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

[Tao Cui]

在 2026/8/12 20:48, Guopeng Zhang 写道:
> From: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/20260310113804.245647-1-ranxiaokai627@xxxxxxx/
> Fixes: a77d6d338685 ("mm/slab: place slabobj_ext metadata in unused space within s->size")
> Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
> ---
>  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 *);
>  }
>  
> @@ -3539,7 +3543,6 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  				  gfp_t flags, unsigned int slab_alloc_flags,
>  				  size_t size, void **p)
>  {
> -	size_t obj_size = obj_full_size(s);
>  	struct obj_cgroup *objcg;
>  	struct slab *slab;
>  	size_t i;
> @@ -3580,6 +3583,7 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  		unsigned long obj_exts;
>  		struct slabobj_ext *obj_ext;
>  		struct obj_stock_pcp *stock;
> +		size_t obj_size;
>  
>  		slab = virt_to_slab(p[i]);
>  
> @@ -3590,6 +3594,8 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  				continue;
>  		}
>  
> +		obj_size = obj_full_size(s, slab);
> +
>  		/*
>  		 * if we fail and size is 1, memcg_alloc_abort_single() will
>  		 * just free the object, which is ok as we have not assigned
> @@ -3632,7 +3638,7 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  void __memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab,
>  			    void **p, int objects, unsigned long obj_exts)
>  {
> -	size_t obj_size = obj_full_size(s);
> +	size_t obj_size = obj_full_size(s, slab);
>  
>  	for (int i = 0; i < objects; i++) {
>  		struct obj_cgroup *objcg;
> 
> base-commit: e7f630142df2afccce90555e4972e60008222311
Reviewed-by: Tao Cui <cuitao@xxxxxxxxxx>