Re: [PATCH V1 2/2] mm/slab: only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

[Hao Li <hao.li@linux.dev>]

On Tue, Jan 27, 2026 at 07:31:51PM +0900, Harry Yoo wrote:
> While SLAB_OBJ_EXT_IN_OBJ allows to reduce memory overhead to account
> slab objects, it prevents slab merging because merging can change
> the metadata layout.
> 
> As pointed out Vlastimil Babka, disabling merging solely for this memory
> optimization may not be a net win, because disabling slab merging tends
> to increase overall memory usage.
> 
> Restrict SLAB_OBJ_EXT_IN_OBJ to caches that are already unmergeable for
> other reasons (e.g., those with constructors or SLAB_TYPESAFE_BY_RCU).
> 
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  mm/slab.h        | 1 +
>  mm/slab_common.c | 3 +--
>  mm/slub.c        | 3 ++-
>  3 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 8593c506cbf1..a5c4f981ee8b 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -388,6 +388,7 @@ extern void create_boot_cache(struct kmem_cache *, const char *name,
>  			unsigned int useroffset, unsigned int usersize);
>  
>  int slab_unmergeable(struct kmem_cache *s);
> +bool slab_args_unmergeable(struct kmem_cache_args *args, slab_flags_t flags);
>  
>  slab_flags_t kmem_cache_flags(slab_flags_t flags, const char *name);
>  
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 904414c3ebb8..d5a70a831a2a 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -174,8 +174,7 @@ int slab_unmergeable(struct kmem_cache *s)
>  	return 0;
>  }
>  
> -static bool slab_args_unmergeable(struct kmem_cache_args *args,
> -				  slab_flags_t flags)
> +bool slab_args_unmergeable(struct kmem_cache_args *args, slab_flags_t flags)
>  {
>  	if (slab_nomerge)
>  		return true;
> diff --git a/mm/slub.c b/mm/slub.c
> index ae9af184a18b..0581847e7dac 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7676,7 +7676,8 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
>  	 */
>  	aligned_size = ALIGN(size, s->align);
>  #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT)
> -	if (aligned_size - size >= sizeof(struct slabobj_ext))
> +	if (slab_args_unmergeable(args, s->flags) &&
> +			(aligned_size - size >= sizeof(struct slabobj_ext)))
>  		s->flags |= SLAB_OBJ_EXT_IN_OBJ;

Hi Harry,

This patch looks reasonable to me. I just noticed a minor point that I
wanted to bring up:

It seems a bit self-referential that SLAB_NEVER_MERGE already includes
SLAB_OBJ_EXT_IN_OBJ, but we're using SLAB_NEVER_MERGE to decide whether to set
SLAB_OBJ_EXT_IN_OBJ.

Do you think it might be helpful to add a comment here for better clarity?

-- 
Thanks,
Hao

>  #endif
>  	size = aligned_size;
> -- 
> 2.43.0
>