> bytes for every object, which leads to over-charging for slabs with
> embedded obj_exts.
> 
> This series optimizes obj_full_size() to check whether obj_exts uses
> slab leftover space or is embedded in the object. If so, only the object
> size is charged. Otherwise, the extra obj_cgroup pointer space is also
> charged.

Hi Ran,

At first look, I'm not sure if it's a good idea - although it's
allocated from wasted space, it's still memory that's needed to
charge objects.

But for "embedded into the slub object" case, yeah,
the metadata is charged twice, as it's already included in s->size.

Not having much expertise on memcg myself,
let's see what memcg folks say :)

> Patch1 moves obj_exts_in_slab() definition to slab.h so it can be
>        called from memcontrol.c.
> Patch2 updates obj_full_size() to avoid over-charging.
> 
> Ran Xiaokai (2):
>   mm/slab: move obj_exts_in_slab() definition to slab.h
>   memcg: fix kmem over-charging for embedded obj_exts array
> 
>  mm/memcontrol.c | 19 ++++++++++++++-----
>  mm/slab.h       | 19 +++++++++++++++++++
>  mm/slub.c       | 19 -------------------
>  3 files changed, 33 insertions(+), 24 deletions(-)
> 
> -- 
> 2.25.1
> 
> 
> 

-- 
Cheers,
Harry / Hyeonggon